The S66 dimers range in size from (H 2 O) 2 up to pentane dimer and are divided into three subsets based on the ratio of the electrostatic energy (E elst ) to the dispersion energy (E disp ), as determined if neither of these condition is met then the system is regarded as having mixed-influence interactions. Although SAPT is used to classify the dimers, benchmark interaction energies have been determined at the CCSD(T)/CBS level. Figure shows the calculated S66 interaction energies for a wide variety of functionals, plotted alongside the reference values. These DFT results include "baseline" functionals that lack any dispersion correction and thus fail to bind most of the dispersion-dominated dimers. Even for the hydrogen-bonded subset of S66, however, these baseline functionals systematically underestimate the interaction energies, by 1 kcal/mol in many cases. (Errors are considerably larger for the other subsets of S66.) In contrast, results obtained using dispersion-inclusive functionals track the reference values exceedingly well, to the point that we have not attempted to break out the performance of individual functionals in Fig. .