12.10.1 Introduction

The “XSAPT” method, which may be regarded either as an acronym for “XPol+SAPT” or for “extended” symmetry adapted perturbation theory (SAPT),^{199, 557} was originally introduced by Jacobson and Herbert, ⁶²¹ Jacobson L. D., Herbert J. M.
J. Chem. Phys.
(2011), 134, pp. 094118. Link ^{, 560} Herbert J. M. et al.
Phys. Chem. Chem. Phys.
(2012), 14, pp. 7679. Link and later by Lao and Herbert, ⁷⁵⁹ Lao K. U., Herbert J. M.
J. Phys. Chem. Lett.
(2012), 3, pp. 3241. Link ^{, 760} Lao K. U., Herbert J. M.
J. Chem. Phys.
(2013), 139, pp. 034107. Link ^{, 762} Lao K. U., Herbert J. M.
J. Phys. Chem. A
(2015), 119, pp. 235. Link ^{, 764} Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 2955. Link ^{, 765} Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 5128. Link 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 SAPT0(KS) calculations. The method was significantly extended by Lao, Herbert, and co-workers, ⁷⁵⁹ Lao K. U., Herbert J. M.
J. Phys. Chem. Lett.
(2012), 3, pp. 3241. Link ^{, 760} Lao K. U., Herbert J. M.
J. Chem. Phys.
(2013), 139, pp. 034107. Link ^{, 762} Lao K. U., Herbert J. M.
J. Phys. Chem. A
(2015), 119, pp. 235. Link ^{, 764} Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 2955. Link ^{, 765} Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 5128. Link ^{, 200} Carter-Fenk K. et al.
J. Phys. Chem. Lett.
(2019), 10, pp. 2706. Link ^{, 845} Liu K.-Y., Carter-Fenk K., Herbert J. M.
J. Chem. Phys.
(2019), 151, pp. 031102. Link ^{, 480} Gray M., Herbert J. M.
J. Chem. Phys.
(2021), 155, pp. 034103. Link with various approximations applied in place of the SAPT0 or SAPT0(KS) dispersion terms, ¹⁹⁹ Carter-Fenk K., Lao K. U., Herbert J. M.
Acc. Chem. Res.
(2021), 54, pp. 3679. Link which are both the least accurate and most expensive contributions to second-order SAPT. A concise overview of XSAPT-based methods can be found in Ref.  199 Carter-Fenk K., Lao K. U., Herbert J. M.
Acc. Chem. Res.
(2021), 54, pp. 3679. Link and a comprehensive review in Ref. 557. In particular, the XSAPT+MBD method ²⁰⁰ Carter-Fenk K. et al.
J. Phys. Chem. Lett.
(2019), 10, pp. 2706. Link stands out as a way to obtain qualitative insight about noncovalent interactions in large systems, backed by quantitative energetics calculations. ¹⁹⁹ Carter-Fenk K., Lao K. U., Herbert J. M.
Acc. Chem. Res.
(2021), 54, pp. 3679. Link In many cases, this type of analysis has upended textbook “conventional wisdom". ⁵⁷⁰ Herbert J. M.
J. Phys. Chem. A
(2021), 125, pp. 7125. Link