Photodissociation of O2 in the Herzberg continuum. I. Ab initio calculation of potential energy curves and properties

We present ab initio complete active space self-consistent-field plus multireference configuration interaction (CASSCF+MRCI) potential energy curves for the eight electronically excited ungerade states of oxygen (A 3Σu+, c 1Σu−, A′ 3Δu, Πu1, Πu3, Πu5, Πu-5, and 2 3Σu+) that correlate with the O(3P)+O(3P) dissociation limit. We also report the R-dependent spin–orbit couplings between these states and the R-dependent radial derivative coupling matrix element 〈2 3Σu+|∂/∂R| A 3Σu+〉. The near degeneracy in the long range of the same-symmetry states 2 3Σu+ and A 3Σu+ may result in unphysical mixing of these states in a CASSCF calculation. We derive the correct asymptotic behavior of these states as dictated by the quadrupole–quadrupole interaction and we show how a correct long range description of these states can be achieved numerically by employing undistorted molecular orbitals. Bound state calculations using Herzberg I, II, and III potentials show excellent agreement with all available spectroscopic data. In the accompanying paper the potentials and couplings will be employed in a semiclassical study of the photodissociation of O2 in the Herzberg continuum.