Justin M. Turney

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 programs 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.

Large-scale symmetry-adapted perturbation theory computations via density fitting and Laplace transformation techniques: Investigating the fundamental forces 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.

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

Although never spectroscopically identified in the laboratory, hydrogenated nitrogen (HN(2)) is thought to be an important species in combustion chemistry. The classical barrier height (10.6+/-0.2 kcal mol(-1)) and exothermicity (3.6+/-0.2 kcal mol(-1)) for the HN(2)-->N(2)+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+/-0.2 kcal mol(-1) and the reaction exothermicity to be 8.8+/-0.2 kcal mol(-1). The importance of these parameters for the thermal NO(x) decomposition (De-NO(x)) process is discussed. The unimolecular rate constant for dissociation of the HN(2) molecule and its lifetime are estimated by canonical transition-state theory and Rice-Ramsperger-Kassel-Marcus theory. The lifetime of the HN(2) molecule is here estimated to be 2.8x10(-10) s at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies (tau=1.5x10(-8) s), albeit consistent with the very rough experimental upper limit (tau<0.5 mus). For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the DN(2) molecule has a longer lifetime than the HN(2) molecule. Thus, DN(2) might be more readily identified experimentally. The ionization potential of the HN(2) molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of HN(2) is predicted to be 8.19+/-0.05 eV, in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothon-radiation-based photoionization mass spectrometry.

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 (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(-1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(-1). For (1)NOH → 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 (3)HNO → (3)NOH. 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 (3)NOH (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 (1)NOH has not yet been observed experimentally.

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

The singlet electronic ground state isomers, X (1)Sigma(g) (+) (AlOAl D(infinityh)) and X (1)Sigma(+) (AlAlO C(infinitynu)), 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 81.9 kcal mol(-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 (1)Sigma(g) (+) AlOAl molecule, omega(1)=517 cm(-1), omega(2)=95 cm(-1), and omega(3)=1014 cm(-1), are in good agreement with experimental values. The harmonic vibrational frequencies for the X (1)Sigma(+) AlAlO structure, omega(1)=1042 cm(-1), omega(2)=73 cm(-1), and omega(3)=253 cm(-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 2.4 kcal mol(-1), respectively. The dissociation energies D(0) for AlOAl (X (1)Sigma(g) (+)) and AlAlO (X (1)Sigma(+))-->AlO (X (2)Sigma(+))+Al ((2)P) were determined to be 130.8 and 48.9 kcal mol(-1), respectively. Thus, both symmetric AlOAl (X (1)Sigma(g) (+)) and asymmetric AlAlO (X (1)Sigma(+)) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X (2)Sigma(+)) + Al ((2)P) and kinetically stable for the isomerization reaction (AlAlO-->AlOAl) at sufficiently low temperatures.

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.

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 HO2∙ (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.

The cis- and trans-formylperoxy radical: fundamental vibrational frequencies and relative energies of the 2A′′ and à 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 ( 2A′′) and first excited (A 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- → cis-) = 2.35 kcal mol−1 and ΔE(trans-A → cis-A) = −2.95 kcal mol−1. On the 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 2A′′ and A 2A′ states are reported for the first time, using the equation of motion (EOM)-CCSD method, which predicts lifetimes for trans-A 2A′ HC(O)OO˙ of 5.4 ms and cis-A 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 and A 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.

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.

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.