Justin M. Turney

Senior Research Scientist

About Me

Originally from southeast Texas, I moved to Athens in 2002 to start my graduate studies under Prof. Henry F. Schaefer. Upon graduation, I was brought on board as a full-time researcher. In 2012, I was named Senior Research Scientist.

Here at the CCQC, I am responsible for the creation of research projects for our graduate students. I then mentor the students through the entire process of a research project; from literature review, performing and analysing computations, and writing a manuscript. I lead students on both application and theoretical research projects.

Areas of Research Interest

The overarching goal of my research is to understand the effects that govern molecular interactions through the application of computational chemistry. This can be summarized as:

  • High-accuracy computational reaction energetics;
  • Development of ab initio quantum mechanical methods that could potentially be applied to presently inaccessible chemical systems applying ab initio quantum chemical techniques to a wide variety of molecular species with multi-reference character
  • Efficient implementations of quantum chemical models on modern high-performance computing hardware.
  • Development of machine learning models for the description of molecular geometries and properties.

Software

I have developed or been a part in the development of the following software packages:

1. Psi4

Psi4 is an open-source suite of ab initio quantum chemistry programs designed for efficient, high-accuracy simulations of a variety of molecular properties. It is very easy to use and has an optional Python interface.
Website     Source Code

2. Psi4NumPy

The overall goal of the Psi4NumPy project is to provide an interactive quantum chemistry framework for reference implementations, rapid prototyping, development, and education. To do this, quantities relevant to quantum chemistry are computed with the Psi4 electronic structure package, and subsequently manipulated using the Numerical Python (NumPy) package. This combination provides an interface that is both simple to use and remains relatively fast to execute.
Source Code     View Paper

3. Ambit

Ambit is a C++ library for the implementation of tensor product calculations through a clean, concise user interface.
Source Code

4. PES-Learn

PES-Learn is a Python library designed to fit system-specific Born-Oppenheimer potential energy surfaces using modern machine learning models. PES-Learn assists in generating datasets, and features Gaussian process and neural network model optimization routines. The goal is to provide high-performance models for a given dataset without requiring user expertise in machine learning.
Source Code     View Paper

5. Janus

A Python library for adaptive QM/MM methods.
Source Code     View Paper

6. v2RDM-CASSCF

A variational 2-RDM-driven CASSCF plugin to Psi4
Source Code     View Paper

7. Psi3

Psi3 is a suite of ab initio quantum chemistry programs designed to compute various molecular properties.
Source Code     View Paper

Publications

42. Energetics and mechanisms for the acetonyl radical + O2 reaction: An important system for atmospheric and combustion chemistry (Pending)

The acetonyl radical (•CH2COCH3) is relevant to atmospheric and combustion chemistry due to its prevalence in many important reaction mechanisms. One such reaction mechanism is the decomposition of Criegee intermediates in the atmosphere that can produce acetonyl radical and OH. In order to understand the fate of the acetonyl radical in these environments and to create more accurate kinetics models, we have examined the reaction system of acetonyl radical with O2 using highly reliable theoretical methods. Structures were optimized using coupled cluster theory with singles, doubles, and perturbative triples [CCSD(T)] with an atomic natural orbital (ANO0) basis set. Energetics were computed to chemical accuracy using the focal point approach involving perturbative treatment of quadruple excitations [CCSDT(Q)] and basis sets as large as cc-pV5Z. The addition of O2 to acetonyl radical produces the acetonylperoxy radical, and multireference computations on this reaction suggest it to be barrierless. No submerged pathways were found for the unimolecular isomerization of the acetonylperoxy radical. Besides dissociation to reactants, the lowest energy pathway available for the acetonylperoxy radical is a 1-5 H shift from the methyl group to the peroxy group through a transition state that is 3.3 kcal mol-1 higher in energy than acetonyl radical + O2. The ultimate products from this pathway are the enol tautomer of acetonyl radical and O2. Multiple pathways are considered that lead to OH formation; however, all of these pathways are predicted to be energetically inaccessible excepting high temperatures.

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41. Reaction mechanisms of a cyclic ether intermediate: cis-2,3-dimethyloxirane (Pending)

Oxiranes are a class of cyclic ethers formed in abundance during low-temperature combustion of hydrocarbons and biofuels, either via chain-propagating steps that occur from unimolecular decomposition of β-hydroperoxyalkyl radicals (β-Q̇OOH) or from reactions of HOȮ with alkenes. cis- and trans- isomers of 2,3-dimethyloxirane are produced as intermediates from n-butane oxidation, and while rate coefficients for β- Q̇OOH → 2,3-dimethyloxirane + ȮH are reported extensively, subsequent reaction mechanisms of the cyclic ethers are not. As a result, chemical kinetics models commonly adopt simplified chemistry to describe the consumption of 2,3- dimethyloxirane isomers by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error – model uncertainty derived from missing chemistry.

The present work provides fundamental insight on reaction mechanisms of 2,3-cisdimethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during Cl-initiated oxidation of 2,3-cis-dimethyloxirane using multiplexed photoionization mass spectrometry (MPIMS). The experiments were conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, stationary point energies were conducted using MP2/CBS//B3LYP/cc-pVTZ with CCSD(T)/aug-cc-pVDZ corrections on Ṙ + O2 potential energy surfaces for the two 2,3-cis-dimethyloxiranyl radical isomers to determine barrier heights for 14 reaction pathways.

Several species were identified from ring-opening of α- and β-2,3-cis-dimethyloxiranyl radicals and neither of the two conjugate alkene isomers from Ṙ + O2 reactions were detected. Products were also identified from decomposition of alternative Q̇OOH radicals, which form when the unpaired electron is localized adjacent to the ether group, causing ring-opening and the formation of a resonance-stabilized carboncentered ketohydroperoxide-type radical. The present work provides the first analysis of 2,3-cis-dimethyloxirane reaction mechanisms and reveals that consumption pathways of alkyl-substituted oxiranes formed as intermediates during hydrocarbon and biofuel combustion are complex and may require expanded sub-mechanisms.

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40. Reaction mechanisms of a cyclic ether intermediate: ethyloxirane (Pending)

Oxiranes are a class of cyclic ethers formed in abundance during low-temperature combustion of hydrocarbons, either via chain-propagating steps that occur from unimolecular decomposition of β-hydroperoxyalkyl radicals (β-Q̇OOH) or from reactions of HOȮ with alkenes. Ethyloxirane is one of three alkyl-substituted isomers produced as an intermediate from n-butane oxidation. While rate coefficients for β- Q̇OOH → ethyloxirane + ȮH are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics models commonly adopt simplified chemistry to describe alkyl-substituted oxirane consumption by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error – model uncertainty derived from missing chemistry.

The present work provides fundamental insight on reaction mechanisms of ethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during Cl-initiated oxidation of ethyloxirane using multiplexed photoionization mass spectrometry (MPIMS). The experiments were conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, stationary point energies were conducted using MP2/CBS//B3LYP/cc-pVTZ with CCSD(T)/aug-cc-pVDZ corrections on Ṙ + O2 potential energy surfaces for the four ethyloxiranyl radical isomers to determine barrier heights for 25 reaction pathways.

