SCM: ADF User's Guide

General FDE options

In addition to the fragment-specific options, there are also a number of options available in FDE calculations that will be described in the following.

FDE
  {kinetic energy functional}
  {CJCORR [rho_cutoff]}
  {GGAPOTXFD exchange functional}
  {GGAPOTCFD correlation functional}
  {FULLGRID}
  {RELAXCYCLES n}
  {RELAXPOSTSCF}
end

kinetic energy functional

There are several approximate kinetic energy functionals available, that can be used for the nonadditive kinetic energy in the effective embedding potential. If no kinetic energy functional is specified, by default the local-density approximation (Thomas-Fermi functional) is used. For an assessment of functionals for weakly interacting systems see Ref. [188]. Based on this study, the use of PW91k is recommended.

THOMASFERMI (default)

Thomas-Fermi LDA functional [194,195]

WEIZ

gradient-dependent von Weizsäcker functional [196] (no LDA part included)

TF9W

Thomas-Fermi functional + 1/9 von Weizsäcker functional

PW91k (recommended functional)

GGA functional based on PW91 exchange functional, reparametrized for the kinetic energy by Lembarki and Chermette [197]

LLP91

GGA functional based on Becke88 exchange functional, reparametrized for the kinetic energy by Lee, Lee, and Parr [198]

PW86k

GGA functional based on PW86 exchange functional [199]

OL91A, OL91B

gradient-dependent functionals OL1 and OL2 by Ou-Yang and Levy [200]

THAKKAR92

gradient-dependent functional by Thakkar [201]

COULOMB

This option does not stand for a kinetic energy functional, but it disables the nonadditive kinetic energy part and the exchange-correlation part in the embedding potential. The remaining embedding potential will only contain the Coulomb interaction with the frozen density. Note that the use of this option is not recommended, it is useful for analysis purposes only.

CJCORR

Option to switch on the long-distance correction. By default this option is not used. As was shown in Ref. [220], with the available approximate kinetic-energy functionals, the embedding potential has the wrong form in the limit of a large separation of the subsystems. In particular, it was shown that this can have serious consequences in the case of FDE(s) calculations (USEBASIS option). In Ref. [220], a correction is proposed that enforces the correct long-distance limit. (See also this reference for limitations of this correction.)

CJCORR [rho_cutoff]

This option switches on the long-distance correction. This option has to be used in combination with one of the above kinetic-energy functionals. By default, a density cut-off of 0.1 is employed.

GGAPOTXFD
GGAPOTCFD

Option to specify the nonadditive exchange-correlation functional. By default, in the construction of the effective embedding potential the exchange-correlation functional that was specified in the XC block is used. It is possible to specify a different functional with the GGAPOTXFD and GGAPOTCFD options. This is particularly useful in combination with the use of model potentials like SAOP, that can not be used in the embedding potential because of their orbital dependence. (For a discussion, see Ref. [189])

GGAPOTXFD exchange functional

The exchange functional is used in the construction of the embedding potential. The same exchange functionals as in the XC key are available.

GGAPOTCFD correlation functional

The correlation functional is used in the construction of the embedding potential. The same correlation functionals as in the XC key are available.

FULLGRID

By default, FULLGRID is not used, and in FDE calculations the integration grid is generated as described in Ref. [185] by including only atoms of the frozen system that are close to the nonfrozen system in the generation of the integration grid. The distance cutoff used is chosen automatically based on the extend of the basis functions of the nonfrozen system. (It can also be chosen manually, see the option qpnear in the INTEGRATION key) This scheme results in a efficient and accurate integration grid. However, it is possible that the default integration scheme is not accurate enough. This can be the case for weakly interacting systems and when the distance between the frozen and the nonfrozen system is large. It is therefore recommended to check the quality of the default integration grid by comparing to results obtained using the full supermolecular grid (FULLGRID option).

If the subkey FULLGRID is included all atoms of the frozen system are included in the generation of the integration grid. This results in the same grid that would be used in a supermolecular calculation of the combined frozen and nonfrozen system. The integration grid generated by this option might be much larger than the default grid. This option should be used to check the quality of the default integration grid.

RELAXCYCLES n

Specifies the maximum number n of freeze-and-thaw iterations that are performed (for frozen fragments with the RELAX) option. If a smaller number of iterations is specified as a fragment-specific option, for this fragment this smaller number is used. Furthermore, if convergence is reached earlier, no more iterations will be performed.

RELAXPOSTSCF

If this option is included, several post-SCF properties will be calculated after each freeze-and-thaw cycle. These are otherwise only calculated in the last cycle.

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