QUASI-DEGENERATE PERTURBATION-THEORY CALCULATION OF EXCITATION-SPECTRA - A CONVERGENCE STUDY ON THE CH+ MOLECULE

Quasidegenerate perturbation theory within the configuration interacti on approach (QDPT CI) has been applied to the calculation of excitatio n energies and transition moments of CH+ and the results are compared with the full configuration interaction (FCI) values in the same basis set. The importance of a proper choice of the zeroth-order space in w hich the most important quasidegenerate correlation effects are descri bed is pointed out and general schemes to generate it are proposed. Th e ability of the QDPT CI approach to treat the dynamical correlation e ffects is shown and satisfactory convergence to the FCI results is rea ched with a limited computational effort.