FDE Input

To invoke a frozen-density embedding calculation, two additional specifications in the input are required. First, one or more frozen fragments have to be included in the FRAGMENTS block, and second, the block key FDE has to be included. In the simplest case, this input should look like this:

FRAGMENTS
  ...
  FragType FragFile type=FDE
  ...
END

FDE
  PW91K
end

In the FRAGMENTS block, for any fragment it is possible to specify the option type=FDE to indicate that the density of this fragment is kept frozen. This density is imported from the file FragFile. The frozen fragments have to be included in addition to the usual, nonfrozen fragments. The atoms of the frozen fragments have to be included in the ATOMS block. As with normal fragments, the fragment found in the file will be rotated and translated to its position specified in the ATOMS block. For more details on specifying fragments, see the section 'fragment files'. In the FDE input block, the recommended PW91k functional is chosen for the nonadditive kinetic energy. For all other options the defaults will be used.

By including more than one frozen fragment, it is possible to use a frozen fragment that is a superposition of the densities of isolated molecules (this was possible in the previous version of ADF using the DENSPREP option). For a discussion and tests of the use of such approximate environment densities, see Ref. [185].

There is no restriction on the use of symmetry in FDE calculations, and usually the correct symmetry will be detected automatically. However, in the preparation of frozen fragments that will be rotated and/or translated in the FDE calculation, for technical reasons one has to include the keyword NOSYMFIT.

In the current implementation, only the electron density of the embedded (nonfrozen) system is calculated. Therefore, only properties that depend directly on the electron density (e.g., dipole moments) are available. In particular, the calculation of interaction energies or of energy gradients is not implemented yet. All quantities given in the output refer (unless explicitly specified otherwise) to the nonfrozen system only.

The TDDFT extension of the FDE formalism allows the calculation of electronic excitation energies and polarizabilities. This extension is automatically activated if FDE is used in combination with the EXCITATIONS or the RESPONSE key.

To employ the extension of FDE to the calculation of NMR shieldings, the file TAPE10 has to be used in the FDE calculation (by including the option SAVE TAPE10), and subsequently the NMR shielding has to be calculated using the program NMR (not with EPR).