(This is also a simple way to understand that dispersion is entirely an electron correlation effect, absent at the mean-field HF level.) For this reason, SAPT0 + δHF and similar methods have sometimes been called "HF plus dispersion". We avoid that terminology for SAPT0 + δHF, however, because it is easily confused with very old approaches that augment a HF calculation with an empirical dispersion correction (e.g., of the form -C 6 /R 6 ), analogous to the manner in which contemporary DFT is typically augmented to obtain dispersion-corrected DFT + D methods. Along those lines, let us pause to discuss terminology. We will sometimes refer to the conventional, HFbased SAPT0 model as "SAPT0(HF)", in order to distinguish it from an analogous method that uses Kohn-Sham (KS) DFT for the monomers. We define SAPT0(KS) as second-order intermolecular perturbation theory based on a generalized description of the monomer wavefunctions, meaning Eq. ( ) with all SAPT terms evaluated using KS orbitals and one-particle energy levels. (There are important caveats concerning which XC functionals should be used, which are discussed in Section 1.2.2.) Ultimately, we want to eliminate E (2) disp in favor of dispersion models that are both more accurate and less expensive (Section 1.2.4). Since SAPT0 implies second-order dispersion, we will refer to these hybrid models as "SAPT(KS)", without the zero. Explicitly, E