Fragment-specific FDE options

For each frozen fragment, several additional options can be applied. To do this, the fragment specification is used as a subblock key by appending a & sign. The subblock is terminated with SubEnd. This subblock key looks, in the most general form, as follow:

FRAGMENTS
  ...
  FragType FragFile type=FDE &
     {FDEOPTIONS [USEBASIS] [RELAX]}
     {FDEDENSTYPE [SCF | SCFexact | SCFfitted | FullSum ]}
     {RELAXCYCLES n}
     {XC [LDA | GGA ggapotx ggapotc | MODEL SAOP]}
  SubEnd
  ...
END

FDEOPTIONS

FDEOPTIONS USEBASIS

If the USEBASIS option is specified, the basis functions of this frozen fragment will be included in the calculation of the nonfrozen subsystem. This allows it to apply the so-called 'supermolecular basis set expansion', also referred to as FDE(s) calculations, which are often useful for benchmarking.

FDEOPTIONS RELAX

If the RELAX option is specified, the density of this frozen fragment will be relaxed in freeze-and-thaw cycles, i.e., the active subsystem is frozen, while this fragment is thawed. This is repeated, until convergence is reached or until the maximum number of iterations has been performed. By relaxing frozen fragments, it is possible to improve a given approximate environment density by including the polarization of the environment due to the embedded system.

FDEOPTIONS USEBASIS RELAX

It is further possible to combine USEBASIS and RELAX. In this case, the basis functions of the nonfrozen fragment will be included when the density of the fragment is relaxed. This allows fully relaxed FDE(s) calculations.

FDEDENSTYPE

The FDEDENSTYPE option can be used to specify which density is read from the fragment file. The possible options are:

FDEDENSTYPE SCF (or FDEDENSTYPE SCFexact)

The exact density (not calculated using the fit functions) is used. This is the default.

FDEDENSTYPE FullSum

The superposition of the densities of the initial fragments is used. This option is only included for compatibility with the DENSPREP option included in the previous version of ADF.

RELAXCYCLES n

This gives the maximum number of freeze-and-thaw cycles that are performed for this fragment. If the maximum number given in the FDE block is smaller, or if convergence is reached earlier, then less cycles are performed.

XC

The XC option can be used to select the exchange-correlation potential that is used for this fragment when it is relaxed. By default, the same potential as for the nonfrozen system is used, but in some cases it might be preferable to use another approximation for certain fragments.

XC LDA

This option selects LDA as exchange-correlation potential for relaxing this fragment.

XC GGA ggapotx ggapotc

This selects a GGA potential for relaxing this fragment. The GGA potential is specified by giving the name of the exchange potential, followed by the name of the correlation potential. The available potentials are listed in the documentation for the XC key.

XC MODEL SAOP

This selects the model potential SAOP for relaxing this fragment.