mirror of https://github.com/abinit/abinit.git
84 lines
3.5 KiB
Markdown
84 lines
3.5 KiB
Markdown
---
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description: How to set parameters related to the exchange and correlation functionals
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authors: FJ
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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 set parameters related to the exchange and correlation functionals with the ABINIT package.
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## Introduction
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Total energy computation in ABINIT is done according to Density Functional
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Theory (DFT). Although formally exact, an approximate exchange-correlation
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(XC) functional must be chosen. This is governed by the input variable
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[[ixc]].
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However, the pseudopotentials (or PAW data sets) are constructed for one
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specific XC functional. If [[ixc]] is not specified, ABINIT will simply take
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the [[ixc]] of the given pseudopoential(s) - hoping they are coherent with
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each others. One introduces an error by using a pseudopotential generated with
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an XC functional that is not the same as the one explicitly specified by
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[[ixc]]. However, ABINIT will nevertheless do the calculation.
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Many exchange-correlation functionals are available (see the list in the
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description of [[ixc]]), through two different implementations: one is the
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native ABINIT implementation, the other is the ETSF library of XC functionals
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(LibXC is a plug-in to ABINIT). In the native ABINIT set, most of the
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important local approximations (LDA) are available, including the Perdew-
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Zunger one. Two different local spin density (LSD) are available, including
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the Perdew Wang 92, and one due to M. Teter. The Perdew-Burke-Ernzerhof, the
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revPBE, the RPBE and the HCTH GGAs (spin unpolarized as well as polarized) are
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also available.
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(The following section is obsolete and should be upgraded. ABINIT is currently
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interfaced with LibXC 6).
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In the LibXC 2.0 library, as interfaced with ABINIT, there are 24 functional
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forms of the 3D LDA type, and over 80 functional forms of the GGA type. They
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can be used with norm-conserving pseudopotentials as well as PAW atomic data.
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Also, some metaGGA can be used with ABINIT (norm-conserving case only).
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They need [[usekden]]=1.
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In particular, the TB09 (not delivering reliable total energies)
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allows one to get cheap corrected band structures (use [[ixc]]=-12208, with HGH pseudopotentials).
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For response-function type calculations, the native ABINIT LDA and GGA kernels can
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be used as well as the LibXC ones.
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#### **Hybrid functionals:**
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The exchange can also be computed on the basis of the Fock expression (exact
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exchange), and the correlation can be computed on the basis of the RPA
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approximation (see the GW section). [[topic:Hybrids|Hybrid functionals]]
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calculations (HSE06, PBE0, B3LYP) can be performed. The implementation of the
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exact exchange, correlation and hybrid does not deliver the forces and
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stresses at present, at the exception of forces with norm-conserving
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pseudopotentials.
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#### **Local exact exchange:**
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When [[useexexch]]=1, the hybrid functional PBE0 is used in PAW, inside PAW
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spheres only, and only for correlated orbitals given by [[lexexch]]. To change
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the ratio of exact exchange, see also [[exchmix]]. The implementation of local
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exact exchange in ABINIT is provided in [[cite:Jollet2009]]. See useful input
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variables [[exchmix]], [[lexexch]] and [[useexexch]].
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#### **Van der Waals functionals:**
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[[topic:vdw|Several Van der Waals functionals]] are available: Grimme (D2, D3,
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D3(Becke-Johnson)), Silvestrelli.
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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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## Tutorials
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* The [[tutorial:base2]] deals with the H2 molecule: convergence studies, LDA versus GGA
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