Symmetry Breaking And Electron Correlation In O-2(-), O-2, And O-2(+): A Comparison Of Coupled Cluster And Quadratic Configuration Interaction Approaches

Potential energy curves are calculated for O-2(-), O-2, and O-2(+) at the CCSD, QCISD, CCSD(T), and QCISD(T) levels of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets with electron correlation built onto inversion symmetry constrained and relaxed UHF wave functions. The spectroscopic constant r(e), omega(e), omega(e), x(e), D-j, and alpha(e) are determined from the potential curves using standard second-order perturbation theory expressions and are compared with experimental values to assess the relative accuracy of the theoretical approaches. Comparison of corresponding symmetry-constrained and symmetry-relaxed calculations indicates that the CCSD method is generally superior to CCSD(T), QCISD, and QCISD(T) in recovering from a symmetry-broken reference function. (C) 1996 John Wiley & Sons, Inc.