Coupled-cluster singles, doubles and triples (CCSDT) calculations of atomization energies

Atomization energies have been calculated for CO, H2O, F2, HF, N2 and CH2 (the 1A1 state) using the coupled-cluster singles, doubles and triples (CCSDT) model as well as the coupled-cluster singles and doubles model with a perturbative correction for triples [CCSD(T)]. The CCSD(T) model provides an excellent approximation to the CCSDT model; at the cc-pV5Z basis set level, the CCSDT valence triples contribution is underestimated by 9.1% (0.8 kJ/mol) for CH2 and overestimated for the remaining molecules by as little as 4.3% (1.3 kJ/mol) for F2 and as much as 8.4% (3.0 kJ/mol) for N2. At the CCSDT level, the agreement with experiment is not improved, suggesting that some cancellation of error occurs between the missing triples contributions at the CCSD(T) level and the contributions from the connected quadruples.