Theoretical studies of intersystem crossing effects in the O+H₂ reaction

A theory of intersystem crossing effects is developed that is applicable to atom-diatom and related chemical reactions where global potential energy surfaces can be developed. The theory is applied to singlet-triplet interactions in the reaction O(³P, ¹D) + H₂ → OH(²Π) + H. This theory uses quasiclassical trajectory surface-hopping methods to determine dynamical information based on a diabatic representation in which the usual nonrelativistic potential surfaces define the diagonal potentials, and matrix elements of the Breit-Pauli spin-orbit operator determine the couplings. A key result is that the spin-orbit matrix elements may be determined adequately using relatively low level theory such as complete active space self-consistent field methods, and with significant truncation of the electronic state basis set. In addition, the spin-orbit matrix elements is found to have a relatively simple dependence on internuclear geometry, such that determining global spin-orbit coupling surfaces is much easier than determining the Born-Oppenheimer surfaces.