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17.5 Empirical damped dispersion correction
Empirical damped dispersion corrections can be calculated in addition
to KohnSham calculations. This is particularly important in cases
where longrange correlation effects become dominant.
The dispersion correction can be added to the DFT energy by using
ks,<func>; disp
The total energy will then be calculated as

(31) 
Currently the default dispersion correction added to the DFT energy
is the D3 dispersion correction developed by Grimme et al.,
see Ref. [1]. The disp
keyword can have the following
additional options:
 FUNC
 Functional name (default:
FUNC='pbe'
).
 VERSION
 Can have values 2 and 3 according to parametrisations
from Refs. [2] and [3] (default:
VERSION=3
)
 ANAL
 Performs a detailed analysis of pair contributions.
 GRAD
 Cartesian gradients are computed. (note that
geometry optimisations with
DFT+dispcorr3
are currently not yet possible).
 TZ
 Use special parameters for calculations with
triplezeta basis sets. Preliminary results in the SI of
Ref. [3] indicate that results are slightly worse than with
the default parameters and QZVP type basis sets. This option
should be carfully tested for future use in very large
computations.
(see also http://toc.unimuenster.de/DFTD3/index.html
for further
documentation).
Alternatively, the D3 dispersion correction can also be calculated
separately from the DFT calculation using the following template:
ks,<func>
eks=energy
dispcorr3
eks_plus_disp=eks+edisp
The older DFTD1 [2] or DFTD2 [3] methods by Grimme can still be used
invoking
ks,<func>
eks=energy
dispcorr
eks_plus_disp=eks+edisp
with the following options to dispcorr
:
 MODE
 Adjusts the parametrisation used:
MODE=1
uses parameters from Ref. [1] and MODE=2
uses parameters from Ref. [2] (default: MODE=1
)
 SCALE
 Overall scaling parameter (see Eq. (34)
and Refs. [2,3] for optimised values).
 ALPHA
 Damping function parameter (see Eq. (37)).
Smaller values lead to larger corrections for intermediate
distances.
In the DFTD1 and DFTD2 method the dispersion energy is calculated
as

(32) 
where
is the total number of
atoms, is the interatomic distance of atoms and ,
is a global scaling parameter depending on the choice of the
functional used and the values are calculated
from atomic dispersion coefficients and in
the following way:
The function
damps the dispersion correction for
shorter interatomic distances and is given by:

(35) 
whith
being the vanderWaals radius for atom
and is a parameter that is usually set to 23 (Ref. [1])
or 20 (Ref. [2]).
References:
S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys. 132, 154104 (2010)
S. Grimme, J. Comp. Chem. 25, 1463 (2004).
S. Grimme, J. Comp. Chem. 27, 1787 (2006).
Next: 17.6 Timedependent density functional
Up: 17 THE DENSITY FUNCTIONAL
Previous: 17.4.4 Implementing new hybridfunctionals
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