Binding energies and bond distances of Ni(CO)_x, x=1-4: An application of coupled-cluster theory

The accuracy of the single and double excitation coupled-cluster (CCSD) method that includes a perturbational estimate of connected triple excitations, denoted CCSD(T), has been tested for some representative transition metal complexes. For both the binding energy and metal to ligand bond distance of NiCO and Ni(CO)₂, the CCSD(T) method yields results in very good agreement with multireference averaged coupled-pair functional (ACPF) calculations. The results are much better than those obtained using either the coupled-pair functional (CPF) or modified CPF (MCPF) methods. The contribution of connected triples to the binding energy is significant for all Ni(CO) _x, x=1-4 ranging from 15 kcal/mol for NiCO to 30 kcal/mol for Ni(CO)4. In contrast, for the geometries the connected triples are only of minor importance. In this case, the correct treatment of disconnected quadruple excitations appears to be more important. For Ni(CO)₄, the calculated binding energy is 125 kcal/mol (expt. 140 kcal/mol) and the bond distance is 3.46 a₀ (exp. 3.45 a₀). Virtually all of the remaining discrepancy, relative to experiment, should be due to limitations in the one-particle basis set. In addition, some test calculations were performed for Ni(C₂H₄), where near degeneracy effects are even more severe than for the nickel carbonyls and the CCSD(T) method gives very accurate results in this case also.