The generalized valence bond description of O₂

Ab initio calculations using the generalized valence bond (GVB) method have been carried out for the lowest triplet and singlet states of O₂ at internuclear distances (R) between 2a₀ and 6a₀. In contrast to other orbital descriptions, GVB leads correctly to ground state oxygen atoms as the bond length is increased to infinity. This proper behavior requires optimization of the spatial orbitals themselves and of the permutational coupling between them as well. Analysis of the results as a function of R is straightforward. Constructing a simple configuration interaction (CI) wavefunction using the GVB orbitals leads to excellent potential curves, accounting for 94% of the bond dissociation energy. The calculated adiabatic separation of the singlet and triplet states is 1.09 eV, which is 0.11 eV above the experimental T_e.