The CuC bond dissociation energy of CuCH₃. A dramatic failure of the QCISD(T) method

The coppercarbon bond dissociation energy of CuCH₃ is calculated using coupled cluster (CC) theory and the quadratic CI approach (QCI) in conjunction with an effective core potential (ECP) for Cu. The results at the CCSD (D_e=45.4 kcal/mol) and CCSD(T) level (D_e=48.0 kcal/mol) are in good agreement with previous high-level calculations using the MCPF approch (D_e=48.4 kcal/mol). The theoretically predicted dissociation energy at the QCISD(T) level shows a dramatic failure of the estimate for the triple excitations in CuCH₃. The QCISD value is D_e=55.4 kcal/mol, the QCISD(T) values is D_e=6.0 kcal/mol. The QCISD(T) method predicts clearly different dissociation energies when the core electrons are explicitly included in the calculations, while the CCSD(T) method gives similar results to the ECP calculations. The predicted dissociation energy using a large all-electron basis set is D_e=52.7 kcal/mol at QCISD and D_e=29.4 kcal/mol at QCISD(T). The CCSD method gives with the same basis set D_e=46.0 kcal/mol; the value at CCSD(T) is D_e=49.7 kcal/mol.