The SCF Component of TAE

For systems devoid of nondynamical correlation effects, this is the largest individual contribution to the molecular binding energy. Its basis set convergence is relatively rapid, yet our discussion will be disproportionately long because a number of the dramatis personae that reappear in the remainder of the story need to be introduced here. 

Martin and Taylor [21] compared numerical SCF energies with extrapolations from calculated SCF/A VQZ, SCF/A V5Z, and SCF/A V6Z energies using the formula 

Petersson et al. had earlier proposed [23] an alternative expression E(n) = Eoo + ]CiSn+i A/(l + 1/2)6 in the context of the CBS methods developed in his group. The summation is carried out numerically in that paper, but in fact an elegant analytical approximation exists for 

This suggests the simple extrapolation formula E(n) = Eqq + A/n5, i.e. Eco = E(n) +, where n is identified with the n-tuple ze- 

In the original W1/W2 paper [1], we opted for the geometric formula in view of the observed geometric convergence behavior. In a subsequent validation study [26] on a much wider variety of systems, we however found the two-point formula to be much more reliable, and we have adopted it henceforth. 

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Extrapolation of the SCF component of TAE from the small , medium , and large basis sets (Wl) or medium , large , and extra-large basis sets (W2), by means of either the geometric... 

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