Configuration interaction studies on low-lying states of O2

We report configuration interaction calculations as a function of internuclear distance for the nine states of O2 corresponding to the (3σg)2 (1πu)4 (1πg)2 and (3σg)2 (1πu)3 (1πg)3 configurations, that is, X 3Σ−g, a 1Δg, b 1Σ+g, c 1Σ+g, C 3Δu, A 3Σ+u, B 3Σ−u, 1Δu, 1Σ+u. By using the generalized valence bond (GVB) orbitals of the X 3Σ−g state, we obtain good quality GVB–CI wavefunctions with only a moderate number of configurations (72 to 98 spatial configurations) despite the use of a large basis set (double-zeta plus d polarization functions). The calculated De for the X 3Σ−g state is 4.88 eV, 93% of the experimental value. The calculated adiabatic excitation energies are on the average about 0.1 eV from the experimental values.