A mutually consistent procedure for excitation energies and transition densities based on the extended Brillouin's theorem

A mutually consistent method to calculate excitation energies and corresponding transition densities is proposed. The method is based on the extended Brillouin's theorem that is derived from the nonstationary variation principle. Within the proposed procedure, the Brillouin's conditions, which appear in this extension, are used as a set of nonlinear equations for molecular orbitals and configuration interaction coefficients of the trial ground- and excited-state functions. The excitation energy is an eigenvalue of the set. To some extent, this procedure is related to the variational treatment of the conventional random-phase approximation within the equation-of-motion method. The basic features of the proposed procedure are discussed and it is illustrated by numerical examples. © 1993 John Wiley & Sons, Inc.