Ground states of BeC and MgC: A comparative multireference Brillouin-Wigner coupled cluster and configuration interaction study

摘要

The competing X(3)Sigma(-) and (5)Sigma(-) states of the experimentally unknown alkaline-earth metal carbides BeC and MgC are examined with the multireference Brillouin-Wigner coupled cluster method restricted to single and double excitations (MRBW-CCSD). The results are compared against the traditional single-reference CCSD approach, as well as with other single and multireference methods. In both molecules, the CCSD (5)Sigma(-) <- X(3)Sigma(-) energy difference is underestimated, leading to an "erroneous" ground-state prediction in BeC. The MRBW-CCSD method corrects this anomalous behavior, leading to fair agreement with multireference configuration interaction (MR-CI) predictions. Our results at the highest levels of theory are extrapolated to the basis set limit, and the core/valence correlation is taken into account, leading to very accurate energetics and spectroscopic constants in both carbides. (c) 2004 Wiley Periodicals, Inc.