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In the context of high-accuracy computational thermochemistry, the valenceCCSD correlation component of molecular atomization energies present the mostsevere basis set convergence problem, followed by the (T) component. In thepresent paper, we make a detailed comparison, for an expanded version of theW4-11 thermochemistry benchmark, between on the one hand orbital-basedCCSD/AV{5,6}Z+d and CCSD/ACV{5,6}Z extrapolation, and on the other handCCSD-F12b calculations with cc-pVQZ-F12 and cc-pV5Z-F12 basis sets. This latterbasis set, now available for H--He, B--Ne, and Al--Ar, is shown to be veryclose to the basis set limit. Apparent differences (which can reach 0.35kcal/mol for systems like CCl4) between orbital-based and CCSD-F12b basis setlimits disappear if basis sets with additional radial flexibility, such asACV{5,6}Z, are used for the orbital calculation. Counterpoise calculationsreveal that, while TAEs with V5Z-F12 basis sets are nearly free of BSSE,orbital calculations have significant BSSE even with AV(6+d)Z basis sets,leading to non-negligible differences between raw and counterpoise-correctedextrapolated limits. This latter problem is greatly reduced by switching toACV{5,6}Z core-valence basis sets, or simply adding an additional zeta to justthe valence orbitals. Previous reports that all-electron approaches like HEATlead to different CCSD(T) limits than valence limit+CV correction approacheslike FPD and W4 theory can be rationalized in terms of the greater radialflexibility of core-valence basis sets. W4-F12 is found to agree slightlybetter than W4 with ATcT (active thermochemical tables) data, at a substantialsaving in CPU time and especially I/O overhead. A W4-F12 calculation on benzeneis presented as a proof of concept.

Toward a W4-F12 approach: Can explicitly correlated and orbital-based ab initio CCSD(T) limits be reconciled?