In addition to products from Q̇OOH → cyclic ether + ȮH and Ṙ + O2 → conjugate alkene + HOȮ, both of which were significant pathways and are prototypical to alkane oxidation, other species were identified from ring-opening of ethyloxiranyl and from decomposition of alternative Q̇OOH radicals. The latter occurs when the unpaired electron is localized adjacent to the ether group, causing the initial Q̇OOH structure to ring-open and form a resonance-stabilized carbon-centered ketohydroperoxide-type radical that subsequently reacts. The present work provides the first analysis of ethyloxirane reaction mechanisms and reveals that consumption pathways are complex and may require expanded sub-mechanisms.

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39. Sulfurous and sulfonic acids: Predicting Infrared spectra and setting the surface straight

Sulfurous acid (H2SO3) is an infamously elusive molecule. Although some theoretical papers have supposed possible roles for it in more complicated systems, it has yet to be experimentally observed. To aid experiment in detecting this molecule, we have examined the H2O + SO2 potential energy surface at the CCSDT(Q)/CBS//CCSD(T)-F12b/cc-pVTZ-F12b level of theory to resolve standing discrepancies in previous reports and predict the gas-phase vibrational spectrum for H2SO3. We find that sulfurous acid has two potentially detectable rotamers, separated by 1.1 kcal mol−1 ΔH0K with a torsional barrier of 1.6 kcal mol−1. The sulfonic acid isomer is only 6.9 kcal mol−1 above the lowest enthalpy sulfurous acid rotamer, but the barrier to form it is 57.2 kcal mol−1. Error in previous reports can be attributed to misidentified stationary points, the use of density functionals that perform poorly for this system, and, most importantly, the basis set sensitivity of sulfur. Using VPT2+K, we determine that the intense S=O stretch fundamental of each species is separated from other intense peaks by at least 25 cm−1, providing a target for identification by infrared spectroscopy.

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38. Important features of the potential energy surface of the methylamine plus O(1D) reaction

This research presents an ab initio characterization of the potential energy surface for the methylamine plus 1D oxygen atom reaction, which may be relevant to interstellar chemistry. Geometries and harmonic vibrational frequencies were determined for all stationary points at the CCSD(T)/aug-cc-pVTZ level of theory. The focal point method along with several additive corrections was used to obtain reliable CCSDT(Q)/CBS potential energy surface features. Extensive conformational analysis and intrinsic reaction coordinate computations were performed to ensure accurate chemical connectivity of the stationary points. Five minima were determined to be possible products of this reaction and three novel transition states were found that were previously unreported or mislabeled in the literature. The pathways we present can be used to guide further searches for NH2 containing species in the interstellar medium.

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37. Characterization of the 2-methylvinoxy radical + O2 reaction: a focal point analysis and composite multireference study

Vinoxy radicals are involved in numerous atmospheric and combustion mechanisms. High-level theoretical methods have recently shed new light on the reaction of the unsubstituted vinoxy radical with O2. The reactions of 1-methylvinoxy radical and 2-methylvinoxy radical with molecular oxygen have experimental high pressure limiting rate constants, k∞, 5–7 times higher than that of the vinoxy plus O2 reaction. In this work, high-level ab initio quantum chemical computations are applied to the 2-methylvinoxy radical plus O2 system, namely, the formation and isomerization of the 1-oxo-2-propylperoxy radical, the immediate product of O2 addition to the 2-methylvinoxy radical. Multireference methods were applied to the entrance channel. No barrier to O2 addition could be located, and more sophisticated treatment of dynamic electron correlation shows that the principal difference between O2 addition to the vinoxy and 2-methylvinoxy radicals is a larger steric factor for 2-methylvinoxy + O2. This is attributed to the favorable interaction between the incoming O2 molecule and the methyl group of the 2-methylvinoxy radical. Via the focal point approach, energetics for this reaction were determined, in most cases, to chemical accuracy. The coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] correlation energy and Hartree–Fock energies were independently extrapolated to the complete basis set limit. A correction for the effect of higher excitations was computed at the CCSDT(Q)/6-31G level. Corrections for the frozen-core approximation, the Born–Oppenheimer approximation, the nonrelativistic approximation, and the zero-point vibrational energy were included. From the 1-oxo-2-propylperoxy radical, dissociation to reactants is competitive with the lowest energy isomerization pathway. The lowest energy isomerization pathway ultimately forms acetaldehyde, CO, and ·OH as the final products.

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36. The addition of methanol to Criegee intermediates

Bimolecular reactions involving stabilized Criegee intermediates (SCI) have been the target of many studies due to the role these molecules play in atmospheric chemistry. Recently, kinetic rates for the addition reaction of the simplest SCI (formaldehyde oxide) and its methylated analogue (acetone oxide) with methanol were reported both experimentally and theoretically. We re-examine the energy profile of these reactions by employing rigorous ab initio methods. Optimized CCSD(T)/ANO1 geometries are reported for the stationary points along the reaction path. Energies are obtained at the CCSD(T)/CBS level of theory. Contributions of full triple and quadruple excitations are computed to assess the convergence of this method. Rate constants are obtained using conventional canonical transition state theory under the rigid rotor harmonic oscillator approximation and with the inclusion of a one-dimensional hindered rotor treatment. These corrections for internal rotations have a significant impact on computed kinetic rate constants. With this approach, we compute rate constants for the addition of methanol to formaldehyde oxide (H2COO) and acetone oxide [(CH3)2COO] at 298.15 K as (1.2 ± 0.8) × 10−13 and (2.8 ± 1.3) × 10−15 cm3 s−1, respectively. Additionally, we investigate the temperature dependence of the rate constant, concluding that the transition state barrier height and tunneling contributions shape the qualitative behaviour of these reactions.

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35. PES-Learn: an open-source software package for the automated generation of machine learning models of molecular potential energy surfaces

We introduce a free and open-source software package (PES-Learn) which largely automates the process of producing high-quality machine learning models of molecular potential energy surfaces (PESs). PES-Learn incorporates a generalized framework for producing grid points across a PES that is compatible with most electronic structure theory software. The newly generated or externally supplied PES data can then be used to train and optimize neural network or Gaussian process models in a completely automated fashion. Robust hyperparameter optimization schemes designed specifically for molecular PES applications are implemented to ensure that the best possible model for the data set is fit with high quality. The performance of PES-Learn toward fitting a few semiglobal PESs from the literature is evaluated. We also demonstrate the use of PES-Learn machine learning models in carrying out high-level vibrational configuration interaction computations on water and formaldehyde.

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34. JANUS: an extensible open-source software package for adaptive QM/MM methods

Adaptive quantum mechanics/molecular mechanics (QM/MM) approaches are able to treat systems with dynamic or nonlocalized active centers by allowing for on-the-fly reassignment of the QM region. Although these approaches have been in active development, the inaccessibility of current software has caused slow adoption and limited applications. Janus seeks to remedy the limitations of current software by providing a free and open-source Python library for adaptive methods that is modular and extensible. Our software has implementations of many existing adaptive methods and a user-friendly input structure that removes the hindrance of complicated setup procedures. A Python API is made available to customize Janus’s capabilities and implement novel adaptive approaches. Janus currently interfaces with Psi4 and OpenMM, but its modular infrastructure enables easy extensibility to other molecular codes without major modifications to either code. The software is freely available at https://github.com/CCQC/janus. Our goal is that Janus will serve as a user-driven platform for adaptive QM/MM methods.

