CO₂⁺: A difficult molecule for electron correlation

Although unrestricted, Hartree-Fock calculations correctly predict a D_∞h equilibrium geometry for CO₂⁺, inclusion of electron correlation by methods limited to double substitutions result in the incorrect prediction of a C_∞v equilibrium geometry. Additional electron correlation recovers the correct equilibrium geometry. This unusual pattern of first having, and then losing, and then regaining symmetry with increasing electron correlation is rationalized in terms balancing the contribution of a valence bond configuration having two C-O double bonds which favors a D_∞h structure and one having a single and a triple C-O bond which favors a C_∞v structure.