mirror of https://github.com/abinit/abinit.git
82 lines
4.2 KiB
Markdown
82 lines
4.2 KiB
Markdown
---
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description: How to use Van der Waals functionals
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authors: YPouillon, BVanTroeye
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---
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<!--- This is the source file for this topics. Can be edited. -->
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This page gives hints on how to use Van der Waals functionals with the ABINIT package.
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## Introduction
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It is well known that long range correlations responsible of van der Waals
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interactions are out of reach for both LDA and GGA approximations to the
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exchange-correlation energy in DFT. In recent years several methods have been
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devised to include such interactions, which can be grouped into two
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strategies, namely _ad hoc_ methods and self-consistent approaches. Currently
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ABINIT can perform calculations based on either the DFT-D methods or the vdW-
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WF methods, as described later, both belonging to the first group.
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A fully customizable implementation of the vdW-DF method [[cite:Dion2004]], a
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self-consistent approach, and an adaptation of the strategy followed by
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G.Roman-Perez _et al._ [[cite:Romanperez2009]] to the case of ABINIT are under
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development. It will offer around 25 ajustable parameters and be delivered
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with graphical tools to help users assess the quality of their kernels. It
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does not only aim at performing production calculations with vdW-DF, but also
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at helping researchers who develop new density functionals optimised for
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systems requiring van-der-Waals interactions.
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The DFT-D methods have been implemented inside ABINIT, namely DFT-D2
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[[cite:Grimme2006]], DFT-D3 [[cite:Grimme2010]] and DFT-D3(BJ)
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[[cite:Grimme2011]]. In these cases, pair-wise terms (and 3-body corrections
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for DFT-D3 and DFT-D3(BJ)) are added to the DFT energy, which are independent
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of the electronic density, in order to mimic the vdW interactions. The
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implementation includes the contributions of these methods to forces and
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stresses, in view of geometry optimization, as well as to first-order response
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functions like dynamical matrices, clamped elastic constants and internal
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strain coupling parameters.
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To activate DFT-D dispersion correction, two keywords are in use: [[vdw_xc]] =
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5/6/7 to choose between DFT-D2, DFT-D3 and DFT-D3(BJ), and [[vdw_tol]], to
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control the inclusion of largely distant pairs (those giving a contribution
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below [[vdw_tol]] are ignored). It is also possible to include 3-body
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corrections [[cite:Grimme2010]] (for ground-state only) with the keyword
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[[vdw_tol_3bt]], which also controls the tolerance over this term.
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Methods based on maximally localized Wannier functions (MLWFs) to calculate
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vdW energy corrections have also been implemented in ABINIT. In this case the
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pair-wise terms come from contributions of pairs of MLWFs rather than from
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atoms. Among the implemented methods in ABINIT it is found vdW-WF1
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[[cite:Silvestrelli2008]], [[cite:Silvestrelli2009]] vdW-WF2
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[[cite:Ambrosetti2012]] and vdW-QHO-WF [[cite:Silvestrelli2013]]. A full
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description of the implementation of vdW-WF1 is reported in
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[[cite:Espejo2012]].
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Selection of one of these 3 methods is achieved by using [[vdw_xc]]=10/11/14
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respectivelly. Since vdW-WF1 and vdW-WF2 methods are approximations for the
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dispersion energy of non overlapping electronic densities, it is necessary to
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define the interacting fragments of the system whose dispersion energy is
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going to be calculated. The latter is achieved by using the input variables
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[[vdw_nfrag]] and [[vdw_typfrag]] to define the number of interacting
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fragments in the unit cell and to assign each atom to a fragment. A given MLWF
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belongs to the same fragment as its closer atom. The need for defining the
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interacting fragments is overridden in the vdW-QHO-WF, for which these input
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variables are not used. When dealing with periodic systems the input variable
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[[vdw_supercell]] controls the number of neighbor unit cells that will be
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included in the calculation. Each one of the 3 components of [[vdw_supercell]]
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indicates the maximum number of cells along both positive or negative
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directions of the corresponding primitive vector. This is useful for studying
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the spacial convergency of the vdW energy. It should be noticed that the user
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must set the variables associated to the calculation of MLWFs and that the
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resulting vdW energies strongly depend on the obtained Wannier functions.
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## Related Input Variables
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{{ related_variables }}
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## Selected Input Files
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{{ selected_input_files }}
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