Diagrammatic many-body perturbation expansion for atoms and molecules: IV. Fourth-order linked diagrams involving quadruply-excited states

Abstract

An algorithm is described for rigorously evaluating the fourth-order energy terms corresponding to linked diagrams involving quadruply-excited intermediate states which arise in the many-body perturbation series for the electron correlation energy of closed-shell atoms and molecules. The algebraic approximation is employed in which the single particle state functions are parametrized by expansion in a finite basis set. The algorithm is noniterative and enables the effect of quadruply-excited configurations on electron correlation energies to be estimated efficiently.