Non-adiabatic coupling matrix elements for large CI wavefunctions

Abstract

Non-adiabatic coupling matrix elements of the type <Ψ z|∂/∂ QΨ β> are obtained for large-scale CI wavefunctions, whereby both contributions to the matrix elements, that originating exclusively from the dependence of the CI coefficients on the nuclear variables and that related to the changes in the MO's themselves, are evaluated by the method of finite differences. comparison is made between this procedure and that which employs an analytical fit for the individual CI coefficients followed by direct differentiation. Numerical applications of the present method include the interaction of two 4A 2 states of CH + 2 as a function of the bending nuclear coordinate, the Rydberg- valence mixing between two 3Π u states in O 2 and the interaction of four states of 3Π g symmetry in the C 2 molecule. The influence of the AO basis and he size of the configuration space on the results is dicussed, and it is shown that contrary to experience reported in prior investigations the part of the matrix element which results from the molecular orbital dependence on the nuclear coordinates can be as important as the corresponding CI term in certain instances.