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33. A comparison between hydrogen and halogen bonding: the hypohalous acid-water dimers, HOXH2O (X=F, Cl, Br)

Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds. The interactions between the HOX monomers (X = F, Cl, Br) and water have been studied at the CCSD(T)/aug-cc-pVTZ level of theory with the spin free X2C-1e method to account for scalar relativistic effects. Focal point analysis was used to determine CCSDT(Q)/CBS dissociation energies. The anti hydrogen bonded dimers were found with interaction energies of −5.62 kcal mol−1, −5.56 kcal mol−1, and −4.97 kcal mol−1 for X = F, Cl, and Br, respectively. The weaker halogen bonded dimers were found to have interaction energies of −1.71 kcal mol−1 and −3.03 kcal mol−1 for X = Cl and Br, respectively. Natural bond orbital analysis and symmetry adapted perturbation theory were used to discern the nature of the halogen and hydrogen bonds and trends due to halogen substitution. The halogen bonds were determined to be weaker than the analogous hydrogen bonds in all cases but close enough in energy to be relevant, significantly more so with increasing halogen size.

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32. Mechanisms of the ethynyl radical reaction with molecular oxygen

The ethynyl radical, •C2H, is a key intermediate in the combustion of various alkynes. Once produced, the ethynyl radical will rapidly react with molecular oxygen to produce a variety of products. This research presents the first comprehensive high level theoretical study of the reaction of the •C2H (2Σ+) radical with molecular oxygen (3Σg–). Correlation methods as complete as CCSDT(Q) were used; basis sets as large as cc-pV6Z were adopted. Focal point analysis was employed to approach relative energies within the bounds of chemical accuracy (≤1 kcal mol–1). Two dominate reaction pathways from the ethynyl peroxy radical include oxygen–oxygen cleavage from the ethynyl peroxy radical that is initially formed to produce HCCO (2A″) and O (3P) and an isomerization of the ethynyl peroxy radical to eventually yield HCO (2A′) and CO (1Σ+). The branching ratio between these two competitive reaction pathways was determined to be 1:1 at 298 K. Minor reaction pathways leading to the production of CO2 (1Σg+) and CH (2Π, 4Σ–, 2Δ) were also characterized. The absence of CCO (3Σ–) and OH (2Π) was explained in terms competition with more accessible reaction pathways.

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31. The non-covalently bound SO…H2O system, including an interpretation of the differences between SO…H2O and O2…H2O

Despite the interest in sulfur monoxide (SO) among astrochemists, spectroscopists, inorganic chemists, and organic chemists, its interaction with water remains largely unexplored. We report the first high level theoretical geometries for the two minimum energy complexes formed by sulfur monoxide and water, and we report energies using basis sets as large as aug-cc-pV(Q+d)Z and correlation effects through perturbative quadruple excitations. One structure of SO⋯H2O is hydrogen bonded and the other chalcogen bonded. The hydrogen bonded complex has an electronic energy of −2.71 kcal mol−1 and a zero kelvin enthalpy of −1.67 kcal mol−1, while the chalcogen bonded complex has an electronic energy of −2.64 kcal mol−1 and a zero kelvin enthalpy of −2.00 kcal mol−1. We also report the transition state between the two structures, which lies below the SO⋯H2O dissociation limit, with an electronic energy of −1.26 kcal mol−1 and an enthalpy of −0.81 kcal mol−1. These features are much sharper than for the isovalent complex of O2 and H2O, which only possesses one weakly bound minimum, so we further analyze the structures with open-shell SAPT0. We find that the interactions between O2 and H2O are uniformly weak, but the SO⋯H2O complex surface is governed by the superior polarity and polarizability of SO, as well as the diffuse electron density provided by sulfur's extra valence shell.

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30. Fundamental vibrational analyses of the HCN monomer, dimer, and isotopologues

In this work we provide high level ab initio treatments of the structures, vibrational frequencies, and electronic energies of the HCN monomer and dimer systems along with several isotopologues. The plethora of information related to this system within the literature is summarized and serves as a basis for comparison with the results of this paper. The geometry of the dimer and monomer are reported at the all electroncoupled‐cluster singles, doubles, and perturbative triples level of theory [AE‐CCSD(T)] with the correlation consistent quadruple‐zeta quality basis sets with extra core functions (cc‐pCVQZ) from Dunning. The theoretical geometries and electronic structures are further analyzed through the use of the Natural Bond Orbital (NBO) method and Natural Resonance Theory (NRT). At the AE‐CCSD(T)/cc‐pCVQZ level of theory, the full cubic with semi‐diagonal quartic force field for nine dimer and four monomer isotopologues (the parent isotopologue along with 15N, 13C, and D derivatives) were obtained to treat the anharmonicity of the vibrations via second order vibrational perturbation theory (VPT2). Lastly, the enthalpy change associated with the formation of the dimer from two monomer units was determined using the focal point analysis. Computations including coupled‐cluster through perturbative quadruples as well as basis sets up to six‐zeta quality, including core functions (cc‐pCVXZ, X=D,T,Q,5,6) were used to extrapolate to the AE‐CCSDT(Q)/CBS energy associated with this hydrogen‐bond forming process. After appending anharmonic zero‐point vibrational, relativistic, and diagonal Born‐Oppenheimer corrections, we report a value of −3.93 kcal mol−1 for the enthalpy of formation. To our knowledge, each set of results (geometries, vibrational frequencies, and energetics) reported in this study represents the highest‐level and most reliable theoretical predictions reported for this system.

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29. This bismuth tetramer Bi4: the 𝜔3 key to experimental observation

The spectroscopic identification of Bi4 has been very elusive. Two constitutional Bi4 isomers of Td and C2v symmetry are investigated and each is found to be a local energetic minimum. The optimized geometries and vibrational frequencies of these two isomers are obtained at the CCSD(T)/cc-pVQZ-PP level of theory, utilizing the Stoll, Metz, and Dolg 60-electron effective core potential. The fundamental frequencies of the Td isomer are obtained at the same level of theory. The focal point analysis method, from a maximum basis set of cc-pV5Z-PP, and proceeding to a maximum correlation method of CCSDTQ, was employed to determine the dissociation energy of Bi4 (Td) into two Bi2 and the adiabatic energy difference between the C2v and Td isomers of Bi4. These quantities are predicted to be +65 kcal mol−1 and +39 kcal mol−1, respectively. Two electron vertical excitation energies between the Td and C2v electronic configurations are computed to be 156 kcal mol−1 for the Td isomer and 9 kcal mol−1 for the C2v isomer. The most probable approach to laboratory spectroscopic identification of Bi4 is via an infrared spectrum. The predicted fundamentals (cm−1) with harmonic IR intensities in parentheses (km mol−1) are 94(0), 123(0.23), and 167(0) for the Td isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi4 has yet to be observed. Through natural bond orbital analysis, the C2v isomer of Bi4 was discovered to exhibit “long-bonding” between the furthest apart ‘wing’ atoms. This long-bonding is postulated to be facilitated by the σ-bonding orbital between the ‘spine’ atoms of the C2v isomer.

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28. Student-friendly guide to molecular integrals

Preceding even the Hartree–Fock method, molecular integrals are the very foundation upon which quantum chemical molecular modeling depends. Discussions of molecular integrals are normally found only in advanced and technical texts or articles. The objective of the present article is to provide less experienced readers, or students in a physical/computational chemistry course, a thorough understanding of molecular integrals. Through a series of detailed Handouts, the student/reader can participate in the derivation of molecular integrals, and in turn implement them in computer code. Hartree–Fock theory is discussed in enough detail to motivate the molecular integrals and address such topics as the atomic orbital basis. An introduction to the programming language of choice, Python3, is provided, tailored toward developing the essential skills necessary for implementing molecular integrals. The article is intended to be useful not only to instructors of physical/computational chemistry, but also to any reader who has independently sought a primer on this elusive subject.

