As suggested in Eq. ( ), induction may be described using various levels of sophistication; these are listed in order of increasing accuracy and cost. By using SAPT0 energy components, electron correlation is excluded in the monomer calculations. This may be undesirable, especially for strong hydrogen bonds where monomer correlation effects can be significant. Within conventional SAPT methodology, this requires terms that are first-and second-order in Ŵ [Eq. ( )], an approach that has been called "SAPT2" and engenders O(N 6 ) scaling, similar to CCSD but noniterative. XSAPT instead uses KS-DFT for the monomers, as a lowcost means to incorporate monomer electron correlation effects. Importantly, the XC functional must have correct asymptotic behavior. This is accomplished using long-range corrected (LRC) density functionals , as discussed in Section 1.2.2. An additional consideration is how to represent the monomer wavefunctions in a system that may contain more than two monomers A, B, C, . . .. Our choice is to represent the SCF wavefunction on A using strictly those atomic orbital basis functions that are centered on A. Elsewhere, this has been called the "absolutely localized" MO (ALMO) basis, which is a starting point for a DFT-based EDA of the same name. This choice is not entirely obvious from the point of view of conventional (dimer) SAPT calculations, where our choice is known as a monomer-centered basis and it is atypical, because more accurate results are obtained using a dimer-centered basis. The latter approach employs the combined basis functions of A and B to compute both |ψ A 0 and |ψ B 0 , which allows for some intermolecular CT. For systems with more than two monomers, however, the appropriate definition for a dimer basis becomes ambiguous. For large collections of monomers, this approach cannot be generalized without prohibitive cost.