Applicability of quasi-degenerate many-body perturbation theory to quasi-degenerate electronic states: The H4 model revisited

The performance of the quasi-degenerate many-body perturbation theory up to the third order is investigated for the ground state, five excited states, and the first quintet of a simple four-electron H₄ model system consisting of two stretched hydrogen molecules, in which the degree of quasi-degeneracy can be continuously varied from a nondegenerate to a full degenerate situation. We employ a DZP basis set. The effect of intruder states is considered and a comparison with other multireference correlation techniques is also provided. Finally, a criterion for the model space to be quasi-degenerate will be reformulated and generalized. © 1995 John Wiley & Sons, Inc.