XPol calculations are enabled by setting the $rem variable XPOL to TRUE. The XPol method can be used in combination with Hartree-Fock theory and with most density functionals, a notable exception being that XPol is not yet implemented for meta-GGA functionals (Section 5.3). Combination of XPol with solvation models (Section 12.2) or external charges ($external_charges) is also not supported. Analytic gradients are available when Mulliken or Löwdin embedding charges are used, but not yet available for ChElPG embedding charges.
Other XPol options are specified via keywords contained in the $xpol section. These keywords are given below.
Especially in the context of SAPT(KS) calculations (see Section 13.12) and XSAPT(KS) calculations (Section 13.13), in which a Kohn-Sham description of the monomers is combined with symmetry-adapted perturbation theory (SAPT), it is essentially that the Kohn-Sham density functional exhibit correct asymptotic behavior.526, 527 Most standard density functionals do not satisfy this criterion, but it can be imposed by using a range-separated hybrid functional in which the range-separation parameter is tuned so as to satisfy the ionization potential theorem of DFT, namely, . This condition should be enforced separately on each monomer within an XPol calculation, which requires that a different value of be used for each monomer. This functionality is requested by setting the DFT-LRC option in the $xpol section. (Note that no value needs to be set with this keyword; if it is present in the $xpol section then this option is enabled.)
If DFT-LRC is specified, then a $lrc_omega input section is also required. This input section simply consists of the values for each monomer, listed one per line in the order that the monomers appear in the $molecule section. These values have the same units as the $rem variable OMEGA that is used in range-separated hybrid functional calculations, namely, OMEGA/1000 in atomic units. See Section 13.13.2 for an example of how the DFT-LRC option and the $lrc_omega input section are used in the context of the XSAPT(KS) method.