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12.14 The XPol+SAPT (XSAPT) Method

12.14.1 Introduction

(December 20, 2021)

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 511 Jacobson L. D., Herbert J. M.
J. Chem. Phys.
(2011), 134, pp. 094118.
Link
, 456 Herbert J. M. et al.
Phys. Chem. Chem. Phys.
(2012), 14, pp. 7679.
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, 630 Lao K. U., Herbert J. M.
J. Phys. Chem. Lett.
(2012), 3, pp. 3241.
Link
, 631 Lao K. U., Herbert J. M.
J. Chem. Phys.
(2013), 139, pp. 034107.
Link
, 633 Lao K. U., Herbert J. M.
J. Phys. Chem. A
(2015), 119, pp. 235.
Link
, 635 Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 2955.
Link
, 636 Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 5128.
Link
, 162 Carter-Fenk K. et al.
J. Phys. Chem. Lett.
(2019), 10, pp. 2706.
Link
, 705 Liu K.-Y., Carter-Fenk K., Herbert J. M.
J. Chem. Phys.
(2019), 151, pp. 031102.
Link
, 390 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, 161 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. Overviews of of XSAPT-based methods can be found in Refs.  633 Lao K. U., Herbert J. M.
J. Phys. Chem. A
(2015), 119, pp. 235.
Link
and 161 Carter-Fenk K., Lao K. U., Herbert J. M.
Acc. Chem. Res.
(2021), 54, pp. 3679.
Link
and implementation details can be found in Refs.  456 Herbert J. M. et al.
Phys. Chem. Chem. Phys.
(2012), 14, pp. 7679.
Link
, 635 Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 2955.
Link
, and 705 Liu K.-Y., Carter-Fenk K., Herbert J. M.
J. Chem. Phys.
(2019), 151, pp. 031102.
Link
. In particular, the XSAPT+MBD method 162 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. 161 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", as reviewed in Ref.  462 Herbert J. M.
J. Phys. Chem. A
(2021), 125, pp. 7125.
Link
.