## 17.5 Empirical damped dispersion correction

Empirical damped dispersion corrections can be calculated in addition to Kohn-Sham calculations. This is particularly important in cases where long-range 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.
Cartesian gradients are computed. (note that geometry optimisations with DFT+dispcorr3 are currently not yet possible).
TZ
Use special parameters for calculations with triple-zeta 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.

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 DFT-D1 [2] or DFT-D2 [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 DFT-D1 and DFT-D2 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:
 (33) (34)

The function damps the dispersion correction for shorter interatomic distances and is given by:
 (35)

whith being the van-der-Waals 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).

molpro@molpro.net 2018-11-20