For the DFT-based second-order methods, most of the energy components exhibit sub-kcal/mol MAEs, with the lone exception being E ind for the electrostaticallydominated dimers described at the SAPT0(KS) + MBD level, for which the MAE is 1.5 kcal/mol. That is reduced to 0.2 kcal/mol using XSAPT + MBD, demonstrating the importance of charge embedding even in a dimer system, at least where hydrogen bonds are concerned. For the dimers that are not dominated by electrostatics, charge embedding makes little difference and the error statistics for both SAPT0(KS) + MBD and XSAPT + MBD are very similar, for each energy component. Notably, error statistics for the induction energy evaluated using SAPT0(KS) + MBD are more similar to the SAPT0(HF) + MBD errors than they are to the XSAPT + MBD errors. Each of these methods uses the same δE HF correction (evaluated in each case using HF theory and not DFT), which again supports the important role that charge embedding plays even dimers, at least where hydrogen bonds are involved. Swapping DFT for HF orbitals reduces the MAE for the hydrogenbonded subset from 2.0 to 1.5 kcal/mol, then adding charge embedding reduces it even more, to 0.2 kcal/mol. 4.7. Hybrid Basis-Set Calculations. We now consider the possibility of a hybrid method in which different energy components are computed with different basis sets, exploiting the separability of the SAPT decomposition. A simple-to-implement version of such a procedure is to focus on the δE HF correction, which requires a dimer HF calculation. This is the only part of an XSAPT calculation that requires an iterative SCF calculation on something larger than a monomer, and it is usually the computational bottleneck step when large basis sets are employed. That δE HF is defined by difference (eq. 4) also suggests that this term might converge more rapidly than other energy components in eq. 1, similar to the way that a correction δ = E CCSD(T) -E MP2 converges more Benchmark SAPT2+(3) calculations were performed using the aug-cc-pVTZ data set and are taken from Ref. 88, whereas SAPT0(HF) + MBD, SAPT0(KS) + MBD, and XSAPT + MBD calculations were performed using the aug-cc-pVQZ basis set, and the latter two methods are indistinguishable except for E ind in (c). Shaded regions delineate the electrostatics-dominated dimers (1-7), the dispersion-dominated dimers (8-15), and the dimers with mixed-influence interactions (16-22). Note that the energy scale is different in each panel.