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27. Psi4NumPy: an interactive quantum chemistry programming environment for reference implementations and rapid development

Psi4NumPy demonstrates the use of efficient computational kernels from the open-source Psi4 program through the popular NumPy library for linear algebra in Python to facilitate the rapid development of clear, understandable Python computer code for new quantum chemical methods, while maintaining a relatively low execution time. Using these tools, reference implementations have been created for a number of methods, including self-consistent field (SCF), SCF response, many-body perturbation theory, coupled-cluster theory, configuration interaction, and symmetry-adapted perturbation theory. Furthermore, several reference codes have been integrated into Jupyter notebooks, allowing background, underlying theory, and formula information to be associated with the implementation. Psi4NumPy tools and associated reference implementations can lower the barrier for future development of quantum chemistry methods. These implementations also demonstrate the power of the hybrid C++/Python programming approach employed by the Psi4 program.

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26. Radicals derived from acetaldehyde and vinyl alcohol

Vinyl alcohol and acetaldehyde are isoelectronic products of incomplete butanol combustion. Along with the radicals resulting from the removal of atomic hydrogen or the hydroxyl radical, these species are studied here using ab initio methods as complete as coupled cluster theory with single, double, triple, and perturbative quadruple excitations [CCSDT(Q)], with basis sets as large as cc-pV5Z. The relative energies provided herein are further refined by including corrections for relativistic effects, the frozen core approximation, and the Born–Oppenheimer approximation. The effects of anharmonic zero-point vibrational energies are also treated. The syn conformer of vinyl alcohol is predicted to be lower in energy than the anti conformer by 1.1 kcal mol−1. The alcoholic hydrogen of syn-vinyl alcohol is found to be the easiest to remove, requiring 84.4 kcal mol−1. Five other radicals are also carefully considered, with four conformers investigated for the 1-hydroxyvinyl radical. Beyond energetics, we have conducted an overhaul of the spectroscopic literature for these species. Our results also provide predictions for fundamental modes yet to be reported experimentally. To our knowledge, the ν3 (3076 cm−1) and ν4 (2999 cm−1) C–H stretches for syn-vinyl alcohol and all but one of the vibrational modes for anti-vinyl alcohol (ν1–ν14) are yet to be observed experimentally. For the acetyl radical, ν6 (1035 cm−1), ν11 (944 cm−1), ν12 (97 cm−1), and accounting for our changes to the assignment of the 1419.9 cm−1 experimental mode, ν10 (1441 cm−1), are yet to be observed. We have predicted these unobserved fundamentals and reassigned the experimental 1419.9 cm−1 frequency in the acetyl radical to ν4 rather than to ν10. Our work also strongly supports reassignment of the ν10 and ν11 fundamentals of the vinoxy radical. We suggest that the bands assigned to the overtones of these fundamentals were in fact combination bands. Our findings may be useful in constructing improved combustion models of butanol and in spectroscopically characterizing these molecules further.

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25. Analytic energy gradients for variational two-electron reduced-density-matrix-driven complete active space self-consistent field theory

Analytic energy gradients are presented for a variational two-electron reduced-density-matrix (2-RDM)-driven complete active space self-consistent field (CASSCF) method. The active-space 2-RDM is determined using a semidefinite programing (SDP) algorithm built upon an augmented Lagrangian formalism. Expressions for analytic gradients are simplified by the fact that the Lagrangian is stationary with respect to variations in both the primal and the dual solutions to the SDP problem. Orbital response contributions to the gradient are identical to those that arise in conventional CASSCF methods in which the electronic structure of the active space is described by a full configuration interaction (CI) wave function. We explore the relative performance of variational 2-RDM (v2RDM)- and CI-driven CASSCF for the equilibrium geometries of 20 small molecules. When enforcing two-particle N-representability conditions, full-valence v2RDM-CASSCF-optimized bond lengths display a mean unsigned error of 0.0060 Å and a maximum unsigned error of 0.0265 Å, relative to those obtained from full-valence CI-CASSCF. When enforcing partial three-particle N-representability conditions, the mean and maximum unsigned errors are reduced to only 0.0006 and 0.0054 Å, respectively. For these same molecules, full-valence v2RDM-CASSCF bond lengths computed in the cc-pVQZ basis set deviate from experimentally determined ones on average by 0.017 and 0.011 Å when enforcing two- and three-particle conditions, respectively, whereas CI-CASSCF displays an average deviation of 0.010 Å. The v2RDM-CASSCF approach with two-particle conditions is also applied to the equilibrium geometry of pentacene; optimized bond lengths deviate from those derived from experiment, on average, by 0.015 Å when using a cc-pVDZ basis set and a (22e,22o) active space.

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24. Psi4 1.1: an open-source electronic structure program emphasizing automation, advanced libraries, and interoperability

Psi4 is an ab initio electronic structure program providing methods such as Hartree–Fock, density functional theory, configuration interaction, and coupled-cluster theory. The 1.1 release represents a major update meant to automate complex tasks, such as geometry optimization using complete-basis-set extrapolation or focal-point methods. Conversion of the top-level code to a Python module means that Psi4 can now be used in complex workflows alongside other Python tools. Several new features have been added with the aid of libraries providing easy access to techniques such as density fitting, Cholesky decomposition, and Laplace denominators. The build system has been completely rewritten to simplify interoperability with independent, reusable software components for quantum chemistry. Finally, a wide range of new theoretical methods and analyses have been added to the code base, including functional-group and open-shell symmetry adapted perturbation theory, density-fitted coupled cluster with frozen natural orbitals, orbital-optimized perturbation and coupled-cluster methods (e.g., OO-MP2 and OO-LCCD), density-fitted multiconfigurational self-consistent field, density cumulant functional theory, algebraic-diagrammatic construction excited states, improvements to the geometry optimizer, and the “X2C” approach to relativistic corrections, among many other improvements.

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23. Ethylperoxy radical: approaching spectroscopic accuracy via coupled cluster theory

Interest in peroxy radicals derives from their central role in tropospheric and low-temperature combustion processes; however, their transient nature limits the scope of possible experimental characterization. As a result, theoretical methods (notably, coupled-cluster theory) have become indispensable in the reliable prediction of properties of such ephemeral open-shell systems. Herein, the [X with combining tilde] and à state conformers of ethylperoxy radical (C2H5O2) have been structurally optimized at the CCSD(T)/ANO2 level of theory. Relative enthalpies at 0 K [including à ← [X with combining tilde] transition origins (T0)] are reported, incorporating CCSD(T) electronic energies extrapolated to the complete basis set limit via the focal point approach. Higher-level computations, employing basis sets as large as cc-pV5Z and post-HF methods up to CCSDT(Q), prove essential in achieving predictions to within 10 cm−1 for experimental T0; we predict 7363 and 7583 cm−1 for the trans and gauche conformers, respectively. Furthermore, predictions of [X with combining tilde] state fundamental transitions incorporating CCSD(T)/ANO0 anharmonic contributions are given. For each conformer, all 21 modes were characterized, improving upon the 16 modes reported in the experimental literature, and providing predictions for the 5 remaining modes.

