rapidly than either E CCSD(T) or E MP2 . To test this, we repeated the XSAPT + MBD calculations on the S66 dimers, using the def2-QZVPD basis set to evaluate all of the terms in eq. 1 except for δE HF , for which we use a smaller basis set. The smaller basis set is used both for the supramolecular HF calculation (for ∆E HF int ) and to solve the coupled-perturbed equations that are used to obtain E