High order correlation effects in the calculation of enthalpies of formation of sulfur compounds, CCSDT vs CCSD[T]

The enthalpies of formation of 24 sulfur molecules have been calculated employing the CCSDT and CCSD[T] methodologies with Dunning correlation consistent basis sets. The observed tendency is that the CCSD[T] method overestimates the correlation energy when we compare it against the CCSDT method. The average CCSDT - CCSD[T] difference is 0.42 kcal/mol with deviations larger than 1 kcal/mol for some molecules, such us SO2, SO3, SCO and CS2. This effect difficults the prediction of enthalpies of formation with chemical accuracy ( I kcal/mol), since, CCSD[T] in general performs better than CCSDT in the calculation of enthalpies of formation. Regarding basis set dependency, we did not appreciate differences between the cc-pVTZ results and the aug-cc-pV(T+D)Z ones. The cc-pVTZ basis set is a good compromise between precision and computational cost. (C) 2003 Published by Elsevier B.V.