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
J. Chem. Phys.
(2011), 134, pp. 094118. , 445 Phys. Chem. Chem. Phys.
(2012), 14, pp. 7679. 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, 612 J. Phys. Chem. Lett.
(2012), 3, pp. 3241. , 613 J. Chem. Phys.
(2013), 139, pp. 034107. , 615 J. Phys. Chem. A
(2015), 119, pp. 235. , 617 J. Chem. Theory Comput.
(2018), 14, pp. 2955. , 618 J. Chem. Theory Comput.
(2018), 14, pp. 5128. , 159 J. Phys. Chem. Lett.
(2019), 10, pp. 2706. , 685 J. Chem. Phys.
(2019), 151, pp. 031102. 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. 615.