13 Fragment-Based Methods

13.13 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 Herbert,414, 369 and later significantly extended by Lao and Herbert,524, 525, 527, 529 as a low-scaling, systematically-improvable method for intermolecular interactions that could be applicable to large systems. The original 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. An overview of XSAPT-based methods can be found in Ref. 527.