XPol calculations are enabled by setting the $rem variable XPOL to TRUE. The XPol method can be used in combination with Hartree-Fock theory and with most density functionals, a notable exception being that XPol is not yet implemented for meta-GGA functionals (Section 5.3). Combination of XPol with solvation models (Section 11.2) or external charges ($external_charges) is also not supported. Analytic gradients are available when Mulliken, Löwdin, or CM5 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. 12.16)
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:
CM5
OPTIONS:
Mulliken
Mulliken charges
Lowdin
Löwdin charges
CHELPG
ChElPG charges
CM5
CM5 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 they
perform slightly worse than CM5 charges in systems with charged monomers.
The CM5 charges are a cost-effective and slightly more accurate choice.
For single-point calculations, CM5 charges are recommended.
print
Specifies the level of output for the XPol code.
INPUT SECTION: $xpol
TYPE:
INTEGER
DEFAULT:
1
OPTIONS:
Desired print level
RECOMMENDATION:
Higher values print additional information
Especially in the context of SAPT(KS) calculations (see Section 12.13) and XSAPT(KS) calculations (Section 12.14), 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.538, 539 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.)
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
(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 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 12.14.2 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.