Brillouin's theorem for excited (nl)q(n'l)q' configurations

Abstract

It is shown that in the non-orthogonal Hartree-Fock approximation Brillouin's theorem holds for all functions Phi _c* obtained from Phi _c by the replacement of one nl electron by n*l, such a replacement affecting only the radial part of the wavefunction. The orthogonality restrictions of the Hartree-Fock approximation prevent the satisfaction of Brillouin's theorem for some replacements such as nln'lLS to nl²LS. A multi-configuration approximation then exists for which the theorem is valid.