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

Abstract

AbstractThe competing X3Σ− and 5Σ− 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 5Σ− ← X3Σ− 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. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005