Kaoru Iguchi

An extension of ab initio molecular orbital theory to nuclear motion

Abstract We propose an extension of the quantum chemical molecular orbital (MO) method to describe the nuclear motion. Both electronic and nuclear wavefunctions are simultaneously solved with the full variational MO method, by which exponents and centers of gaussian-type function (GTF) basis sets are optimized as well as the linear combination of GTF coefficients. Applications of the method to [F − ; e + ], FH and FD systems are carried out. The calculated bond lengths and harmonic frequencies agree well with the experimental values.

Binding energies of positronium fluoride and positronium bromide by the model potential quantum Monte Carlo method

A method previously used by the authors in an accurate calculation of the binding energy of positronium chloride [Phys. Rev. Lett. 68, 3281 (1992)] is applied to positronium fluoride and positronium bromide. The binding energies obtained with this method are PsF, 1.98±0.17 eV; PsCl, 1.91±0.16 eV; PsBr, 1.14±0.11 eV.

Ab initio calculation of [OH−;e+] system with consideration of electron correlation effect

Ab initio calculations are made to examine theoretically the possibility of stable existence of [OH−;e+] system. Diffuse functions are added to the conventional 6‐31G basis set, considering the wide spread of positron orbital. Moreover, the Mo/ller–Plesset perturbation of the second order is calculated to take the electron correlation into account. These two improvements are found to be very effective for the stable existence of the system. The positron affinity of OH− is computed to be 4.9 eV, and the binding energy of positronium to OH as 0.7 eV which is in good agreement with experimental estimate.

Theory of the Solvated Electron in Polar Liquids. I. Oriented Dipole Model

A model is proposed for the solvated electron in polar liquids, in which molecular dipoles are assumed to orient spherically around the electron. The potential field due to this orientation, the energy levels, the absorption peak and its temperature dependence, and the heat of solvation are calculated at various temperatures. Calculated absorption peak E = 1.69 eV (300°K), 1.89 eV (180°K), and ∂ΔE / ∂T = 0.16 eV deg−1 are compared with experimental data, and they show a fairly good agreement.

Ab initio intermolecular potential of the ethylene dimer

The intermolecular potential of the ethylene dimer in the ground state has been computed nonempirically with the exchange perturbation theory. The principal terms of interaction energy considered are the Coulombic and exchange energies as the first order ones, then the dispersion energy as the second order one, and other higher terms are neglected. Computations were made exactly without multipole expansion approximation, to the order of the square of intermolecular overlap integrals. Nine orientations of molecules are considered. Three kinds of basis set, i.e., STO‐3G, 4‐31G, and 4‐31G* were employed, and the results were compared to each other and also with those by other authors. The potential minimun was obtained for all orientations against results of other authors, and the most stable one has the slightly shifted parallel structure near to that found in the ethylene crystal.

Nonadditivity effects in the molecular interactions of H2O and HF trimers by the symmetry‐adapted perturbation theory

Nonadditivity contribution to the three‐body interaction energy is studied in terms of the symmetry‐adapted perturbation theory for many‐electron systems. Each component of energy, particularly the second‐order exchange dispersion and exchange induction contributions, is given by a combination of electrostatic interaction energies in Longuet–Higgins representation of the intermolecular charge distribution. The formulas of these energies are derived with the Hartree–Fock approximation and by taking triple‐electronic exchanges among three monomers into account. Numerical calculation has been performed for the cyclic planar H2O and HF trimers, considering only single‐electronic exchanges between molecules. The three‐body effect of the second‐order exchange energy has been found to be repulsive, while the main part of attractive contribution is due to the induction. The ratio of three‐body energy to two‐body one for the dispersion is much smaller than that for the induction, though the latter decreases more r...

Electron affinity of Cl: A model potential‐quantum Monte Carlo study

The MP‐QMC procedure, which is a combination of the model potential (MP) and quantum Monte Carlo (QMC) method, is employed to determine the electron affinity of Cl, together with the fixed‐node approximation. Only valence electrons are treated explicitly, while core electrons are replaced by an effective potential constructed with Gaussian‐type functions. The expectation value of electron affinity of Cl atom is obtained from subtracting the valence energy of Cl− from that of Cl. The result is in very good agreement with experiment.

Quantum Monte Carlo method with the model potential

Quantum Monte Carlo (QMC) method, which is a stochastic procedure to solve the Schrodinger equation by diffusing random walk of particles, has been combined with the model potential (MP) to calculate efficiently the electronic states of many‐electron atoms. This scheme has been applied to compute the valence energies of atoms such as Mg, Ca, Sr, and their cations, and ionization potentials of these atoms are evaluated. Results are compared with experimental values and they are found to be in excellent agreement.

Variational Monte Carlo method in the connected moments expansion: H, H−, Be, and Li2

The connected moments expansion with use of variational Monte Carlo technique (CMX‐VMC) is applied to the calculation of the ground state energies of H, H−, Be, and Li2. Exponential‐type wave functions for H, Hylleraas‐type wave functions for H−, and a Hartree–Fock single determinant constructed with a single‐zeta Slater‐type orbital multiplied by a pair correlation factor of Jastrow‐type for Be and Li2 are employed as approximate trial wave function. The results of the present computation are found to agree with the corresponding exact values quite well. The overlap between the approximate and exact wave function is also estimated simultaneously by the new technique.

Connected moments expansion with variational Monte Carlo technique

Abstract The variational Monte Carlo (VMC) method, which evaluates multi-dimensional integrals by random walk sampling based on the Metropolis algorithm, has been implemented in the connected moments expansion (CMX). The Hartree-Fock single determinant multiplied by the Jastrow function has been employed for a trial function. Pilot calculations on the hydrogen molecule at various internuclear separations have been performed. The results are found to be in good agreement with those by Kolos and Wolniewicz.