A b i n i t i o third order effective valence shell Hamiltonian calculations for first row diatomic hydrides

Abstract

A b initio effective valence shell Hamiltonian ℋv calculations are presented through third order for the correlation energies of CH, NH, and OH at their equilibrium internuclear distance. The valence shell consists of the 2σ, 3σ, 1π, and 4σ orbitals. This represents the first quasidegenerate third order many-body perturbation calculation of ℋv for molecules with more than two valence electrons. As in the atomic case, a single ℋv calculation provides correlation energies for all neutral valence states as well as for the positive and negative ion valence states of the hydrides. Calculated vertical excitation energies are in good agreement with those obtained from large configuration interaction calculations. Four-electron effective interactions, present in the third order expansion of ℋv, are evaluated. These four-electron terms make significant contributions to the energies of many states. Technical difficulties are discussed which are related to ℋv calculations with large valence spaces that violate the quasidegeneracy requirements of the theory. One method which alleviates the associated divergence problems is shown to be useful in third order calculations.