Multireference many-body perturbation theory: Application to O2 potential energy surfaces

A modification of multi-reference many-body perturbation theory is introduced and applied to potential surface calculations of the first 3Σ−g, 1Δg, and 1Σ+g bound states of O2. The modification enhances the convergence and efficiency of both Rayleigh–Schrödinger and Brillouin–Wigner expansions by pre-diagonalizing the primary space and then contracting to a smaller dimension space before calculating perturbative corrections.