The FDE-Man job control with respect to embedding parameters is accomplished via options in the $fde input section. The format of the $fde section requires key and value pairs separated by a space character:
$fde <Keyword> <parameter> $end
Note: The following job control variables belong only in the $fde section. Do not place them in the $rem section.
The supermolecular expansion (SE) uses the full basis set of the supersystem for calculations on each fragment. Because of the computational cost this option should only be used for small to medium sized supersystems. Note that for visualization of orbitals or densities SE only supports the generation of volumetric data via MAKE_CUBE_FILES (MolDen files are not supported, i.e. MOLDEN_FORMAT should be avoided). On the other hand, the monomer expansion (ME) only uses the basis set of each individual fragment. This choice of basis expansion is recommended for daily basis embedding calculations.
Analogous to a regular DFT calculation in Q-Chem (by using METHOD) the exchange-correlation functional combination can either be selected with one keyword XC_Func,or by defining X_Func and C_Func (similar to EXCHANGE and CORRELATION). To include only the electrostatic terms in the embedding potential one can set all the functional keywords to None.
It is recommended to employ the same level of approximation for the non-additive kinetic and exchange-correlation functional, for instance TF/SVWN or PW91k/PW91.
The minimal specifications required to successfully run a FDET calculation are:
Functionals for embedding potential: T_Func, XC_Func (or X_Func and C_Func),
Basis expansion: Expansion,
Method for environment density: rhoB_method
When the selected environment comprises many atoms and molecules, a sensible approximation is to construct the total environment density as the sum of the densities of the individual molecules. This can be done with the keywords Superposition_B or Import_Superposition_B. In the first case, the molecular densities are computed in the same calculation, and assembled into one single density matrix. The molecular densities and the superposed density matrices are saved into text files.
In the second case, the individual density matrices are expected to be already calculated independently and are required to be present in the same folder of the calculation.
Note: No embedding calculation is performed if these functions are selected. In both cases, only the geometry of fragment B has to be given, and each molecule needs to be specified as independent fragment in the MOLECULE section.
When using the superposition options, the file of the superposed density matrix of the environment (B) is saved with the name Densmat_B.txt, and can be used with the import_rhoB option to subsequently perform an embedding calculation (see Sec. 18.104.22.168 below).
In FDE-Man the user has the option to import the density matrices used to generate the embedding potential. This alternative is particularly useful if the desired method to generate the densities for the embedding potential is not available in the current version, for instance, or when the superposition of densities is used to generate . The density matrices must be given in AO basis in a text file with the following format:
<nspin> <nbas> <nbas> <value> <value> ...
where <nspin> has two possible values (1 or 2) depending on the number of spin densities contained in the file. The first line should also contain the dimensions of the density matrix (<nbas>).
Formally the exactness of FDET equations requires the condition
to be satisfied,
whereby denotes the exact density of the supersystem.
J. Phys. Chem.
(1993), 97, pp. 8050. , 1167 Phys. Rev. A
(2008), 77, pp. 012504. , 792 Phys. Rep.
(2010), 489, pp. 1. This condition can, however, not be assured a priori. In practice, deficiencies with the initial choice of can be cured to some extent by pre-polarizing the environment density. 933 Phys. Chem. Chem. Phys.
(2018), 20, pp. 26053. Pre-polarization schemes available in FDE-Man can be turn on with the prepol keyword. The different pre-polarization options are described below.
Note: All basis used within the FDE-Man module use PURECART = 2111.
If the user desires to only save the embedding potential without performing an embedding calculation, it can be done by setting to TRUE simultaneously the variables debug and Vemb_only. In this case, the individual components of the embedding potential are saved to disk into text files as mentioned above in the debug.