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, N-2, F-2, and CO. For N-2 and F-2, 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 N-2, 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. (C) 2003 Elsevier Science B.V. All rights reserved.