Finally, we have shown that canonical extrapolation (T CutPNO → 0) is often ill-posed in augmented basis sets, because diffuse basis functions sometimes lead to errors that behave erratically as a function of T CutPNO . Operationally, diffuse functions may reduce the absolute DLPNO error while simultaneously introducing behavior that precludes extrapolation to T CutPNO → 0. For small complexes, such as those in the S66 data set, addition of diffuse functions modifies interaction energies by < 5% upon two-point extrapolation to the MP2/CBS limit, and even the modest cc-pV[D/T]Z extrapolation procedure affords sub-kcal/mol accuracy, without requiring either diffuse basis functions or quadruple-ζ basis sets. However, it is not yet clear whether this result generalizes to larger supramolecular complexes. More work is needed on this front, to decide whether diffuse functions can sensibly be avoided in benchmark-quality calculations involving large van der Waals complexes.