CALCULATION OF EXCITATION-ENERGIES USING QUASI-DEGENERATE VARIATIONAL PERTURBATION-THEORY AND MULTIREFERENCE-AVERAGED COUPLED-PAIR FUNCTIONAL THEORY

Results are presented for the ground and excited states of Be and SiH2 from ab initio multireference singles and doubles configuration interaction, quasidegenerate variational perturbation theory, and multireference-averaged coupled-pair functional theory calculations. The results are compared with previously obtained full configuration interaction studies. It is seen that quasidegenerate variational perturbation theory and multireference-averaged coupled-pair functional theory yield accurate total energies and excitation energies, even for states that are not the lowest in a given symmetry. Increased accuracy is obtained for excitation energies as the reference space is expanded. In these systems we find that for a fixed small reference space, multireference-averaged coupled-pair functional theory yields total energies closer to full configuration interaction values than either quasidegenerate variational perturbation theory or multireference singles and doubles configuration interaction. As the reference space size is increased averaged coupled-pair functional theory and configuration interaction yield comparable errors in the total energy. Comparison is also made with Davidson-corrected multireference singles and doubles configuration interaction results. Similar results to those given by quasidegenerate variational perturbation theory were obtained.