Wigner's (2n+ 1) rule in MBPT

Wigner's (2n + 1) rule (stating that the 2nth and/or (2n + 1)th order perturbation contributions to a non-degenerate energy can be evaluated from a knowledge of the wavefunction through nth order) is applied to many-body perturbation theory (MBPT). This approach essentially simplifies the calculation of correlation energy to an order higher than the third and reduces the number of required diagrams.