Developments in the n-electron valence state perturbation theory

A short summary of the n-electron valence state perturbation theory (NEVPT) is provided, and the defects inherent in cases of quasi-degeneration are underscored. A new version of the theory, called QD-NEVPT, based on quasi-degenerate perturbation theory, is presented. The new theory introduces an effective interaction among the zero-order wave functions and overcomes the difficulties of state-specific NEVPT. An application concerning the interaction among the pi->pi* and Rydberg singlets of formaldehyde is presented.