Looking back at the errors in total interaction energies (Fig. ), we see that none of the double-ζ basis sets achieves a MAE below 1 kcal/mol. Based on the analysis presented in this section, we ascribe this failure primarily to inadequate description of E elst and E exch , since E ind and E disp are reasonably well described in double-ζ basis sets so long as diffuse functions are included. The def2-TZVP basis set affords a mean accuracy < 1 kcal/mol despite the absence of diffuse functions, without benefiting significantly from error cancellation. With minimal augmentation (def2-ma-TZVP), the MAE drops below 0.5 kcal/mol and the average deviation in each energy component is < 3% of the def2-QZVPD value. The def2ma-TZVP basis set therefore appears to afford converged XSAPT + MBD results, at least for the S66 dimers.