Examining the individual energy components in Fig. , we note that the relatively low-quality 6-31+G(d) and 6-311+G(d,p) basis sets both overestimate E elst (mak- ing it too attractive) but underestimate E ind (making it insufficiently attractive). Partial cancellation of these errors is responsible for the reasonable interaction energies that are obtained in either basis set. Errors in both E elst and E ind are larger for 6-31+G(d), which therefore relies more heavily on error cancellation. In contrast, the 6-311+G(3df,2pd) basis set affords individual energy components that are fully converged with respect to def2-TZVPD values, so the accuracy of this approach does not rely on error cancellation, and in fact the maximum deviation with respect to the def2-TZVPD interaction energies is only 0.4 kcal/mol when this high-quality Pople basis set is used. Interestingly, the exchange energies appear to be converged in any triple-ζ basis set but also in def2-SVPD, although they are somewhat overestimated when 6-31+G(d) is used. Timing data in Table demonstrate that the use of 6-311+G(3df,2pd) affords a 2× speedup relative to def2-TZVPD. Interestingly, the latter actually comprises fewer basis functions. For Cl -(H 2 O) 28 , which is the average size of the clusters examined in this section, the use of def2-TZVPD involves 1,670 basis functions versus 1,923 for 6-311+G(3df,2pd). The smaller timings for the larger calculation provides an unambiguous indication of the much better efficiency of Pople basis sets, provided that the integrals engine can exploit sp shells.