Degeneracy nondegenerate perturbation theory

If there is no degeneracy in the MO energies of the unperturbed system, the nondegenerate perturbation theory leads to the results. 

Quasi-degeneracy Effects.—The convergence properties of the non-degenerate formulation of the many-body perturbation theory deteriorate when quasidegeneracy is present in the reference spectrum. In view of its simplicity, however, there is considerable interest in exploring the range of applicability of the nondegenerate formalism. 

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