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 follows:
FRAGMENTS
...
FragType FragFile type=FDE &
{FDEOPTIONS [USEBASIS] [RELAX or FREEZEANDTHAW]}
{FDEDENSTYPE [SCF | SCFexact | SCFfitted ]}
{RELAXCYCLES n or FREEZEANDTHAWCYCLES 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 embedded subsystem. This allows to expand the density of the embedded subsystem using not only atom-centered basis sets localized in the embedded subsystem but also the ones in the environment Ref. [238]. In large-scale simulations using the embedding potential, this option is recommended to be used in the preparation stage to investigate the basis set dependence of the results (chapter 5.3 in Ref. [205]). This option is also an indispensable element in the procedure introduced in Ref. [238] to test approximants to the kinetic-energy component of the embedding potential introduced by Wesolowski and Warshel.
FDEOPTIONS RELAX or FREEZEANDTHAW
If the RELAX option (or equivalent FREEZEANDTHAW option) is specified, the density of this frozen fragment will be relaxed
in freeze-and-thaw cycles [Ref. 240], i.e., the embedded 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.
This option is recommended to be used in the preparation stage of a large-scale numerical simulation.
The freeze-and-thaw calculations lead to a pair of electron densities (embedded system and environment) that minimizes the total energy.
As a consequence, the electron density of the environment derived from the freeze-and-thaw calculations
can be used as a reference to verify the adequacy of the assumed electron density for the environment
in a large-scale simulation.
FDEOPTIONS USEBASIS RELAX or FDEOPTIONS USEBASIS FREEZEANDTHAW
It is further possible to combine USEBASIS and RELAX or FREEZEANDTHAW. 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 calculations with supermolecular expansion of the electron density of each subsystem. This option is to be used to test approximants to the kinetic-energy component of the embedding potential introduced by Wesolowski and Warshel by means of the procedure introduced in [Ref. 238].
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 SCFfitted
The fitted density is used. This is less accurate but can be significantly faster.
RELAXCYCLES n or FREEZEANDTHAWCYCLES 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 fewer cycles are performed. For historical reasons, two equivalent keywords are available.
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. An example is given in Ref. [189].
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.
