12 Fragment-Based Methods

12.14 The XPol+SAPT (XSAPT) Method

The “XSAPT” method, which may be regarded either as an acronym for “XPol+SAPT” or for “extended” symmetry adapted perturbation theory (SAPT), was originally introduced by Jacobson and Herbert422, 375 as a low-scaling, systematically-improvable method for intermolecular interactions that could be applicable to large systems. The idea was to replace the need for empirical parameters in the XPol method with on-the-fly evaluation of exchange-repulsion and dispersion interactions via pairwise-additive SAPT. Stated differently, XSAPT uses XPol to evaluate many-body (non-pairwise-additive) polarization effects, but then assumes that dispersion and exchange-repulsion interactions are pairwise additive, and evaluates them via pairwise SAPT0 or SAPT(KS) calculations. The method was significantly extended by Lao, Herbert, and co-workers,536, 537, 539, 541, 542, 127, 596 with various approximations applied in place of the SAPT0 or SAPT(KS) dispersion terms, which are both the least accurate and most expensive contributions to second-order SAPT. An overview of XSAPT-based methods can be found in Ref. 539.