4.1. Tests of Traditional SAPT0 Using S66. The basis-set convergence of traditional SAPT methods, including SAPT0 but also higher-order methods, has been reported previously for several small-molecule data sets. Nevertheless we include our own SAPT0 convergence tests here, because they establish a baseline to which we can later compare the XSAPT methods, whose alternative descriptions of dispersion lead to accelerated basis-set convergence. Table provides the mean absolute errors (MAEs) for the S66 database at SAPT0/calcc-pVXZ levels of theory, where X = D, T, or Q and cal = jun, jul, or aug. As previously reported, the jun-cc- SAPT0 errors increase both with increasing cardinality of the basis set (double-, triple-, or quadruple-ζ) and with increasing augmentation. The reason for this behavior is that the MP2-like second-order approximation that is used in SAPT0 tends to overestimate dispersion significantly, yet dispersion in particular converges very slowly to the CBS limit. More complete basis sets therefore afford increasingly poor dispersion energies and the use of jun-cc-pVDZ represents something of a "Pauling point", balancing slow convergence against overestimation of the result. It is a remarkably robust compromise in small molecules, although it may fare worse in larger systems. For example, in the L7 set of large dispersion-bound complexes, the MAE for SAPT0/juncc-pVDZ interaction energies is 4.8 kcal/mol, and the maximum error is 10.3 kcal/mol. SAPT0 dispersion energies for all of the S66 dimers are plotted in Fig. using the jun-cc-pVXZ basis sets for X = D, T, and Q. These data make it clear that the triple-ζ dispersion energies are systematically much larger than double-ζ values, and quadruple-ζ dispersion energies are a bit larger still. As will become clear from the XSAPT data that are presented below, energy components other than dispersion are essentially converged at the triple-ζ level.