A b i n i t i o calculations on the He–O2 potential energy surface. Hartree–Fock instability of O2

The interaction energy between He and O2 (3∑−g) is obtained for five angles and eight distances, keeping the O–O distance fixed. The energies are calculated at the Hartree–Fock and MCSCF level, corrected for basis set superposition errors and augmented by an estimate of the dispersion energy from second-order perturbation theory. The Hartree–Fock wave function for O2 is unstable with respect to breaking the D∞h symmetry. This makes a multiconfigurational description of the O2 π system essential to obtain uniform reliability at all angles. The MCSCF+dispersion surface shows minima at 90° (−0.17 mhartree) and 0° (−0.16 mhartree) and a barrier in between at 47° (−0.13 mhartree).