The issue for XSAPT + ai D in these large complexes is dispersion nonadditivity arising from the presence of polarizable centers that screen the C 6,ab (and higher-order) dispersion coefficients within a molecule, relative to what would be predicted based on isolated-atom C 6 coefficients, using the combination rule in Eq. ( ). Dobson has called this "type-B" dispersion nonadditivity 134 and to account for it, Grimme's D3 and D4 corrections incorporate three-body (triatomic) dispersion corrections of the Axilrod-Teller-Muto (ATM) "triple-dipole" form. This is a different usage of "many-body" as compared to that discussion in Section 1.2.3, where the "bodies" were monomers. Here they are atoms, reflecting the atom-atom pairwise starting point in Eq. (25).