XSAPT was originally intended as a method for molecular clusters and liquids, in which many-body polarization effects would be treated self-consistently (via the XPol procedure) while resorting to a pairwise approximation for other energy components, i.e., XPol + SAPT. That design reflects the reality that nonadditive effects are generally small, except for polarization. 109 For example, three-body (trimer) dispersion effects contribute no more than 5-6% (and typically < 2%) of the total interaction energy in small clusters composed of small molecules. In crystalline benzene, three-body effects contribute 0.8-0.9 kcal/mol to the lattice energy (about 7%), with three-body dispersion as the largest contribution. 114 In contrast, three-body polarization ranges from -9 to -13 kcal/mol in the stable structures of (H 2 O) 6 . In water trimers extracted from a liquid simulation, the total three-body energies range from -6 kcal/mol to +1 kcal/mol. These effects are not negligible and to describe them at a cost that remains tractable for sizable clusters we use the XPol method of Gao and co-workers. XPol starts from single-monomer SCF calculations, following which a set of atomic point charges is computed from those SCF wavefunctions and subsequent monomer SCF calculations are performed in the presence of these embedding charges. This entire procedure is then iterated to self-consistency, with an outer loop over monomers and an inner loop consisting of SCF iterations on a given monomer, in the presence of point charges representing the other N -1 monomers.