# Vdw

This page gives hints on how to use Van der Waals functionals with the ABINIT package.

## Introduction¶

It is well known that long range correlations responsible of van der Waals
interactions are out of reach for both LDA and GGA approximations to the
exchange-correlation energy in DFT. In recent years several methods have been
devised to include such interactions, which can be grouped into two
strategies, namely *ad hoc* methods and self-consistent approaches. Currently
ABINIT can perform calculations based on either the DFT-D methods or the vdW-
WF methods, as described later, both belonging to the first group.

A fully customizable implementation of the vdW-DF method [Dion2004], a
self-consistent approach, and an adaptation of the strategy followed by
G.Roman-Perez *et al.* [Romanperez2009] to the case of ABINIT are under
development. It will offer around 25 ajustable parameters and be delivered
with graphical tools to help users assess the quality of their kernels. It
does not only aim at performing production calculations with vdW-DF, but also
at helping researchers who develop new density functionals optimised for
systems requiring van-der-Waals interactions.

The DFT-D methods have been implemented inside ABINIT, namely DFT-D2 [Grimme2006], DFT-D3 [Grimme2010] and DFT-D3(BJ) [Grimme2011]. In these cases, pair-wise terms (and 3-body corrections for DFT-D3 and DFT-D3(BJ)) are added to the DFT energy, which are independent of the electronic density, in order to mimic the vdW interactions. The implementation includes the contributions of these methods to forces and stresses, in view of geometry optimization, as well as to first-order response functions like dynamical matrices, clamped elastic constants and internal strain coupling parameters.

To activate DFT-D dispersion correction, two keywords are in use: vdw_xc = ⅚/7 to choose between DFT-D2, DFT-D3 and DFT-D3(BJ), and vdw_tol, to control the inclusion of largely distant pairs (those giving a contribution below vdw_tol are ignored). It is also possible to include 3-body corrections [Grimme2010] (for ground-state only) with the keyword vdw_tol_3bt, which also controls the tolerance over this term.

Methods based on maximally localized Wannier functions (MLWFs) to calculate vdW energy corrections have also been implemented in ABINIT. In this case the pair-wise terms come from contributions of pairs of MLWFs rather than from atoms. Among the implemented methods in ABINIT it is found vdW-WF1 [Silvestrelli2008], [Silvestrelli2009] vdW-WF2 [Ambrosetti2012] and vdW-QHO-WF [Silvestrelli2013]. A full description of the implementation of vdW-WF1 is reported in [Espejo2012].

Selection of one of these 3 methods is achieved by using vdw_xc=10/11/14 respectivelly. Since vdW-WF1 and vdW-WF2 methods are approximations for the dispersion energy of non overlapping electronic densities, it is necessary to define the interacting fragments of the system whose dispersion energy is going to be calculated. The latter is achieved by using the input variables vdw_nfrag and vdw_typfrag to define the number of interacting fragments in the unit cell and to assign each atom to a fragment. A given MLWF belongs to the same fragment as its closer atom. The need for defining the interacting fragments is overridden in the vdW-QHO-WF, for which these input variables are not used. When dealing with periodic systems the input variable vdw_supercell controls the number of neighbor unit cells that will be included in the calculation. Each one of the 3 components of vdw_supercell indicates the maximum number of cells along both positive or negative directions of the corresponding primitive vector. This is useful for studying the spacial convergency of the vdW energy. It should be noticed that the user must set the variables associated to the calculation of MLWFs and that the resulting vdW energies strongly depend on the obtained Wannier functions.

## Related Input Variables¶

*compulsory:*

*basic:*

- vdw_nfrag Van Der Waals Number of interacting FRAGments
- vdw_supercell Van Der Waals correction from Wannier functions in SUPERCELL
- vdw_tol_3bt Van Der Waals TOLerance for 3-Body Term
- vdw_typfrag Van Der Waals TYPe of FRAGment

*useful:*

*expert:*

- vdw_df_acutmin vdW-DF MINimum Angular CUT-off
- vdw_df_aratio vdW-DF Angle RATIO between the highest and lowest angles.
- vdw_df_damax vdW-DF Delta for Angles, MAXimum
- vdw_df_damin vdW-DF Delta for Angles, MINimum
- vdw_df_dcut vdW-DF D-mesh CUT-off
- vdw_df_dratio vdW-DF, between the highest and lowest D, RATIO.
- vdw_df_dsoft vdW-DF Distance for SOFTening.
- vdw_df_gcut vdW-DF G-space CUT-off
- vdw_df_ndpts vdW-DF Number of D-mesh PoinTS
- vdw_df_ngpts vdW-DF Number of G-mesh PoinTS
- vdw_df_nqpts vdW-DF Number of Q-mesh PoinTS
- vdw_df_nrpts vdW-DF Number of R-PoinTS
- vdw_df_nsmooth vdW-DF Number of SMOOTHening iterations
- vdw_df_phisoft vdW-DF PHI value SOFTening.
- vdw_df_qcut vdW-DF Q-mesh CUT-off
- vdw_df_qratio vdW-DF, between highest and lowest Q, RATIO.
- vdw_df_rcut vdW-DF Real-space CUT-off
- vdw_df_rsoft vdW-DF radius SOFTening.
- vdw_df_threshold vdW-DF energy calculation THRESHOLD
- vdw_df_tolerance vdW-DF global TOLERANCE.
- vdw_df_tweaks vdW-DF TWEAKS.
- vdw_df_zab vdW-DF ZAB parameter

## Selected Input Files¶

*v7:*

*vdwxc:*

*wannier90:*