(2) exch-ind,resp in eq. 4. Table shows the error statistics for this hybrid scheme, averaged over the S66 data set and using various choices for the smaller (δE HF ) basis set, and Fig. shows the individual errors across S66 for a selection of basis sets. Even when δE HF is evaluated with a basis set as small as 6-31G*, the errors (with respect to the full def2-QZVPD result) are uniformly < 1 kcal/mol. Average errors with respect to the CCSD(T)/CBS benchmarks, using def2-QZVPD for all the terms except δE HF , are all ≈ 0.4 kcal/mol (see Table ), using a variety of small basis sets to evaluate δE HF . Timing data for these hybrid calculations are shown in Fig. using the largest of the S66 systems, uracil dimer. These data demonstrate the dramatic speedups that are afforded by this simple hybrid procedure. If δE HF is evaluated using def2-ma-SVPD or 6-31+G(d), with all other parts of the calculation performed using def2-QZVPD, then the total cost of the calculation is reduced by 3.8× when either 6-31+G(d) or def2-ma-SVPD is substituted for def2-QZVPD to evaluate δE HF . This substitution introduces an error of < 0.1 kcal/mol. The speedup can be expected to improve as system size increases, since the cost of the δE HF correction scales with dimer size whereas other parts of the XSAPT procedure scale only with monomer size, meaning that the δE HF calculation will constitute an increasing fraction of the total cost as the system grows larger. This will be evident in the timing data presented below for a much larger supramolec- and a DNA-ellipticine intercalation complex 100 that has become something of a standard test system in noncovalent quantum chemistry. The latter system contains 157 atoms including the ligand, two hydrogenbonded base pairs, and the sugar/phosphate backbone.