Abstract
Atomization energies have been calculated for CO, H 2O, F 2, HF, N 2 and CH 2 (the 1 A 1 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 CH 2 and overestimated for the remaining molecules by as little as 4.3% (1.3 kJ/mol) for F 2 and as much as 8.4% (3.0 kJ/mol) for N 2. 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.
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