Multireference CI gradients and MCSCF second derivatives

Abstract

A set of simple and efficient formulas for the calculation of multireference CI gradients and MCSCF second derivatives is presented. The CI gradient formalism is extended to include a general class of references in the CI. This extension is necessary for the calculation of gradients for a commonly employed class of CI wave functions for which the reference configurations are selected from a generalized CAS MCSCF wave function. In addition, we report the first general multireference CI gradient calculations. The calculations are for the reaction Be+H2→BeH2 constrained to C 2v symmetry. Structures of the reactant and transition state and the activation energy calculated at the selected reference CI level compare favorably to the full second order CI results. MCSCF second derivatives are found to be useful for the optimization of the CI structures.