Theoretical study of the low-lying bound states of O2

Ab initio results are presented for the low-lying bound states of O₂ that dissociate to ground-state atoms. Spectroscopic constants and dissociation energies (D_e) are reported for the X ³Σ^-_g, a ¹Δ_g, b ¹Σ⁺_g, c ¹Σ^-_u, A' ³Δ_u, A ³Σ⁺_u, ³Π_u, ⁵Σ^-_u, ⁵Π_u, and ⁵Π_g states. For the first six states, which have been experimentally characterized, we obtain accurate results at the multireference configuration-interaction level after correction for higher excitations. For example, our computed D_e value for the X ³Σ^-_g state corrected for basis-set superposition error is only 0.7 kcal/mol less than the experimental value. The ⁵Π_g state is estimated to have a D_e of 0.16±0.03 eV, suggesting that the importance of this state in the nightglow should perhaps be reconsidered.