On the evaluation of nonadiabatic coupling matrix elements using SA-MCSCF/CI wave functions and analytic gradient methods. I

Abstract

A method for the efficient evaluation of nonadiabatic coupling matrix elements of the form 〈Ψ I ‖∂/∂R αΨ J 〉 is presented. The wave functions Ψ I and Ψ J are assumed to be multiconfiguration self‐consistent field (MCSCF) wave functions optimized within the state averaged approximation. The method, which can treat several states simultaneously, derives its efficiency from the direct solution of the coupled perturbed state averaged MCSCF equations and the availability of other appropriate derivative integrals. An extension of this approach to SA‐MCSCF/CI wave functions is described. Here it is shown that computational efficiencies can be achieved by exploiting analogies with analytic CI gradient methods. Numerical examples for C 2v approach of Mg to H2 are presented.