The cc-pV5Z-F12 basis set: Reaching the basis set limit in explicitly correlated calculations

We have developed and benchmarked a new extended basis set for explicitly correlated calculations, namely cc-pV5Z-F12. It is offered in two variants, cc-pV5Z-F12 and cc-pV5Z-F12(rev2), the latter of which has additional basis functions on hydrogen not present in the cc-pVnZ-F12 (n = D,T,Q) sequence. A large uncontracted reference basis set is used for benchmarking. cc-pVnZ-F12 (n = D-5) is shown to be a convergent hierarchy. Especially the cc-pV5Z-F12(rev2) basis set can yield the valence CCSD (coupled cluster with all single and double substitutions) component of total atomisation energies, without any extrapolation, to an accuracy normally associated with aug-cc-pV{5,6}Z extrapolations. Hartree-Fock self-consistent field (SCF) components are functionally at the basis set limit, while the MP2 limit can be approached to as little as 0.01 kcal/mol without extrapolation. The determination of (T) appears to be the most difficult of the three components and cannot presently be accomplished without extrapolation or scaling. (T) extrapolation from cc-pV{T,Q}Z-F12 basis sets, combined with CCSD-F12b/cc-pV5Z-F12 calculations, appears to be an accurate combination for explicitly correlated thermochemistry. For accurate work on noncovalent interactions, the basis set superposition error with the cc-pV5Z-F12 basis set is shown to be so small that counterpoise corrections can be neglected for all but the most exacting purposes.