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 coupled cluster single and double excitation method with a perturbational estimate of triple excitations, 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 functional binding energies is 1.5 kcal/mol for CrCH2+, with the remaining systems agreeing to within 1.0 kcal/mol.