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22. The fate of the tert-butyl radical in low-temperature autoignition reactions

Alkyl combustion models depend on kinetic parameters derived from reliable experimental or theoretical energetics that are often unavailable for larger species. To this end, we have performed a comprehensive investigation of the tert-butyl radical (R• in this paper) autoignition pathways. CCSD(T)/ANO0 geometries and harmonic vibrational frequencies were obtained for key stationary points for the R• + O2 and QOOH + O2 mechanisms. Relative energies were computed to chemical accuracy (±1 kcal mol−1) via extrapolation of RCCSD(T) energies to the complete basis-set limit, or usage of RCCSD(T)-F12 methods. At 0 K, the minimum energy R• + O2 pathway involves direct elimination of HO∙2 (30.3 kcal mol−1 barrier) from the tert-butyl peroxy radical (ROO•) to give isobutene. This pathway lies well below the competing QOOH-forming intramolecular hydrogen abstraction pathway (36.2 kcal mol−1 barrier) and ROO• dissociation (35.9 kcal mol−1 barrier). The most favorable decomposition channel for QOOH radicals leads to isobutene oxide (12.0 kcal mol−1 barrier) over isobutene (18.6 kcal mol−1 barrier). For the QOOH + O2 pathways, we studied the transition states and initial products along three pathways: (1) α-hydrogen abstraction (42.0 kcal mol−1 barrier), (2) γ-hydrogen abstraction (27.0 kcal mol−1 barrier), and (3) hydrogen transfer to the peroxy moiety (24.4 kcal mol−1 barrier). The barrier is an extensive modification to the previous 18.7 kcal mol−1 value and warrants further study. However, it is still likely that the lowest energy QOOH + O2 pathway corresponds to pathway (3). We found significant spin contamination and/or multireference character in multiple stationary points, especially for transition states stemming from QOOH. Lastly, we provide evidence for an A∼–X∼ surface crossing at a Cs-symmetric, intramolecular hydrogen abstraction structure.

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21. Structural distortions accompanying noncovalent interactions: methane-water, the simplest C-H hydrogen bond

Neglect of fragment structural distortions resulting from noncovalent interactions is a common practice when examining a potential energy surface (PES). Herein, we make quantitative predictions concerning the magnitude of such distortions in the methane–water system. Coupled cluster methods up to perturbative quadruples [CCSDT(Q)] were used in the structural optimizations to the complete basis set limit (using up to cc-pV6Z basis sets). Our results show that the interaction energy differences between the fully optimized and nonoptimized structures are on the order of 0.02 kcal mol–1. These findings imply that scanning the PES of a very weakly bound noncovalent system, while neglecting intramolecular distortions, is a reasonable approximation for points other than the minima.

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20. Investigating the ground-state rotamers of n-propylperoxy radical

The n-propylperoxy radical has been described as a molecule of critical importance to studies of low temperature combustion. Ab initio methods were used to study this three-carbon alkylperoxy radical, normal propylperoxy. Reliable CCSD(T) (coupled-cluster theory, incorporating single, double, and perturbative triple)/ANO0 geometries were predicted for the molecule’s five rotamers. For each rotamer, energetic predictions were made using basis sets as large as the cc-pV5Z in conjunction with coupled cluster levels of theory up to CCSDT(Q). Along with the extrapolations, corrections for relativistic effects, zero-point vibrational energies, and diagonal Born–Oppenheimer corrections were used to further refine energies. The results indicate that the lowest conformer is the gauche-gauche (GG) rotamer followed by the gauche-trans (0.12 kcal mol−1 above GG), trans-gauche (0.44 kcal mol−1), gauche′-gauche (0.47 kcal mol−1), and trans-trans (0.57 kcal mol−1). Fundamental vibrational frequencies were obtained using second-order vibrational perturbation theory. This is the first time anharmonic frequencies have been computed for this system. The most intense IR features include all but one of the C–H stretches. The O–O fundamental (1063 cm−1 for the GG structure) also has a significant IR intensity, 19.6 km mol−1. The anharmonicity effects on the potential energy surface were also used to compute vibrationally averaged rg,0K bond lengths, accounting for zero-point vibrations present within the molecule.

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19. Spin-adapted formulation and implementation of density cumulant functional theory with density-fitting approximation: application to transition metal compounds

Density cumulant functional theory (DCT) has recently emerged as an attractive ab initio approach for the treatment of electron correlation. In its orbital-optimized formulation (ODC-12) [J. Chem. Phys. 139, 204110 (2013)], DCT has been shown to provide reliable results for a variety of challenging chemical systems. Among the attractive properties of DCT are its size-consistency and size-extensivity, as well as the efficient computation of the molecular properties and analytic gradients. In this work, we present a new formulation and implementation of DCT that takes advantage of spin adaptation and the density-fitting approximation (DF-ODC-12). Our new spin-adapted DF-ODC-12 implementation is more efficient than the previous ODC-12 implementation with up to a ∼12-fold speed-up. We demonstrate the capabilities of DF-ODC-12 with a study of transition metal compounds, which require high levels of electron correlation treatment. For transition metal carbonyl complexes [Fe(CO)5, Cr(CO)6] and the ferrocene molecule [Fe(Cp)2], the DF-ODC-12 equilibrium parameters and bond dissociation energies extrapolated to the complete basis set limit are in very good agreement with reference data derived from experiment.

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18. The cis- and trans-formylperoxy radical: fundamental vibrational frequencies and relative energies of the X 2A’’ and A 2A’ states

Acylperoxy radicals [RC(O)OO˙] play an important catalytic role in many atmospheric and combustion reactions. Accordingly, the prototypical formylperoxy radical [HC(O)OO˙] is characterized here using high-level ab initio coupled-cluster theory. Important experiments have been carried out on this system, but have not comprehensively described the properties of even the ground electronic state. We report cis and trans geometries for the ground ([X with combining tilde] 2A′′) and first excited (à 2A′) state equilibrium conformers and the torsional saddle point on the ground state surface at the CCSD(T)/ANO2 level of theory. Relative energies of these ground- and excited-state stationary points were obtained using coupled cluster theory with up to perturbative quadruple excitations, extrapolated from the sextuple zeta basis set to the complete basis set limit. These methods predict conformational energy differences ΔE(trans-[X with combining tilde] → cis-[X with combining tilde]) = 2.35 kcal mol−1 and ΔE(trans-à → cis-Ã) = −2.95 kcal mol−1. On the [X with combining tilde] surface, the transition state for the conformational change lies 8.42 kcal mol−1 above the trans ground state minima. The adiabatic electronic excitation energies from the ground state isomers are predicted to be 18.17 ± 0.10 (trans) and 13.03 ± 0.10 kcal mol−1 (cis). The former is in excellent agreement with the 18.1 ± 1.4 kcal mol−1 transition found by Lineberger and coworkers. Additionally, transition properties between the [X with combining tilde] 2A′′ and à 2A′ states are reported for the first time, using the equation of motion (EOM)-CCSD method, which predicts lifetimes for trans-à 2A′ HC(O)OO˙ of 5.4 ms and cis-à 2A′ HC(O)OO˙ of 20.5 ms. Second-order vibrational perturbation theory was utilized to determine the fundamental frequencies at the CCSD(T)/ANO2 level of theory for the cis and trans conformers of the [X with combining tilde] and à states and five ground state isotopologues of both conformers: H13C(O)OO˙, HC(18O)OO˙, HC(O)18O18O˙, DC(O)OO˙, and DC(O)18O18O˙. This study provides high accuracy predictions of vibrational frequencies, helping to resolve large uncertainties and disagreements in the experimental values. Furthermore, we characterize experimentally unassigned vibrational frequencies and transition properties.

