The XSAPT methodology was originally envisaged as "XPol + SAPT", using a combination of XPol to describe many-body polarization in a cluster or molecular liquid and pairwise-additive SAPT0 for the remaining components of the intermolecular interaction energy. Alternative dispersion models quickly became the focus, and there have been many applications to dimer systems. The methodology is efficient enough that a relatively large ion-water cluster, X -(H 2 O) n , can be handled by treating (H 2 O) n as a single monomer, and results up to X -(H 2 O) 70 are used in Section 2.2.2 to establish how the cost scales with system size. Nevertheless, the methodology is formulated for many-body systems and that formalism is described next. By definition, the XSAPT energy for a collection of N monomers is given by a pairwise sum,