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Xc

This page gives hints on how to set parameters related to the exchange and correlation functionals with the ABINIT package.

Introduction

Total energy computation in ABINIT is done according to Density Functional Theory (DFT). Although formally exact, an approximate exchange-correlation (XC) functional must be chosen. This is governed by the input variable ixc.
However, the pseudopotentials (or PAW data sets) are constructed for one specific XC functional. If ixc is not specified, ABINIT will simply take the ixc of the given pseudopoential(s) - hoping they are coherent with each others. One introduces an error by using a pseudopotential generated with an XC functional that is not the same as the one explicitly specified by ixc. However, ABINIT will nevertheless do the calculation.

Many exchange-correlation functionals are available (see the list in the description of ixc), through two different implementations: one is the native ABINIT implementation, the other is the ETSF library of XC functionals (LibXC is a plug-in to ABINIT). In the native ABINIT set, most of the important local approximations (LDA) are available, including the Perdew- Zunger one. Two different local spin density (LSD) are available, including the Perdew Wang 92, and one due to M. Teter. The Perdew-Burke-Ernzerhof, the revPBE, the RPBE and the HCTH GGAs (spin unpolarized as well as polarized) are also available.

(The following section is obsolete and should be upgraded. ABINIT is currently interfaced with LibXC 6). In the LibXC 2.0 library, as interfaced with ABINIT, there are 24 functional forms of the 3D LDA type, and over 80 functional forms of the GGA type. They can be used with norm-conserving pseudopotentials as well as PAW atomic data. Also, some metaGGA can be used with ABINIT (norm-conserving case only). They need usekden=1. In particular, the TB09 (not delivering reliable total energies) allows one to get cheap corrected band structures (use ixc=-12208, with HGH pseudopotentials). For response-function type calculations, the native ABINIT LDA and GGA kernels can be used as well as the LibXC ones.

Hybrid functionals:

The exchange can also be computed on the basis of the Fock expression (exact exchange), and the correlation can be computed on the basis of the RPA approximation (see the GW section). Hybrid functionals calculations (HSE06, PBE0, B3LYP) can be performed. The implementation of the exact exchange, correlation and hybrid does not deliver the forces and stresses at present, at the exception of forces with norm-conserving pseudopotentials.

Local exact exchange:

When useexexch=1, the hybrid functional PBE0 is used in PAW, inside PAW spheres only, and only for correlated orbitals given by lexexch. To change the ratio of exact exchange, see also exchmix. The implementation of local exact exchange in ABINIT is provided in [Jollet2009]. See useful input variables exchmix, lexexch and useexexch.

Van der Waals functionals:

Several Van der Waals functionals are available: Grimme (D2, D3, D3(Becke-Johnson)), Silvestrelli.

basic:

  • ixc Index of eXchange-Correlation functional

useful:

expert:

  • densty initial DENSity for each TYpe of atom
  • intxc INTerpolation for eXchange-Correlation
  • ixc_sigma Index of eXchange-Correlation functional used for self-energy calculations (SIGMA)
  • ixcrot Index of the XC ROTation method used to calculate first-order exchange-correlation potential in non-collinear DFPT calculations
  • optnlxccc OPTion for the calculation of Non-Linear eXchange-Correlation Core Correction
  • usekden USE Kinetic energy DENsity
  • xc_denpos eXchange-Correlation - DENsity POSitivity value
  • xc_taupos eXchange-Correlation - TAU kinetic energy DENsity POSitivity value
  • xc_tb09_c Value of the c parameter in the eXchange-Correlation TB09 functional

internal:

  • %xclevel eXchange Correlation functional LEVEL

Selected Input Files

gpu_omp:

v1:

v5:

v7:

v9:

Tutorials

  • The base2 tutorial deals with the H2 molecule: convergence studies, LDA versus GGA