Improving the convergence of the many-body perturbation expansion through a convenient summation of exclusion principle violating diagrams

Abstract

All fourth-order and some higher-order EPV (exclusion principle violating) diagrams may be summed up through a simple modification of the energy denominators in the second-order corrections. The procedure simply requires supplementary arrays of dimension N3 (N being the number of orbitals). The typical divergence of the second-order corrections for near-degeneracy cases, occuring for bond lengthenings, is slowed down; the improvement is especially clear for the Epstein–Nesbet partition of the Hamiltonian. The ground state spectroscopic constants are improved as shown on Li2, Li4, N2, and H2O; the procedure may also be used in chemical reaction studies. The above correction results in a straightforward improvement of the Davidson’s correction, which is known to overestimate the effect of quadruple excitations.