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17. The ethyl radical in superfluid helium nanodroplets: Rovibrational spectroscopy and ab initio computations

The ethyl radical has been isolated and spectroscopically characterized in 4He nanodroplets. The band origins of the five CH stretch fundamentals are shifted by < 2 cm−1 from those reported for the gas phase species [S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000); T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006)]. The symmetric CH2 stretching band (v1) is rotationally resolved, revealing nuclear spin statistical weights predicted by G12 permutation-inversion group theory. A permanent electric dipole moment of 0.28 (2) D is obtained via the Stark spectrum of the v1 band. The four other CH stretch fundamental bands are significantly broadened in He droplets and lack rotational fine structure. This broadening is attributed to symmetry dependent vibration-to-vibration relaxation facilitated by the He droplet environment. In addition to the five fundamentals, three a1′ overtone/combination bands are observed, and each of these have resolved rotational substructure. These are assigned to the 2v12, v4 + v6, and 2v6 bands through comparisons to anharmonic frequency computations at the CCSD(T)/cc-pVTZ level of theory.

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16. Psi4: an open-source ab initio electronic structure program

The Psi4 program is a new approach to modern quantum chemistry, encompassing Hartree–Fock and density‐functional theory to configuration interaction and coupled cluster. The program is written entirely in C++ and relies on a new infrastructure that has been designed to permit high‐efficiency computations of both standard and emerging electronic structure methods on conventional and high‐performance parallel computer architectures. Psi4 offers flexible user input built on the Python scripting language that enables both new and experienced users to make full use of the program's capabilities, and even to implement new functionality with moderate effort. To maximize its impact and usefulness, Psi4 is available through an open‐source license to the entire scientific community.

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15. The lowest-lying electronic singlet and triplet potential energy surfaces for the HNO-NOH system: Energetics, unimolecular rate constants, tunneling and kinetic isotope effects for the isomerization and dissociation reactions

The lowest-lying electronic singlet and triplet potential energy surfaces (PES) for the HNO–NOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zero-point vibrational energies, diagonal Born-Oppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the 1HNO → 1NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol−1. For the barrierless decomposition of 1HNO to H + NO, the dissociation energy is estimated to be 47.48 ±  0.2 kcal mol−1. For 1NOH → H + NO, the reaction endothermicity and barrier are 5.25 ±  0.2 and 7.88 ± 0.2 kcal mol−1. On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ±  0.2 and 39.31 ± 0.2 kcal mol−1 for the isomerization reaction 3HNO → 3NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ±  0.2 and 32.41 ± 0.2 kcal mol−1. Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol−1 for the dissociation reaction of 3NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlet–triplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol−1 for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous first-order ordinary-differential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to 400 K. Hence, our theoretical findings clearly explain why 1NOH has not yet been observed experimentally.

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14. Large-scale symmetry-adapted perturbation theory computations via density fitting and Laplace transformation techniques: Investigating the fundamental force of DNA-intercalator interactions

Symmetry-adapted perturbation theory (SAPT) provides a means of probing the fundamental nature of intermolecular interactions. Low-orders of SAPT (here, SAPT0) are especially attractive since they provide qualitative (sometimes quantitative) results while remaining tractable for large systems. The application of density fitting and Laplace transformation techniques to SAPT0 can significantly reduce the expense associated with these computations and make even larger systems accessible. We present new factorizations of the SAPT0 equations with density-fitted two-electron integrals and the first application of Laplace transformations of energy denominators to SAPT. The improved scalability of the DF-SAPT0 implementation allows it to be applied to systems with more than 200 atoms and 2800 basis functions. The Laplace-transformed energy denominators are compared to analogous partial Cholesky decompositions of the energy denominator tensor. Application of our new DF-SAPT0 program to the intercalation of DNA by proflavine has allowed us to determine the nature of the proflavine-DNA interaction. Overall, the proflavine-DNA interaction contains important contributions from both electrostatics and dispersion. The energetics of the intercalator interaction are are dominated by the stacking interactions (two-thirds of the total), but contain important contributions from the intercalator-backbone interactions. It is hypothesized that the geometry of the complex will be determined by the interactions of the intercalator with the backbone, because by shifting toward one side of the backbone, the intercalator can form two long hydrogen-bonding type interactions. The long-range interactions between the intercalator and the next-nearest base pairs appear to be negligible, justifying the use of truncated DNA models in computational studies of intercalation interaction energies.

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13. Reaction energetics for the abstraction process C2H3 + H2 -> C2H4 + H

The fundamentally important combustion reaction of vinyl radical with hydrogen has been studied in the laboratory by at least five experimental groups. Herein, the reaction C2H3 + H2 → C2H4 + H has been examined using focal-point analysis. Molecular energies were determined from extrapolations to the complete basis-set limit using correlation-consistent basis sets (cc-pVTZ, cc-pVQZ, and cc-pV5Z) and coupled-cluster theory with single and double excitations (CCSD), perturbative triples [CCSD(T)], full triples [CCSDT], and perturbative quadruples [CCSDT(Q)]. Reference geometries were optimized at the all-electron CCSD(T)/cc-pCVQZ level. Computed energies were also corrected for relativistic effects and the Born–Oppenheimer approximation. The activation energy for hydrogen abstraction is predicted to be 9.65 kcal mol–1, and the overall reaction is predicted to be exothermic by 5.65 kcal mol–1. Natural resonance theory (NRT) analysis was performed to verify the reaction pathway and describe bond-breaking and bond-forming events along the reaction coordinate.

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12. Quadratically convergent algorithm for orbital-optimized coupled-cluster doubles methods and in orbital-optimized second-order Moller-Plesset perturbation theory

Using a Lagrangian-based approach, we present a more elegant derivation of the equations necessary for the variational optimization of the molecular orbitals (MOs) for the coupled-cluster doubles (CCD) method and second-order Møller-Plesset perturbation theory (MP2). These orbital-optimized theories are referred to as OO-CCD and OO-MP2 (or simply “OD” and “OMP2” for short), respectively. We also present an improved algorithm for orbital optimization in these methods. Explicit equations for response density matrices, the MO gradient, and the MO Hessian are reported both in spin-orbital and closed-shell spin-adapted forms. The Newton-Raphson algorithm is used for the optimization procedure using the MO gradient and Hessian. Further, orbital stability analyses are also carried out at correlated levels. The OD and OMP2 approaches are compared with the standard MP2, CCD, CCSD, and CCSD(T) methods. All these methods are applied to H2O, three diatomics, and the O+4 molecule. Results demonstrate that the CCSD and OD methods give nearly identical results for H2O and diatomics; however, in symmetry-breaking problems as exemplified by O+4, the OD method provides better results for vibrational frequencies. The OD method has further advantages over CCSD: its analytic gradients are easier to compute since there is no need to solve the coupled-perturbed equations for the orbital response, the computation of one-electron properties are easier because there is no response contribution to the particle density matrices, the variational optimized orbitals can be readily extended to allow inactive orbitals, it avoids spurious second-order poles in its response function, and its transition dipole moments are gauge invariant. The OMP2 has these same advantages over canonical MP2, making it promising for excited state properties via linear response theory. The quadratically convergent orbital-optimization procedure converges quickly for OMP2, and provides molecular properties that are somewhat different than those of MP2 for most of the test cases considered (although they are similar for H2O). Bond lengths are somewhat longer, and vibrational frequencies somewhat smaller, for OMP2 compared to MP2. In the difficult case of O+4 , results for several vibrational frequencies are significantly improved in going from MP2 to OMP2.

