  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,
T, Q, 5) is shown to be a convergent hierarchy. Especially the cc-
pV5Z-F12(rev2) basis set can yield the valence CCSD component of total
atomization energies (TAEs), without any extrapolation, to an accuracy normally
associated with aug-cc-pV{5,6}Z extrapolations. 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, 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.

Chemical Physics;Computational Physics