Diffuse Orbitals in Lower States of the Oxygen Molecule

An ab initio SCF CI calculation was carried out for several lower states of O2 with various basis sets including diffuse pπ orbitals. Extensive CI calculations with basis sets including diffuse 2s and 2pπ functions show that the B 3Σu− state is essentially a valencelike state as well as the c 1Σu−, C 3Δu, and A 3Σu+ states, e.g., in the X 3Σg− state 〈z2〉 = 7.51 and in the B 3Σu− state 〈z2〉 = 8.84 a.u. Various levels of calculations are compared with existing theoretical results. The oscillator strength of the B 3Σu− ← X 3Σg− transition is calculated to be 0.167 in agreement with the experiment. A qualitative discussion of the molecular dimension of the B state is also made.