Perturbation theory for steady states of molecules in a one-electron approximation

Abstract

An important problem in quantum chemistry and molecular spectrosopy is the study of the interaction between molecules and external fields. Calculation of the wave functions of a many-electron electron system situated in an external field is possible only by using approximate methods. The most widely used in such calculations is the method of perturbation theory, within the framework of which the interaction between the system and the external field can be described using different kinds of polarizability and susceptibility. However, in practice, the use of perturbation theory for many-electron systems runs into considerable difficulties, due to the fact that the unperturbed wave functions are calculated approximately. Completely satisfactory accuracy for the ground states of the molecules is given by the Hartree--Fock approximation. In the case of perturbed states, the three most widely used variants of the theory in a one-electron approximation are: the configuration interaction (CI) method (sometimes called the Tamm-Dancoff approximation), the random phase method, and the frequency-stability method [i]. A review of different variants of perturbation theory for many-electron systems is given in [2]. In the case of a static perturbation, out of the whole set of variants of perturbation theory for the ground state in a one-electron approximation, most investigatorshave chosen to consider the "bound" Hartree--Fock approximation [3-7]. In [8] Rebane proposed a method for calculating a second-order correction to the energy of the molecules, which has been called variation perturbation theory [i, 9]. In this method, the change in the interelectron interaction under the action of an external field is taken into consideration by the decomposition of the perturbed wave function of the system into single-perturbation configurations, built-up of self-consistent molecular orbitals (MO).