Large-amplitude quantum mechanics in polyatomic hydrides. II. A particle-on-a-sphere model for XH(n) (n=4,5).

This paper describes the application of a relatively simple, but computationally tractable, "particle-on-a-sphere" (POS) model for quantum-mechanical calculation of large-amplitude, H atom dynamics in polyatomic hydrides (XH(n)), based on radially relaxed, two-dimensional angular motion of H atoms on the surface of a sphere. This work focuses on systems with many degrees of freedom, i.e.

Multireference configuration interaction calculations for the F(2P)+HCl-->HF+Cl(2P) reaction: a correlation scaled ground state (1 2A') potential energy surface.

This paper presents a new ground state (1 (2)A(')) electronic potential energy surface for the F((2)P)+HCl-->HF+Cl((2)P) reaction. The ab initio calculations are done at the multireference configuration interaction+Davidson correction (MRCI+Q) level of theory by complete basis set extrapolation of the aug-cc-pVnZ (n=2,3,4) energies. Due to low-lying charge transfer states in the transition state region, the molecular orbitals are obtained by six-state dynamically weighted multichannel self-consistent field methods.

A simple picture for the rotational enhancement of the rate for the F + HCl --> HF + Cl reaction: a dynamical study using a new ab initio potential energy surface.

Quantum scattering calculations for the reaction F + HCl --> HF + Cl are performed on a new ground-state ab initio potential energy surface. The reagent rotation is found to have a dramatic effect on the reaction probability. Furthermore, the exit channel rotational thresholds leave a strong imprint on the reaction probabilities and even on the cumulative reaction probability.

Large amplitude quantum mechanics in polyatomic hydrides. I. A particles-on-a-sphere model for XH(n).

A framework is presented for converged quantum mechanical calculations on large amplitude dynamics in polyatomic hydrides (XH(n)) based on a relatively simple, but computationally tractable, "particles-on-a-sphere" (POS) model for the intramolecular motion of the light atoms. The model assumes independent two-dimensional (2D) angular motion of H atoms imbedded on the surface of a sphere with an arbitrary interatomic angular potential. This assumption permits systematic evolution from "free rotor" to "tunneling" to "quasi-rigid" polyatomic molecule behavior for small, but finite, values of total angular momentum J.

Dynamically weighted multiconfiguration self-consistent field: multistate calculations for F+H2O-->HF+OH reaction paths.

A novel method of dynamically adjusted weighting factors in state-averaged multiconfiguration self-consistent-field calculations (SA-MCSCF) is described that is applicable to systems of arbitrary dimensionality. The proposed dynamically weighted approach automatically weights the relevant electronic states in each region of the potential energy surface, smoothly adjusting between these regions with an energy dependent functional. This method is tested on the F(2P)+H2O-->HF+OH(2Pi) reaction, which otherwise proves challenging to describe with traditional SA-MCSCF methods due to (i) different asymptotic degeneracies of reactant (threefold) and product (twofold) channels, and (ii) presence of low-lying charge transfer configurations near the transition state region.