Very accurate ab initio binding energies - a comparison between empirical corrections and extrapolation methods

The experimentally very accurately known total atomization energies (TAEs; Sigma D-e values) of 14 small polyatomic molecules have been computed at the CCSD(T) level using basis sets of up to [7s6p5d4f3g2h/5s4p3d2flg] quality. Core correlation was corrected for through separate calculations using a special core-correlation basis set. A comparative study of extrapolations to the infinite-basis set limit and empirical correction treatments for basis set incompleteness was made. Up to basis sets of [spdfg] quality, the Martin 3-parameter empirical correction formula significantly outperforms extrapolation techniques based on the asymptotic l-dependence of the correlation energy: for still larger basis sets, the two approaches yield comparable results. The effect of core correlation can be fairly well absorbed in the parameters of the correction, but for very large valence basis sets, it becomes worthwhile to treat it explicitly. 3-parameter-corrected CCSD(T)/aug'-cc-pVQZ calculations represent a particularly good compromise between computational cost and accuracy (mean absolute error 0.23 kcal mol(-1); maximum absolute error 0.63 kcal mol(-1)). (C) 1997 Elsevier Science B.V.