Applicability of non-degenerate many-body perturbation theory to quasidegenerate electronic states: A model study

The nondegenerate finite-order many-body perturbation theory is applied to simple model systems in which the degree of quasidegeneracy can be continuously varied over a wide range. Three ab initio minimum basis set models involving four hydrogen atoms in various spatial arrangements are considered. The results are compared with the exact full configuration-interaction approach, double-excitation configuration-interaction and the coupled-pair many-electron theory.