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12.10 The XPol+SAPT (XSAPT) Method

12.10.1 Introduction

(May 21, 2025)

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, 602 Jacobson L. D., Herbert J. M.
J. Chem. Phys.
(2011), 134, pp. 094118.
Link
, 539 Herbert J. M. et al.
Phys. Chem. Chem. Phys.
(2012), 14, pp. 7679.
Link
and later by Lao and Herbert, 737 Lao K. U., Herbert J. M.
J. Phys. Chem. Lett.
(2012), 3, pp. 3241.
Link
, 738 Lao K. U., Herbert J. M.
J. Chem. Phys.
(2013), 139, pp. 034107.
Link
, 740 Lao K. U., Herbert J. M.
J. Phys. Chem. A
(2015), 119, pp. 235.
Link
, 742 Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 2955.
Link
, 743 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, 737 Lao K. U., Herbert J. M.
J. Phys. Chem. Lett.
(2012), 3, pp. 3241.
Link
, 738 Lao K. U., Herbert J. M.
J. Chem. Phys.
(2013), 139, pp. 034107.
Link
, 740 Lao K. U., Herbert J. M.
J. Phys. Chem. A
(2015), 119, pp. 235.
Link
, 742 Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 2955.
Link
, 743 Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 5128.
Link
, 193 Carter-Fenk K. et al.
J. Phys. Chem. Lett.
(2019), 10, pp. 2706.
Link
, 817 Liu K.-Y., Carter-Fenk K., Herbert J. M.
J. Chem. Phys.
(2019), 151, pp. 031102.
Link
, 460 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, 192 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.  192 Carter-Fenk K., Lao K. U., Herbert J. M.
Acc. Chem. Res.
(2021), 54, pp. 3679.
Link
and a comprehensive review in Ref.  . In particular, the XSAPT+MBD method 193 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. 192 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". 549 Herbert J. M.
J. Phys. Chem. A
(2021), 125, pp. 7125.
Link