Generalized Mo/ller–Plesset perturbation theory: Second order results for two-configuration, open-shell excited singlet, and doublet wave functions

The recently proposed consistent generalization of the Mo/ller–Plesset perturbation theory to arbitrary reference states is reviewed and compared to previous methods. We have implemented the second order of this theory (GMP2) for systems which can be described by a two-configuration wave function. GMP2 is applied here to bond-breaking potential curves: HF, F2, CH3–H, and twisted ethylene, as well as to the geometry and dipole moment of ozone, to the electron affinity of the cyano radical, to the vertical ionization potentials of ethylene, and to the excited singlet states of formaldehyde. In general, the performance of GMP2 is very good and comparable to the two-configuration based doubles CI method. However, like all methods based on a single reference function, GMP2 is not applicable in nearly degenerate situations. We also discuss the continuity of potential surfaces in Monte Carlo self-consistent-field (MC-SCF) theory.