On the role of single excitations in quasi-degenerate perturbation theory

In multiconfiguration perturbation theory, the generalized Brillouin theorem is normally fulfilled, which imposes constraints on the matrix elements involved in the description of single excitations. This can be considered as guaranteeing coherence of the configurations and orbitals describing the zeroth order wavefunction. In quasi-degenerate perturbation theory (QDPT) single excitations are treated independently, which causes a loss of coherence. It is argued that a loss of orbital coherence leads to unreasonable QDPT correlation energies.