We next consider how to compute individual monomer wavefunctions. Although SAPT's double perturbation formalism is flexible enough to include monomer correlation from many-body theory, the simplest method that does so is SAPT2, which exhibits O(N 6 ) cost but does not afford benchmark-quality results for dispersion. As such, we restrict XSAPT to SCF wavefunctions for the monomers. These could be HF wavefunctions, or we could attempt to incorporate monomer electron correlation effects at low cost using KS-DFT, in which case the asymptotic behavior of the XC potential (υ xc ) must be considered carefully. That discussion will occupy most of this section.