Quasidegenerate perturbation theory applied to the calculation of excitation spectra

Abstract

Second-order quasidegenerate perturbation theory within the configuration interaction approach is applied to the calculation of excitation spectra to test its capability in treating correctly both the dynamical and nondynamical correlation effects. The systems examined are Be and SiH 2 for which full configuration interaction results in the same basis set employed in the present work are available for comparison. Particular attention is paid to the choice of the zeroth order space, which is very important for the fast convergence of the method, and some schemes capable of taking into account the strongest correlation effects are proposed for its generation. Good results, with a smooth convergence to the exact values and a limited computational effort, are obtained for the systems examined, both for the excitation energies and the oscillator strengths.