13.11 The Explicit Polarization (XPol) Method

13.11.3 Job Control Variables for XPol

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.

XPOL
       Perform a self-consistent XPol calculation.
TYPE:
       BOOLEAN
DEFAULT:
       FALSE
OPTIONS:
       TRUE Perform an XPol calculation. FALSE Do not perform an XPol calculation.
RECOMMENDATION:
       NONE

Other XPol options are specified via keywords contained in the $xpol section. These keywords are given below.

embed
       Specifies which type of electrostatic embedding will be used.
INPUT SECTION: $xpol
TYPE:
       STRING
DEFAULT:
       Charges
OPTIONS:
       None No embedding charges. Charges Atomic point charges (standard XPol method). Density Fragment densities (as in the FMO method; see Sec. 13.15)
RECOMMENDATION:
       The standard XPol method uses atomic point charges.

charges
       Specifies which type of atomic point charges to use.
INPUT SECTION: $xpol
TYPE:
       STRING
DEFAULT:
       Lowdin
OPTIONS:
       Mulliken Mulliken charges Lowdin Löwdin charges CHELPG ChElPG charges
RECOMMENDATION:
       Problems with Mulliken charges in extended basis sets can lead to XPol convergence failure. Löwdin charges tend to be somewhat more stable, while ChElPG charges are quite robust and provide an accurate electrostatic embedding. However, ChElPG charges are more expensive to compute, and analytic energy gradients are not yet available for this choice. For single-point calculations, ChElPG charges are recommended.

print
       Specifies the level of output for the XPol code.
INPUT SECTION: $xpol
TYPE:
       INTEGER
DEFAULT:
       1
OPTIONS:
       n Desired print level
RECOMMENDATION:
       Higher values print additional information

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 ω is tuned so as to satisfy the ionization potential theorem of DFT, namely, IE(ω)=-ϵHOMO(ω). This condition should be enforced separately on each monomer within an XPol calculation, which requires that a different value of ω 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.)

DFT-LRC
       Specifies whether monomer-specific range-separated hybrid functionals are to be used
INPUT SECTION: $xpol
TYPE:
       None
DEFAULT:
       Not specified
OPTIONS:
       If the keyword is present, this option is enabled.
RECOMMENDATION:
       Placing this keyword into the $xpol section indicates that monomer-specific values of ω (the range-separation parameter) are to be used, which then requires a $lrc_omega input section.

If DFT-LRC is specified, then a $lrc_omega input section is also required. This input section simply consists of the values ω1,ω2, 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, ω= 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.