An orthogonal orbital MCSCF plus Cl treatment of molecular symmetry breaking. I. The lowest ³B₂ and ³A₂ states of CO₂

A general, orthogonal orbital, multiconfuguration self-consistent field (MCSCF) plus configuration interaction (CI) procedure has been developed for treating open shell molecules subject to symmetry breaking (doublet instability, etc.) in the restricted Hartree-Fock approximation. The chemical origin of symmetry breaking was analyzed using a simple valence bond picture and the lowest 3A2 and 3B2 states of CO 2 as examples. This analysis provided the necessary information to design MCSCF wave functions which possessed the symmetry of the nuclear frame, and changed smoothly between high and low symmetry nuclear configurations. Near qualitative potential energy surfaces near the potential minima of the two lowest triplet states of CO2 were obtained by CI calculations using the MCSCF orbitale and reference configurations. Both states were found to have C2, potential minima with the 3A2 state lying approximately 0.9 eV above the 3B2 state. The equilibrium CO bond lengths and OCO bond angles are 1.24 Å, 118° and 1.26 Å, 127° for the 3B2 and 3A2 states, respectively. © 1983 American Institute of Physics.