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11. The barrier height, unimolecular rate constant, and lifetime of the dissociation of HN2 (Correction)

Although never spectroscopically identified in the laboratory, hydrogenated nitrogen (HN2) is thought to be an important species in combustion chemistry. The classical barrier height (10.6±0.2kcalmol−1) and exothermicity (3.6±0.2kcalmol−1) for the HN2→N2+H reaction are predicted by high level ab initio quantum mechanical methods [up to CCSDT(Q)]. Total energies are extrapolated to the complete basis set limit applying the focal point analysis. Zero-point vibrational energies are computed using fundamental (anharmonic) frequencies obtained from a quartic force field. Relativistic and diagonal Born–Oppenheimer corrections are also taken into account. The quantum mechanical barrier with these corrections is predicted to be 6.4±0.2kcalmol−1 and the reaction exothermicity to be 8.8±0.2kcalmol−1. The importance of these parameters for the thermal NOx decomposition (De-NOx) process is discussed. The unimolecular rate constant for dissociation of the HN2 molecule and its lifetime are estimated by canonical transition-state theory and Rice–Ramsperger–Kassel–Marcus theory. The lifetime of the HN2 molecule is here estimated to be 2.8×10−10s at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies (τ=1.5×10−8s), albeit consistent with the very rough experimental upper limit (τ<0.5μs). For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the DN2 molecule has a longer lifetime than the HN2 molecule. Thus, DN2 might be more readily identified experimentally. The ionization potential of the HN2 molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of HN2 is predicted to be 8.19±0.05eV , in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothon-radiation-based photoionization mass spectrometry.

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10. The barrier height, unimolecular rate constant, and lifetime of the dissociation of HN2

Although never spectroscopically identified in the laboratory, hydrogenated nitrogen (HN2) is thought to be an important species in combustion chemistry. The classical barrier height (10.6±0.2kcalmol−1) and exothermicity (3.6±0.2kcalmol−1) for the HN2→N2+H reaction are predicted by high level ab initio quantum mechanical methods [up to CCSDT(Q)]. Total energies are extrapolated to the complete basis set limit applying the focal point analysis. Zero-point vibrational energies are computed using fundamental (anharmonic) frequencies obtained from a quartic force field. Relativistic and diagonal Born–Oppenheimer corrections are also taken into account. The quantum mechanical barrier with these corrections is predicted to be 6.4±0.2kcalmol−1 and the reaction exothermicity to be 8.8±0.2kcalmol−1. The importance of these parameters for the thermal NOx decomposition (De-NOx) process is discussed. The unimolecular rate constant for dissociation of the HN2 molecule and its lifetime are estimated by canonical transition-state theory and Rice–Ramsperger–Kassel–Marcus theory. The lifetime of the HN2 molecule is here estimated to be 2.8×10−10s at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies (τ=1.5×10−8s), albeit consistent with the very rough experimental upper limit (τ<0.5μs). For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the DN2 molecule has a longer lifetime than the HN2 molecule. Thus, DN2 might be more readily identified experimentally. The ionization potential of the HN2 molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of HN2 is predicted to be 8.19±0.05eV , in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothon-radiation-based photoionization mass spectrometry.

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9. Toward the observation of quartet states of the ozone radical cation: Insights from coupled cluster theory

Since the discovery of ozone depletion, the doublet electronic states of the ozone radical cation have received much attention in experimental and theoretical investigations, while the low-lying quartet states have not. In the present research, viable pathways to the quartet states from the lowest three triplet states of ozone, A23, B23, and B13, and excitations from the A12 and B22 states of the ozone radical cation have been studied in detail. The potential energy surfaces, structural optimizations, and vibrational frequencies for several states of ozone and its radical cation have been thoroughly investigated using the complete active space self-consistent field, unrestricted coupled cluster theory from a restricted open-shell Hartree-Fock reference including all single and double excitations (UCCSD), UCCSD method with the effects of connected triple excitations included perturbatively, and unrestricted coupled cluster including all single, double, and triple excitations with the effects of connected quadruple excitations included perturbatively. These methods used Dunning’s correlation-consistent polarized core-valence basis sets, cc-pCVXZ (X=D, T, Q, and 5). The most feasible pathways (symmetry and spin allowed transitions) to the quartet states are A14←A23, A24←A23, A14←B23, A24←B13, B24←B13, A24←A11, B24←A11, and A14←A11 with vertical ionization potentials of 12.46, 12.85, 12.82, 12.46, 12.65, 13.43, 13.93, and 14.90eV , respectively.

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8. Vibrational energy levels for the electronic ground state of the diazocarbene (CNN) molecule

The vibrational energy levels of diazocarbene (diazomethylene) in its electronic ground state, CNN, have been predicted using the variational method. The potential energy surfaces of CNN were determined by employing ab initio single reference coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], multi-reference complete active space self-consistent-field (CASSCF), and internally contracted multi-reference configuration interaction (ICMRCI) methods. The correlation-consistent polarised valence quadruple zeta (cc-pVQZ) basis set was used. Four sets of vibrational energy levels determined from the four distinct analytical potential functions have been compared with the experimental values from the laser-induced fluorescence measurements of Wurfel et al. obtained in 1992. The CCSD, CCSD(T), and CASSCF potentials have not provided satisfactory agreement with the experimental observations. In this light, the importance of both non-dynamic (static) and dynamic correlation effects in describing the ground state of CNN is emphasised. Our best theoretical fundamental frequencies at the cc-pVQZ ICMRCI level of theory, ν1 = 1230, ν2 = 394, and ν3 = 1420 cm− 1, are in excellent agreement with the experimental values of ν1 = 1235, ν2 = 396, and ν3 = 1419 cm− 1, and the mean absolute deviation between the 23 calculated and experimental vibrational energy levels is only 7.4 cm− 1. It is shown that the previously suggested observation of the ν3 frequency at about 2847 cm− 1 was in fact the first overtone 2ν3.

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7. The deprotonation energies of BH5 and AlH5: Comparisons to GaH5

Hypercoordinate boron is most unusual, leading to considerable theoretical and experimental research on the parent BH5 molecule. The deprotonation energies of BH5 and the related molecules AlH5 and GaH5 have been of particular interest. Here the energy differences for are computed to be 332.4 and 326.3 kcal mol−1, respectively, with an aug-cc-pVQZ basis set at the CCSD(T) level of theory. Vibrational frequencies for and are also reported as 1098, 1210, 2263, and 2284 cm−1 and 760, 779, 1658, and 1745 cm−1, respectively, again at the CCSD(T) aug-cc-pVQZ level of theory. Comparisons with the valence isoelectronic GaH5 molecule are made.

