Coupled-cluster connected-quadruples corrections to atomization energies

Using the CCSDTQ method, we have examined the contributions from the connected quadruple excitations to the atomization energies of CH2, H2O, HF, N2, F2, and CO. For N2 and F2, the quadruples contribution is larger than 3 kJ/mol. On average, the quadruples corrections to the CCSDT atomization energies reduce the mean and maximum errors from 2.3 and 5.0 kJ/mol to 0.5 and 1.1 kJ/mol, respectively, for these six molecules. Moreover, the largest error occurs for N2, where the quintuple contribution is shown to account for a large part of this deviation. The calculations presented here establish unequivocally that the small errors in the CCSD(T) atomization energies in part arise from a cancellation of the errors resulting from the approximate treatment of the connected triples and from the neglect of connected quadruples.