Sets. Errors for correlationconsistent basis sets and "calendar" variants thereof are provided in Fig. , with statistics compiled in Table 2. Again we find that some augmentation with diffuse functions is necessary to obtain high-quality results for hydrogen-bonded systems, despite the fact that all of the monomers in S66 are charge-neutral. In the absence of any diffuse functions at all, even the cc-pVTZ basis set affords unacceptably large errors (up to 5.7 kcal/mol), specifically for the hydrogen-bonded complexes. These errors are substantially reduced, even at the double-ζ level, by minimal augmentation, e.g., jun-cc-pVDZ or jun-cc-pVTZ, although some of the errors for hydrogenbonded complexes remain larger than 1 kcal/mol even at the aug-cc-pVTZ level, where the average error is 0.5 kcal/mol but there are outliers up to 2.6 kcal/mol for hydrogen-bonded complexes. In terms of the diffuse functions that are required, the use of jun-cc-pVXZ (for X = D, T, or Q) affords very similar results to aug-cc-pVXZ, indicating that the full complement of diffuse functions is not required even for hydrogen-bonded complexes. This is an extremely useful observation because jun-cc-pVXZ is more scalable to large systems as compared to aug-cc-pVXZ, as the difference (for second-row atoms) is a set of diffuse f functions.