The first column of Table gives the error statistics for the IL16 database of cation-anion pairs, and these errors are also plotted in Fig. for the Dunning and Karlsruhe basis sets. Notably, 6-311++G(3df,2pd) proves to be on par with the other triple-ζ basis sets in systems containing either cations (CHBH) or anions (AHB21) with an MAEs of 2.95 and 1.74 kcal/mol respectively. However, when cations and anions are in the system (IL16), the MAE increases substantially to 8.31 kcal/mol Perhaps serendipitously, the best-performing basis set for IL16 is jun-cc-pVTZ with a MAE of 0.8 kcal/mol although the def2-ha-SVP and def2-TZVPPD basis sets afford comparable MAEs, and def2-TZVPPD affords a smaller maximum error. (The def2-TZVPD basis set is nearly as good as def2-TZVPPD.) In contrast to the behavior for S66, here we see no systematic improvement in accuracy for larger basis sets, although neither do the errors get significantly larger. For example, the XSAPT + ai D3/aug-cc-pVXZ methods with X = D, T, and Q afford MAEs of 1.3, 1.7, and 1.7 kcal/mol, respectively, and maximum errors of 3.5, 3.8, and 3.7 kcal/mol. Similar trends hold for the Karlsruhe sequence of basis sets (def2-SVPD, def2-TZVPD, and def2-QZVPD), although the errors are slightly but systematically smaller in the Karlsruhe basis sets as compared to the Dunning basis sets. As noted in a previous SAPT study, the most challenging systems in the ion-containing data sets that are examined here require a "δMP2" correction for higher-order induction (and not just a δHF correction), in order to achieve sub-kcal/mol accuracy with respect to CCSD(T)/CBS benchmarks. As such, errors of 1.7 kcal/mol (MAE) or 3.7 kcal/mol (maximum) obtained at the XSAPT + ai D3/aug-cc-pVQZ level likely reflect the inherent accuracy of this particular XSAPTbased method as applied to these very challenging ionpair systems. Some error cancelation is then responsible for the somewhat better performance of the smaller basis sets mentioned above.