Average induction energies are presented in Fig. . Whereas the def2-SVP value of E ind is underestimated by 2.0 kcal/mol (17%), all of the other values lie within 0.2 kcal/mol (8%) of the def2-QZVPD result. Remarkably, a double-ζ basis set with diffuse functions affords values of E ind that are close to def2-QZVPD values, and in a triple-ζ basis set the diffuse functions are not needed to obtain a value that is negligibly different from the def2-QZVPD value. This may suggest a tractable computational scheme for large systems, in which the induction energy is computed in a smaller basis set as compared to electrostatics or exchange. First, however, we will more rigorously examine this conclusion in Section 4 .5, using ion-water clusters that exhibit much larger induction energies as compared to the S66 systems.