THE CU-C BOND-DISSOCIATION ENERGY OF CUCH3 - A DRAMATIC FAILURE OF THE QCISD(T) METHOD

The copper-carbon 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 approach (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 De(=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.