Diagrammatic many-body perturbation expansion for atoms and molecules V. Fourth-order linked diagrams involving triply-excited states

Abstract

An algorithm is described for rigorously evaluating the fourth-order energy terms corresponding to linked diagrams involving triply-excited intermediate states which arise in the many-body perturbation expansion for the electron correlation energy of closed-shell atoms and molecules. The algebraic approximation is employed, in which the single particle state functions are parameterised by expansion in a finite basis set. The algorithm is non-iterative and enables the effect of triply-excited states on electron correlation energies to be determined efficiently. Spin-free expressions for triple-excitation energy components are given. The use of vector processing computers, such as the CRAY 1, in this work is discussed.