Rollin A. King

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.

PSI3: An open‐source Ab Initio electronic structure package

PSI3 is a program system and development platform for ab initio molecular electronic structure computations. The package includes mature programming interfaces for parsing user input, accessing commonly used data such as basis‐set information or molecular orbital coefficients, and retrieving and storing binary data (with no software limitations on file sizes or file‐system‐sizes), especially multi‐index quantities such as electron repulsion integrals. This platform is useful for the rapid implementation of both standard quantum chemical methods, as well as the development of new models. Features that have already been implemented include Hartree‐Fock, multiconfigurational self‐consistent‐field, second‐order Møller‐Plesset perturbation theory, coupled cluster, and configuration interaction wave functions. Distinctive capabilities include the ability to employ Gaussian basis functions with arbitrary angular momentum levels; linear R12 second‐order perturbation theory; coupled cluster frequency‐dependent response properties, including dipole polarizabilities and optical rotation; and diagonal Born‐Oppenheimer corrections with correlated wave functions. This article describes the programming infrastructure and main features of the package. PSI3 is available free of charge through the open‐source, GNU General Public License.

Locally correlated equation-of-motion coupled cluster theory for the excited states of large molecules

We report an extension of the local correlation concept to electronically excited states via the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method. We apply the same orbital domain structure used successfully for ground-state CCSD by Werner and co-workers and find that the resulting localized excitation energies are in error generally by less than 0.2 eV relative to their canonical EOM-CCSD counterparts, provided the basis set is flexible and includes Rydberg-like functions. In addition, we account for weak-pair contributions efficiently using a correction to local-EOM-CCSD transition energies based on the perturbative (D) correction used with configuration interaction singles (CIS).

THE ELECTRON AFFINITIES OF THE SILICON FLUORIDES SIFN(N=1-5)

Several independent density functional methods have been employed to determine the molecular structures and total energies of SiFn and SiF−n (n=1–5). Three significant measures of neutral–anion separation are reported: the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The first Si–F ligand dissociation energies D(Fn−1Si–F), D(Fn−1Si−–F), and D(Fn−1Si–F−) as well as the harmonic vibrational frequencies of SiFn and SiF−n are also reported. Trends in the predictions of the different DFT methods are discussed. Self‐consistent Kohn–Sham orbitals were obtained using various exchange correlation functionals and a double‐ζ plus polarization basis set augmented with diffuse s‐type and p‐type functions. The method (BHLYP) based upon the Becke half‐and‐half exchange functional and the Lee–Yang–Parr correlation functional predicts molecular geometries in best agreement with experiment, while the other methods tend to produce bond lengths that are slightl...

Coupled cluster methods including triple excitations for excited states of radicals

We report an extension of the coupled cluster iterative-triples model, CC3, to excited states of open-shell molecules, including radicals. We define the method for both spin-unrestricted Hartree-Fock (UHF) and spin-restricted open-shell Hartree-Fock (ROHF) reference determinants and discuss its efficient implementation in the PSI3 program package. The program is streamlined to use at most O(N(7)) computational steps and avoids storage of the triple-excitation amplitudes for both the ground- and excited-state calculations. The excitation-energy program makes use of a Lowdin projection formalism (comparable to that of earlier implementations) that allows computational reduction of the Davidson algorithm to only the single- and double-excitation space, but limits the calculation to only one excited state at a time. However, a root-following algorithm may be used to compute energies for multiple states of the same symmetry. Benchmark applications of the new methods to the lowest valence (2)B(1) state of the allyl radical, low-lying states of the CH and CO(+) diatomics, and the nitromethyl radical show substantial improvement over ROHF- and UHF-based CCSD excitation energies for states with strong double-excitation character or cases suffering from significant spin contamination. For the allyl radical, CC3 adiabatic excitation energies differ from experiment by less than 0.02 eV, while for the (2)Sigma(+) state of CH, significant errors of more than 0.4 eV remain.

On apparent quantized transition-state thresholds in the photofragmentation of acetaldehyde

Recent photofragmentation experiments have observed stepwise increases in the dissociation rate for CH3CHO (T1)→CH3 (X 2A2″)+HCO (X 2A′) as a function of excitation energy. In accord with the Rice–Ramsperger–Kassel–Marcus (RRKM) form of transition-state theory, these steps were interpreted as corresponding to vibrational levels of the fragmentation transition state on the triplet surface. We have investigated this acetaldehyde dissociation using coupled cluster (CC) and density functional (DFT) methods with [C,O/H] atomic-orbital basis sets ranging in quality from [4s2p1d/2s1p] to [6s5p4d3f2g1h/5s4p3d2 f1g]. A high-level focal point analysis, along with harmonic force field computations, results in predictions of the dissociation energy, D0=1583 cm−1, and the association barrier height, V0*=3149 cm−1. With a basis set of triple-ζ plus double-polarization plus f(TZ2Pf ) quality, the DFT method UB3LYP and the CC method RCCSD predict barrier frequencies of 355i cm−1 and 516i cm−1, respectively, while the empirical value inferred from RRKM models is only 60i cm−1. The RRKM-derived frequencies for the degrees of freedom orthogonal to the reaction path are more reasonable but still not in convincing agreement with electronic structure theory. Thus, while the experimental steps in the dissociation rate of acetaldehyde (as well as ketene) have yet to be satisfactorily explained, proven ab initio methods provide strong evidence that simple RRKM fits to the k(E) profile provide misleading vibrational frequencies of the transition state on the corresponding triplet potential energy surface.Recent photofragmentation experiments have observed stepwise increases in the dissociation rate for CH3CHO (T1)→CH3 (X 2A2″)+HCO (X 2A′) as a function of excitation energy. In accord with the Rice–Ramsperger–Kassel–Marcus (RRKM) form of transition-state theory, these steps were interpreted as corresponding to vibrational levels of the fragmentation transition state on the triplet surface. We have investigated this acetaldehyde dissociation using coupled cluster (CC) and density functional (DFT) methods with [C,O/H] atomic-orbital basis sets ranging in quality from [4s2p1d/2s1p] to [6s5p4d3f2g1h/5s4p3d2 f1g]. A high-level focal point analysis, along with harmonic force field computations, results in predictions of the dissociation energy, D0=1583 cm−1, and the association barrier height, V0*=3149 cm−1. With a basis set of triple-ζ plus double-polarization plus f(TZ2Pf ) quality, the DFT method UB3LYP and the CC method RCCSD predict barrier frequencies of 355i cm−1 and 516i cm−1, respectively, while the e...