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6. The nearly degenerate triplet electronic ground state isomers of lithium nitroxide

The triplet electronic ground state potential energy surface of lithium nitroxide has been systematically investigated using convergent quantum mechanical methods. Equilibrium structures and physical properties for five stationary points (three minima and two transition states) have been determined employing highly correlated coupled cluster theory with four correlation-consistent polarized-valence (cc-pVXZ and aug-cc-pVXZ, X = T and Q) and two core correlation-consistent polarized-valence (cc-pCVXZ, X = T and Q) basis sets. The global minimum, roughly L-shaped Li-O-N, is predicted to lie 6.5 kcal mol-1 below the linear LiON minimum and 2.4 kcal mol-1 below the linear LiON minimum. The barrier to isomerization from the global minimum to LiON was found to be 7.4 kcal mol-1 and with regard to LiNO 6.9 kcal mol-1. The dissociation energies, D0, with respect to Li + NO, have been predicted for all minima and for the global minimum was found to be 34.9 kcal mol-1.

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5. Characterization of singlet ground and low-lying electronic excited states of phosphaethyne and isophosphaethyne

The singlet ground (X̃ Σ+1) and excited (Σ−1,Δ1) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum (X˜Σ+1HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of ω1=3344 cm−1, ω2=689 cm−1, and ω3=1298 cm−1. Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol−1 above the global minimum HCP. The dissociation energy D0[HCP(X̃ Σ+1)→H(S2)+CP(XΣ+2)] of HCP is predicted to be 119.0 kcal mol−1, which is very close to the experimental lower limit of 119.1 kcal mol−1. Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, Σ−1→A″1, with excitation energies Te of 101.4 kcal mol−1 (A″1HCP), 104.6 kcal mol−1 (Σ−1HCP), 122.3 kcal mol−1 (A″1HPC), and 171.6 kcal mol−1 (Σ−1HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the A″1 state with a predicted bond angle of 129.5° compare well with the experimentally reported first singlet state (Ã A″1). The excitation energy predicted for this excitation is T0=99.4 kcal mol−1 (34800 cm−1, 4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol−1 (34746 cm−1, 4.308 eV). For the second lowest-lying excited singlet surface, Δ1→A′1, four stationary points were found with Te values of 111.2 kcal mol−1 (2A′1 HCP), 112.4 kcal mol−1 (Δ1HPC), 125.6 kcal mol−1(2A′1HCP), and 177.8 kcal mol−1(Δ1HPC). The predicted CP bond length and frequencies of the 2A′1 state with a bond angle of 89.8° (1.707 Å, 666 and 979 cm−1) compare reasonably well with those for the experimentally reported C̃ A′1 state (1.69 Å, 615 and 969 cm−1). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol−1 and 89.8° versus experimental values of 115.1 kcal mol−1 and 113°.

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4. Rovibrational energy levels for the electronic ground state of AlOH

The vibrational–rotational energy levels of aluminum monohydroxide in its electronic ground state, AlOH, have been predicted using the variational method. The potential energy surface of the ground state of AlOH was determined employing the ab initio coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)] and the correlation-consistent polarized valence quadruple zeta (cc-pVQZ) basis set. Low-lying J = 0 and J = 1 vibrational levels are reported. These are analyzed in terms of the quasilinearity of the molecule. Coriolis effects are shown to be significant. We hope that our predictions will be of value in the future when assigning rovibrational transitions in spectroscopic studies.

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3. Diode laser spectroscopy of the nu2 fundamental band of cis-HOPO

The absorption spectrum of the ν2 fundamental band of the cis-conformer of the transient molecule HOPO, namely the terminal PO stretching mode, has been detected and measured using diode laser spectroscopy. The molecule was generated in a discharge flow system containing hydrogen and white phosphorus vapour (P4) and a trace of oxygen. The spectrum has the appearance of an a-type band of a near prolate asymmetric top. Above Ka = 5 the spectrum is perturbed and transitions terminating on these higher Ka levels were excluded from the fit. The vibrational frequency and rotational constants derived from the unperturbed parts of the spectrum are compatible with new high precision ab initio calculations reported here. A combined fit of the ν2 band and the ν4 band data, measured earlier, was carried out. The ν2 band origin was determined to be 1258.539525(32) cm−1, approximately 5.5 cm−1 higher than the matrix value.

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2. Does GaH5 exist?

The existence or nonexistence of GaH5 has been widely discussed [N. M. Mitzel, Angew. Chem. Int. Ed. 42, 3856 (2003)]. Seven possible structures for gallium pentahydride have been systematically investigated using ab initio electronic structure theory. Structures and vibrational frequencies have been determined employing self-consistent field, coupled cluster including all single and double excitations (CCSD), and CCSD with perturbative triples levels of theory, with at least three correlation-consistent polarized-valence-(cc-pVXZ and aug-cc-pVXZ) type basis sets. The X̃ A′1 state for GaH5 is predicted to be weakly bound complex 1 between gallane and molecular hydrogen, with Cs symmetry. The dissociation energy corresponding to GaH5→GaH3+H2 is predicted to be De=2.05kcalmol−1. The H–H stretching fundamental is predicted to be v=4060cm−1, compared to the tentatively assigned experimental feature of Wang and Andrews [J. Phys. Chem. A 107, 11371 (2003)] at 4087cm−1. A second Cs structure 2 with nearly equal energy is predicted to be a transition state, corresponding to a 90° rotation of the H2 bond. Thus the rotation of the hydrogen molecule is essentially free. However, hydrogen scrambling through the C2v structure 3 seems unlikely, as the activation barrier for scrambling is at least 30kcalmol−1 higher in energy than that for the dissociation of GaH5 to GaH3 and H2. Two additional structures consisting of GaH3 with a dihydrogen bond perpendicular to gallane (C3v structure 4) and an in-plane dihydrogen bond [Cs(III) structure 5] were also examined. A C3v symmetry second-order saddle point has nearly the same energy as the GaH3+H2 dissociation limit, while the Cs(III) structure 5 is a transition structure to the C3v structure. The C4v structure 6 and the D3h structure 7 are much higher in energy than GaH3+H2 by 88 and 103kcalmol−1, respectively.

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1. The singlet electronic ground state isomers of dialuminum monoxide: AlOAl and AlAlO, and the transition state connecting them

J.M. Turney, L Sari, Y. Yamaguchi, H. F. Schaefer, Journal of Chemical Physics 122, 094304 (2005).

The singlet electronic ground state isomers, X˜Σg+1 (AlOAl D∞h) and X˜Σ+1 (AlAlO C∞ν), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9kcalmol−1 below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X˜Σg+1 AlOAl molecule, ω1=517cm−1, ω2=95cm−1, and ω3=1014cm−1, are in good agreement with experimental values. The harmonic vibrational frequencies for the X˜Σ+1 AlAlO structure, ω1=1042cm−1, ω2=73cm−1, and ω3=253cm−1, presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl→AlAlO and AlAlO→AlOAl reactions were predicted to be 84.3 and 2.4kcalmol−1, respectively. The dissociation energies D0 for AlOAl (X˜Σg+1) and AlAlO (X˜Σ+1)→AlO(XΣ+2)+Al(P2) were determined to be 130.8 and 48.9kcalmol−1, respectively. Thus, both symmetric AlOAl (X˜Σg+1) and asymmetric AlAlO (X˜Σ+1) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (XΣ+2)+Al(P2) and kinetically stable for the isomerization reaction (AlAlO→AlOAl) at sufficiently low temperatures.

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