A comparison of the coupled cluster and internally-contracted averaged coupled-pair-functional levels of theory for the calculation of the MCH2+ binding energies for M=Sc to Cu

The correlation contribution to the M–C binding energy for the MCH2+ systems can exceed 100 kcal/mol. At the self-consistent field (SCF) level, these systems can be more than 50 kcal/mol above the fragment energies. In spite of the poor zeroth-order reference, the CCSD(T), method is shown to provide an accurate description of these systems. The maximum difference between the CCSD(T) and internally contracted averaged coupled-pair function binding energies is 1.5 kcal/mol for CrCH2+, with the remaining systems agreeing to within 1.0 kcal/mol.