Tests of using large valence spaces in quasidegenerate many-body perturbation theory: Calculations of O2 potential curves

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Abstract
Potential curves are calculated for the oxygen molecule using the effective valence shell Hamiltonian H v method based on quasidegenerate many‐body perturbation theory (QDMBPT). Spectroscopic constants of the 12 bound valence states are compared with those obtained from experiment and those calculated by Saxon and Liu from extensive MCSCF and CI calculations. The excellent agreement indicates that the quasidegenerate many‐body perturbation scheme is a reliable ab initio method even with larger valence spaces in which the quasidegenerate condition is strongly violated. Approximations to the correlated effective valence shell Hamiltonian full valence shell CI matrix are tested, and new simplifications are found to give results very similar to the complete calculations, thereby providing useful approximation schemes for larger systems. A remarkable property of the QDMBPT‐H v method is the fact that all the states at a given internuclear distance are calculated using a common set of core and valence orbitals; the perturbation formulation accounts for the valence orbital readjustments normally required in traditional configuration interaction calculations.