Considering the electrostatic energies (Fig. ), the def2-QZVPD basis set affords an S66-averaged value E elst = -6.7 kcal/mol and all of the basis sets except def2-SVP come within ±0.2 kcal/mol of this value, including the rather compact def2-ma-SVP basis set. In contrast to this, when the cardinality of the basis set is increased to triple-or quadruple-ζ level, augmentation does not appear to be necessary in order to obtain a converged result for electrostatics; the def2-TZVP and def2-QZVP values of E elst are both within 0.05 kcal/mol of the def2-QZVPD value. S66-averaged exchange energies are given in Fig. and these prove to be more sensitive to the choice of basis set. The def2-SVP basis is clearly inadequate and differs by about 4 kcal/mol (or 44%) from the def2-QZVPD value of E exch . This should be contrasted with the behavior of E elst , where the entire collection of Karlsruhe basis sets spans a range of less than 1.5 kcal/mol. Also in contrast to the electrostatics case, for exchange it does not seem to be possible to obtain a converged result simply by adding diffuse functions to a double-ζ basis set. However, a triple-ζ basis (with or without diffuse functions) does appear to be adequate: the def2-TZVP result differs by just 0.6 kcal/mol (8%) from the def2-QZVPD result and the def2-TZVPD value differs by just 0.1 kcal/mol (1%). What is similar to the case of electrostatics, and perhaps surprising, is that triple-and quadruple-ζ calculations of the exchange energy do not appear to benefit at all from diffuse basis functions. This is reflected in the overall error statistics in the interaction energies (Fig. ).