Some surprising failures of Brueckner coupled cluster theory

Brueckner coupled cluster (B-CC) methods have seen a considerable rise in p opularity over the last decade thanks, in part, to their apparent propensit y for avoiding artifactual symmetry-breaking problems that sometimes plague Hartree-Fock-based approaches. Recent B-CC applications to problematic sys tems such as the tetraoxygen cation have provided encouraging examples of t he success of this theory. In the present work, we examine the performance of the Brueckner technique for a number of other well-known symmetry-breaki ng problems, including the formyloxyl radical, the first excited state of N O2 and the nitrate radical. In these cases, B-CC methods are found to fail dramatically, predicting broken-symmetry equilibrium geometries in conflict with experimental and/or higher-level theoretical results. A framework is developed which indicates that these errors can be attributed to artificial ly exaggerated second-order Jahn-Teller interactions with nearby electronic states. Hence, in spite of their initial successes, Brueckner methods cann ot be considered a panacea for symmetry-breaking problems. (C) 2000 America n Institute of Physics. [S0021-9606(00)30718-8].