The most expensive terms in the SAPT0 energy expression are E (20) disp and E (20) exch-disp , which scale as O(N 4 ) and O(N 5 ), respectively. Other terms including δE HF scale no worse than O(N 3 ), which is the same asymptotic scaling as DFT. Thus, what makes SAPT0 expensive is the same second-order dispersion that precludes it from being a benchmark-quality method. Recognizing this, we have developed hybrid SAPT-based models in which E (2) disp is replaced by a more accurate and less costly dispersion model. Several such models have been explored, leading to a family of methods that we call "extended" (X)SAPT. In early work, the same approach as called "XPol + SAPT", for reasons that will become clear in Section 1.2.3 when we discuss XPol-based charge embedding.