ON THE ROLE OF SINGLE EXCITATIONS IN QUASI-DEGENERATE PERTURBATION-THEORY

In multiconfiguration perturbation theory, the generalized Brillouin t heorem is normally fulfilled, which imposes constraints on the matrix elements involved in the description of single excitations. This can b e considered as guaranteeing coherence of the configurations and orbit als describing the zeroth order wavefunction. In quasi-degenerate pert urbation theory (QDPT) single excitations are treated independently, w hich causes a loss of coherence. It is argued that a loss of orbital c oherence leads to unreasonable QDPT correlation energies.