Nikita Matsunaga

General atomic and molecular electronic structure system

A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed‐shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines.

Degenerate perturbation theory corrections for the vibrational self-consistent field approximation: Method and applications

A new algorithm for computing anharmonic vibrational states for polyatomic molecules is proposed. The algorithm starts with the vibrational self-consistent field (VSCF) method and uses degenerate perturbation theory to correct for effects of correlation between different vibrational modes. The algorithm is developed in a version that computes the anharmonic vibrational spectroscopy directly from potential energy surface points calculated by using ab initio codes. The method is applied to several molecules where near degeneracies occur for excited vibrational states, including HOOH, HSSH, and HOOOH. The method yields results in very good accordance with experiments and generally provides improvements over nondegenerate perturbation corrections for VSCF.

Relativistic potential energy surfaces of XH2 (X=C, Si, Ge, Sn, and Pb) molecules: Coupling of 1A1 and 3B1 states

Potential energy surfaces of the 1A1 and 3B1 states for XH2 molecules (X=C, Si, Ge, Sn, Pb) are investigated with ab initio full valence multiconfigurational self‐consistent field wave functions, using effective core potentials. Spin–orbit coupling is also calculated to construct relativistic potential energy surfaces. The relativistic potential energy surfaces are compared with the adiabatic nonrelativistic potentials. Simple one dimensional Landau–Zener transition probabilities are calculated at the minimum energy crossing points of XH2 molecules to estimate the intersystem crossing probability.

Accurate ab initio potential energy curve of O2. II. Core-valence correlations, relativistic contributions, and vibration-rotation spectrum

In the first paper of this series, a very accurate ab initio potential energy curve of the (3)Sigma(g)(-) ground state of O(2) has been determined in the approximation that all valence shell electron correlations were calculated at the complete basis set limit. In the present study, the corrections arising from core electron correlations and relativity effects, viz., spin-orbit coupling and scalar relativity, are determined and added to the potential energy curve. From the 24 points calculated on this curve, an analytical expression in terms of even-tempered Gaussian functions is determined and, from it, the vibrational and rotational energy levels are calculated by means of the discrete variable representation. We find 42 vibrational levels. Experimental data (from the Schumann-Runge band system) only yield the lowest 36 levels due to significant reduction in the transition intensities of higher levels. For the 35 term values G(v), the mean absolute deviation between theoretical and experimental data is 12.8 cm(-1). The dissociation energy with respect to the lowest vibrational energy is calculated within 25 cm(-1) of the experimental value of 41,268.2+/-3 cm(-1). The theoretical crossing between the (3)Sigma(g)(-) state and the (1)Sigma(g)(+) state is found to occur at 2.22 A and the spin-orbit coupling in this region is analyzed.

Accurate Ab Initio Potential Energy Curve of F2. III. The Vibration Rotation Spectrum

An analytical expression is found for the accurate ab initio potential energy curve of the fluorine molecule that has been determined in the preceding two papers. With it, the vibrational and rotational energy levels of F(2) are calculated using the discrete variable representation. The comparison of this theoretical spectrum with the experimental spectrum, which had been measured earlier using high-resolution electronic spectroscopy, yields a mean absolute deviation of about 5 cm(-1) over the 22 levels. The dissociation energy with respect to the lowest vibrational energy is calculated within 30 cm(-1) of the experimental value of 12 953+/-8 cm(-1). The reported agreement of the theoretical spectrum and dissociation energy with experiment is contingent upon the inclusion of the effects of core-generated electron correlation, spin-orbit coupling, and scalar relativity. The Dunham analysis [Phys. Rev. 41, 721 (1932)] of the spectrum is found to be very accurate. New values are given for the spectroscopic constants.

Energies and derivative couplings in the vicinity of a conical intersection. II. CH2(2 3A″,3 3A″) and H2S(1 1A″,2 1A″), unexpected results in an ostensibly standard case

The 2 3A′′−3 3A′′ and the 1 1A′′−2 1A′′ seams of conical intersection in CH2 and H2S, respectively, are considered. The nuclear coordinate dependence of the seam of conical intersection, the energy of the lower adiabatic potential energy surface along closed loops containing the conical intersection, and the nonremovable part of the derivative coupling in the region contained within the closed loops are studied. The energetics and derivative couplings in the vicinity of the conical intersections are analyzed in terms of the characteristic parameters of a conical intersection, determined at the configuration interaction level using analytic gradient techniques. The characteristic parameters are found to predict, in a qualitative manner, the energetics at moderate distances from the conical intersection. Loops containing the conical intersection that exhibit and do not exhibit the geometric phase effect are considered. An unusual trifurcation of the C2v seam of conical intersection in CH2 into a C2v branch ...

Accurate ab initio potential energy curve of F2. II. Core-valence correlations, relativistic contributions, and long-range interactions

The nonrelativistic, valence-shell-only-correlated ab initio potential energy curve of the F(2) molecule, which was reported in the preceding paper, is complemented by determining the energy contributions that arise from the electron correlations that involve the core electrons as well as the contributions that are due to spin-orbit coupling and scalar relativistic effects. The dissociation curve rises rather steeply toward the energy of the dissociated atoms because, at larger distances, the atomic quadrupole-quadrupole repulsion and spin-orbit coupling counteract the attractive contributions from incipient covalent binding and correlation forces including dispersion.

A comparative study of the bonding in heteroatom analogues of benzene

SummaryInorganic benzenes X3Y3H6 are investigated, with X and Y chosen from Zn, B, Al, Ga, C, Si, Ge, N, P, As, O, and S such that there are a total of 6 π electrons. Geometries and bond orders are used to qualitatively assess the degree of aromatic π bonding in these species. Bond orders are extracted from the CI density matrix over localized molecular orbitals, using methods pioneered by Ruedenberg. Second row elements C, N, O are found to be more effective at this bonding. The aromatic bonding is poorest when X and Y have a large electronegativity difference.

Energies and derivative couplings in the vicinity of a conical intersection 3. The 'most' diabatic basis

It is shown that in the immediate vicinity of an arbitrary conical intersection at R x all the derivative coupling, except for the small part due to the finiteness of the basis sets, is removable by the orthogonal transformation generated by the angle α(ρ, θ, z) = λ(θ) /2 + ρmρ (θ) /q(θ) + zmz (θ) /q(θ), where ρ,θ,z are cylindrical polar coordinates centred at R x . Expressions for λ(θ), q(θ) mρ (θ) and mz (θ) are given. The implications of this result for numerical studies that (i) determine the ‘most’ diabatic basis using Poissons equation and (ii) assess approximate diabatization schemes are discussed.

Enthalpies of Formation of Hydrocarbons by Hydrogen Atom Counting. Theoretical Implications.

Standard enthalpies of formation at 298 K of unstrained alkanes, alkenes, alkynes, and alkylbenzenes can be expressed as a simple sum in which each term consists of the number of hydrogen atoms n of one of eight different types (n1-n8) multiplied by an associated coefficient (c1-c8) derived from the known enthalpy of formation of a typical molecule. Alkylbenzenes require one additive constant for each benzene ring, accounting for a possible ninth term in the sum. Terms are not needed to account for repulsive or attractive 1,3 interactions, hyperconjugation, or for protobranching, rendering them irrelevant. Conjugated eneynes and diynes show thermodynamic stabilizations much smaller than that observed for 1,3-butadiene, bringing into question the usual explanation for the thermodynamic stabilization of conjugated multiple bonds (p orbital overlap, pi electron delocalization, etc.).