Calculation of effective hamiltonians using quasi-degenerate Rayleigh-Schrödinger perturbation theory (QD-RSPT)

Abstract

The quasi-degenerate Rayleigh-Schrödinger perturbation theory (QD-RSPT) proposed by Brandow [3] is rederived in matrix form. An approximate computational method is proposed which includes the diagonal energy renormalizations (the diagonal folded diagrams) to infinite order. Several numerical examples are presented which illustrate the numerical stability of the method.