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.