DFTB
ResourcesDir relativepath
{RadialExtrapolation none|linear|improved|original|bezier}
{SCC
{iterations NIter}
{thirdOrder}
{converge charge=QDiff}
End}
{UseSymmetry yes|no}
{Repulsion
forcePolynomial
End}
{Occupation hund|fermi {temperature=FermiTemp}}
End
This mandatory key allows to specify and control different aspects of the DFTB evaluation engine.
ResourcesDir
Allows to specify the path (relative to $ADFRESOURCES/DFTB) of the directory containing DFTB parameter files. Different parameters may be suitable for different DFTB evaluations. It is important to choose the appropriate parameter set for the type of calculation and molecular system under study.
RadialExtrapolation
Advanced control option. Overrides the extrapolation method for Slater-Koster grid values between the end of the tabulated grid and the cutoff distance (value for which atoms are considered too far to interact). Depending on the structure of your Slater Koster tables, a different radial extrapolation method may be needed in order to guarantee correct behavior, in particular for large and periodic systems. Five different extrapolation strategies are available:
none
Performs no extrapolation, the value being forced to zero at distances greater than the grid last position.
linear
performs a linear interpolation between the last point of the grid and the value of zero, at cutoff distance.
improved (default)
Perform a 9th grade polynomial interpolation between 6 points of the grid and three zeros. This interpolation may prove unstable for particular Slater Koster data
original
same as improved, but reproducing behavior of previous reference programs. Should not be used in general.
bezier
Uses a bezier curve passing through the last grid point and the cutoff point, guaranteeing continuity and smoothness. This is the suggested method in case of unexpected behavior.
SCC
The SCC key is optional. Its presence enables the Self Consistent Charge iterative procedure in DFTB. If the key is not present, DFTB will perform a non-Self Consistent Charge evaluation. SCC is not available for periodic evaluations. Additional optional keys can be used to control the SCC procedure
iterations NIter
Allows to specify the maximum number of SCC iterations. Default is 100 iterations, which is a very high number. Most computations will converge in less iterations. Lack of convergence within this limit may be due to use of hund Occupation. fermi occupation may improve convergence. See the Occupation key below.
thirdOrder
When this optional keyword is specified, a third order correction will be applied to the SCC evaluation.
converge charge=QDiff
Specifies the tolerance for convergence on the variation of the atomic charges. The default is 1.0e-8.
UseSymmetry yes|no
Enables or disables the use of symmetry during the evaluation. The key is optional, and in its absence the default is yes.
Repulsion
This key allows to specify some details about the Repulsion contribute evaluation. It accepts only one sub-key "forcePolynomial", which forces the use of polynomials as defined in the Slater Koster parameter files, in place of the Spline description.
Occupation
This optional key allows to specify the fill strategy to use for the orbitals. It can be either "hund", to fill the orbitals according to the hund rule, or "fermi", to perform electronic charge distribution over the orbitals. If "fermi" is specified, a further "temperature" option must be present, specifying the fermi temperature in kelvin. If this key is absent, the default is fermi occupation with a temperature of 5 kelvin.
