4.2.4. Summary. Several of the best-performing basis sets from each of the categories discussed above are compared in Fig. . It is clear that even the bestperforming Pople basis sets are outperformed by both Dunning and Karlsruhe basis sets, with absolute errors that are larger by up to 2 kcal/mol. All of the largest discrepancies are found amongst the hydrogen-bonded subset of S66, hence the problem is likely the inadequate description of induction energies. In contrast, the best of the Dunning and Karlsruhe basis sets are nearly identical in their performance, with MAEs < 0.5 kcal/mol. The Karlsruhe basis sets achieve this level of accuracy with fewer basis functions and for that reason we will focus on the Karlsruhe basis sets in much of the rest of this work.