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 (3σ_g)² (1π_u)² (1π_g)² and (3σ_g)² (1π_u)³ (1π_g)³ configurations, that is, X³Σ_g^-, a ¹Δ _g, b ¹Σ_g⁺, c ¹Σ_g⁺, C ³Δ_u, A ³Σ_u⁺, B ³Σ _u^-, ¹Δ_u, 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 D_e for the X ₃³Σ_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.