whose terms are explicated elsewhere. Superscripts n and m in Eq. ( ) indicate orders in perturbation theory, for either the intermolecular perturbation ( VAB ÂAB ) or for monomer electron correlation ( Ŵ ), respectively. The terms E (nm) pol constitute the polarization expansion (valid at long range) but each is accompanied by an exchange contribution E (nm) exch , arising from the antisymmetrizer. The simplest form of SAPT sets m = 0 and thus does not include monomer electron correlation. Further limiting the expansion to n ≤ 2 affords the simplest form of SAPT that includes all of the energy components identified in Eq. (1), albeit in different guises. Somewhat confusingly, this second-order (in VAB ÂAB ) approach is often called "SAPT0", because it is zeroth-order in Ŵ . It is akin to a Hartree-Fock (HF) description of the monomers combined with dispersion resembling that of second-order Møller-Plesset perturbation theory (MP2). Regrouping the relevant terms in the polarization expansion, this approximation for E int is E SAPT0 int = E (10) elst + E (10) exch + E (20) disp + E (20) exch-disp