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

Quasidegenerate perturbation theory within the configuration interaction approach (QDPT CI) has been applied to the calculation of excitation 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 which the most important quasidegenerate correlation effects are described is pointed out and general schemes to generate it are proposed. The ability of the QDPT CI approach to treat the dynamical correlation effects is shown and satisfactory convergence to the FCI results is reached with a limited computational effort.