The structures, electron affinities, and energetic stabilities of TiOn and TiOn− (n=1–3)

By use of density functional and coupled cluster methods, we report energetic and structural information concerning the ground states of TiOn and TiOn− (n=1–3), much of which has not previously been observed experimentally or predicted theoretically. This study establishes the following geometrical symmetries and electronic ground states: X 1A′ TiO3 (Cs), X 2A2′ or 2B2 TiO3− (D3h or C2v), and X 2A1 TiO2− (C2v). In addition, the electronic ground state of TiO− is established as 2Δ, arising from the 9σ21δ configuration. This finding is contrary to the suggestion of Wu and Wang, contained in their report of recent photoelectron experiments, that ground state TiO− has a 9σ1δ2 electronic configuration and 4Σ− symmetry. The ground state minimum-energy structure for TiO3− contains no oxygen–oxygen bonds and has D3h or C2v symmetry. The first theoretical adiabatic electron affinities, as predicted by the CCSD(T)//B3LYP level of theory, for TiO, TiO2, and TiO3 are 1.25 eV, 1.60 eV, and 3.34 eV, while Wu and Wan...

A benchmark study of the vertical electronic spectra of the linear chain radicals C2H and C4H

The ability of coupled-cluster models to predict vertical excitation energies is tested on the electronic states of carbon-chain radicals of particular relevance to interstellar chemistry. Using spin-unrestricted and -restricted reference wave functions, the coupled-cluster singles and doubles (CCSD) model and a triples-including model (CC3) are tested on the sigma radicals C(2)H and C(4)H. Both molecules exhibit low-lying excited states with significant double-excitation character (as well as states of quartet multiplicity) and are thus challenging cases for excited-state approaches. In addition, we employ two diagnostics for the reliability of the CC results: the approximate excitation level (AEL) relative to the ground state and the difference between excitation energies obtained with spin-unrestricted and spin-restricted reference wave functions (the U-R difference). We find that CCSD yields poor excitation energies for states with AEL significantly larger than ca. 1.1 and/or large U-R differences, as well as for certain states exhibiting large spin contamination or other inadequacies in the reference determinant. In such cases, connected triple excitations can be included in the model and generally provide improved results. Furthermore, we find that large discrepancies exist between CC and multireference (MR) results for certain states. These disagreements are not related to basis-set effects, but likely arise from the lack of spin adaptation in conventional spin-orbital CC implementations and active space selection in the MR models.

Experimental and ab initio study of the infrared spectra of ionic species derived from PF5, PF3, and F3PO and trapped in solid neon

When a Ne:PF5 or a Ne:PF3 mixture is codeposited at 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid shows a complicated pattern of new absorptions. Little fragmentation of PF5 into PF3 occurs, but several of the absorptions can be tentatively attributed to PF4. The results of extensive ab initio calculations of the vibrational spectra of the neutral, cation, and anion species of formula PFn are presented, in order to aid in the product identification. Several absorptions of PF4+, PF3+, and PF2+ are identified, with only PF3+ common to both systems. Other prominent absorptions are contributed by PF5−, PF4−, and PF3−. In all of the Ne:PF5 experiments and some of the Ne:PF3 experiments, F3PO was a major contaminant. Therefore, ab initio calculations were also conducted for most of the neutral and charged species that can result from F3PO. There is strong evidence supporting the identification of two of the vibrational fundamentals of...

On the accuracy of spin-component-scaled perturbation theory (SCS-MP2) for the potential energy surface of the ethylene dimer

The bimolecular interaction potentials for various configurations of the ethylene dimer computed with coupled-cluster and spin-component-scaled second-order Møller–Plesset perturbation theory (SCS-MP2) are reported. With a triple-ζ basis set including diffuse functions, SCS-MP2 improves over the results of conventional MP2. However, when the resulting energies are counterpoise-corrected for overbinding due to basis-set superposition error or obtained with a quadruple-ζ basis set, the MP2 results are superior. Alternative scaling parameters for SCS-MP2 have been determined from computations performed across the potential energy surface. These scaling parameters are found to be relatively insensitive to the relative configuration of the monomers and to include a slightly larger relative weighting of the same-spin electron pairs than originally proposed by Grimme [J. Chem. Phys. 118, 9095 (2003)]. The opposite-spin and same-spin parameters that minimize errors in the SCS-MP2 results for the ethylene dimer are C OS = 1.28 and C SS = 0.50, respectively. With these alternative scaling parameters, the CCSD(T) potential energy curves for ethylene dimer are well reproduced for all configurations.