SYMMETRY-BREAKING IN O-4- AN APPLICATION OF THE BRUECKNER COUPLED-CLUSTER METHOD( )

A recent calculation of the antisymmetric stretch frequency for the re ctangular structure of quartet O4+ using the QCISD(T) method gave a va lue of 3710 cin-1. This anomalous frequency is shown to be a consequen ce of symmetry-breaking effects, which occur even though the QCISD(T) solution derived from a delocalized SCF reference function lies energe tically well below the two localized (symmetry-broken) solutions at th e equilibrium geometry. The symmetry breaking is almost eliminated at the CCSD level of theory, but the small remaining symmetry-breaking ef fects are magnified at the CCSD (T) level of theory so that the antisy mmetric stretch frequency is still significantly in error. The Brueckn er coupled-cluster method, however, leads to a symmetrical solution wh ich is free of symmetry-breaking effects, with an antisymmetric stretc h frequency of 1322 cm-1, in good agreement with our earlier calculati ons using the CASSCF/CASSI method.