Showing papers by "Yasuyuki Ishikawa published in 1999"

Ab initio monte carlo simulated annealing study of hcl(h2o)n (n = 3, 4) clusters

Abstract: In the past decade the study of small HCl-H{sub 2}O clusters has been an active area of research, for its intrinsic interest and for the role such clusters play in the ozone depletion cycle. Heterogeneous reactions of chlorine compounds on polar stratospheric cloud (PSC) particles are important in the destruction of ozone in the antarctic and the arctic. Low-lying structures of HCl (H{sub 2}O){sub n} (n = 3, 4) clusters have been studied by ab initio Monte Carlo simulated annealing (MCSA), a procedure which efficiently samples minima on a potential energy surface. In the Monte Carlo simulated annealing procedure, energies were computed ab initio at each Monte Carlo step by the B3-LYP density functional method with 6--31 G{sup *} basis sets. All geometries of the isomers found for each cluster were refined in full conventional geometry optimizations, and frequency analyses were performed at both the B3-LYP and MP2 levels with 6-311 + G{sup **} basis sets. The stability of the B3-LYP and MP2 energy orderings was tested in single point QCISD(T) calculations performed at the MP2 optimized geometries. Only isomers with strong H-Cl interaction were found for HCl(H{sub 2}O){sub 3}. However, both associated- and dissociated-HCl structures of HCl(H{sub 2}O){sub 4}more » were found.« less

Relativistic multireference many-body perturbation theory for quasidegenerate systems: Energy levels of ions of the oxygen isoelectronic sequence

Abstract: A relativistic multireference many-body perturbation theory for quasidegenerate systems with multiple open valence shells is developed and implemented with analytic basis sets of Gaussian spinors. The theory employs a general class of multiconfigurational Dirac-Fock self-consistent-field wave functions as reference functions, and thus is applicable to open-shell systems with near degeneracy of a manifold of strongly interacting configurations. A procedure is described by which to perform multireference second-order M\o{}ller-Plesset perturbation calculations for a general class of reference functions constructed from one-particle Dirac spinors. Multireference perturbation calculations are reported for the ground and low-lying excited states of oxygen and oxygenlike ions with up to a nuclear charge of $Z=60$ in which the near degeneracy of a manifold of strongly interacting configurations mandates a multireference treatment.