Effective valence shell Hamiltonian and potential curves of the oxygen molecule from quasidegenerate many-body perturbation theory

Abstract

The effective valence shell Hamiltonian (Hv) is calculated for O2 using quasidegenerate many-body perturbation theory with an eight orbital valence space. A comparison is made of the accuracy of Hv results from a second vs third order truncation of the perturbation expansion. Potential curves for ten low lying valence states show that second order calculations produce dissociation energies and harmonic frequencies that are systematically too large. However, the third order Hv calculations correct the deviations present in second order. Our third order ground state spectroscopic constants compared well with those from a full configuration interaction calculation using the same basis set. Hv calculations are also performed using a second set of orbitals constrained such that the molecular valence space is the union of atomic valence spaces. The constrained orbital Hv calculations are designed for comparison with model valence shell Hamiltonians of semiempirical methods. Comparison of second and third order constrained calculations enables a determination of the reliable range of internuclear distances of the individual constrained Hv matrix elements. Third order constrained Hv matrix elements in the atomic orbital basis set are least squares fit to simple functions of inverse internuclear separation or orbital overlap for comparison with the forms used in semiempirical methods. Functional forms employed for second order Hv matrix elements are compared with previous fits to second order Hv matrix elements for S2 and CH in order to present systematic trends.