The cc-pV[T/Q]Z and aug-cc-pV[T/Q]Z extrapolation schemes prove to be negligibly different (see Fig. ), with a mean absolute difference of just 0.04 kcal/mol, suggesting that diffuse functions are unnecessary if augcc-pVQZ calculations are feasible. Comparing the cc-pV[D/T]Z and aug-cc-pV[D/T]Z extrapolation procedures (Fig. ), we observe differences ranging up to 0.7 kcal/mol (12% of |∆E|) and averaging 0.2 kcal/mol (0.9% of |∆E|), meaning that diffuse functions matter only at the sub-kcal/mol level. Table provides a statistical summary of all four extrapolation procedures in comparison to well-converged MP2/CBS benchmarks. The most affordable extrapolation scheme (cc-pV[D/T]Z) yields sub-kcal/mol accuracy for every single S66 dimer, with an average deviation of 0.3 kcal/mol from MP2/ CBS reference values. This is notable given the considerable computational expense of augmenting the basis set. For the largest S66 system (pentane dimer), the RI-MP2/aug-cc-pVDZ calculation took 1.2 h on 12 processors compared to only 0.2 h for RI-MP2/cc-pVDZ on the same hardware. For small systems, this additional effort may or may not be justifiable to capture < 1 kcal/mol of interaction energy, but for larger systems the addition of diffuse basis functions may simply be impossible. Regardless, the contribution of the diffuse functions to ∆E is likely extensive with system size, and it remains to be determined whether diffuse functions are important in sizable van der Waals complexes. Our own experience with proteinligand interaction energies computed using DFT suggests this may be the case. However, more effort is needed along these lines in order to establish reliable computational protocols for non-covalent interactions in sizable supramolecular complexes.