AEXX: Difference between revisions

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{{TAGDEF|AEXX|[real]}}
{{TAGDEF|AEXX|[real]}}
{{DEF|AEXX|0.25|if {{TAG|LHFCALC}}{{=}}.TRUE.|0|otherwise}}
{{DEF|AEXX|0.25|if {{TAG|LHFCALC}}{{=}}.TRUE.|0|if {{TAG|LHFCALC}}{{=}}.FALSE.}}


Description: {{TAG|AEXX}} specifies the fraction of exact exchange in a Hartree-Fock-type/hybrid-functional calculation.
Description: {{TAG|AEXX}} specifies the fraction of exact exchange in a Hartree-Fock-type/hybrid-functional calculation.
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{{NB|important|The sum of the fraction of the exact exchange and local-density-approximation (LDA) exchange is always 1.0, i.e., it is not possible to independently set the amount of LDA exchange.}}
{{NB|mind|
For instance, if {{TAG|AEXX}}=0.25, <math>1/4</math> of the exact exchange is used and <math>3/4</math> of the LDA exchange is added. For {{TAG|AEXX}}=0.5, half of the exact exchange is used, and one-half of the LDA exchange is added. In contrast, for semi-local generalized-gradient approximation (GGA), the amount of GGA exchange ({{TAG|AGGAX}}) and GGA correlation ({{TAG|AGGAC}}) can be set independently. In fact, some [[List_of_hybrid_functionals|popular hybrid functionals]] use for instance only 0.8 of the gradient contribution to the exchange. The tags {{TAG|AGGAX}} and {{TAG|AGGAC}} are only used if GGA is already selected. For LDA calculations, no gradient correction will be added regardless of the value set for {{TAG|AGGAX}} and {{TAG|AGGAC}}.
*For versions of VASP prior to 6.4.0, {{TAG|ALDAX}} was constrained to be equal to 1.0-{{TAG|AEXX}}. This constraint is lifted since VASP.6.4.0.
{{NB|mind| The defaults are chosen such that the PBE0 functional is selected.}}  
*For {{TAG|AEXX}}{{=}}1.0, VASP switches off correlation by default ({{TAG|ALDAC}}{{=}}0.0, {{TAG|AGGAC}}{{=}}0.0, and {{TAG|AMGGAC}}{{=}}0.0) and thus runs a full Hartree-Fock calculation.}}
That is, the PBE0 functional contains 25% of the exact exchange, and  75% of the PBE exchange, and 100% of the PBE correlation energy. The resulting expression for the exchange-correlation energy then takes the following form:


<math>
== Related tags and articles ==
E_{\mathrm{xc}}^{\mathrm{PBE0}}=\frac{1}{4}~E_{\mathrm{x}}~+
{{TAG|ALDAX}},
~\frac{3}{4}~ E_{\mathrm{x}}^{\mathrm{PBE}}
{{TAG|ALDAC}},
+~E_{\mathrm{c}}^{\mathrm{PBE}}
</math>
 
A comprehensive evaluation of the performance of the PBE0 functional, as compared to PBE, can be found in Ref. {{cite|paier:jcp:05}}.
 
For {{TAG|AEXX}}=1.0, VASP automatically sets {{TAG|ALDAC}}=0.0 and {{TAG|AGGAC}}=0.0, and thus runs a full Hartree-Fock-type calculation.
 
== Related Tags and Sections ==
{{TAG|AGGAX}},
{{TAG|AGGAX}},
{{TAG|AGGAC}},
{{TAG|AGGAC}},
{{TAG|ALDAC}},
{{TAG|AMGGAX}},
{{TAG|AMGGAC}},
{{TAG|LHFCALC}},
{{TAG|LHFCALC}},
[[List_of_hybrid_functionals|List of hybrid functionals]]
{{TAG|HFSCREEN}},
[[List_of_hybrid_functionals|List of hybrid functionals]],
[[Hybrid_functionals:_formalism|Hybrid functionals: formalism]]


{{sc|AEXX|Examples|Examples that use this tag}}
{{sc|AEXX|Examples|Examples that use this tag}}
== References ==
== References ==
<references/>
<references/>


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[[Category:INCAR]][[Category:XC Functionals]][[Category:Hybrids]]
[[Category:INCAR tag]][[Category:Exchange-correlation functionals]][[Category:Hybrid_functionals]]

Latest revision as of 08:28, 18 October 2023

AEXX = [real] 

Default: AEXX = 0.25 if LHFCALC=.TRUE.
= 0 if LHFCALC=.FALSE.

Description: AEXX specifies the fraction of exact exchange in a Hartree-Fock-type/hybrid-functional calculation.


Mind:
  • For versions of VASP prior to 6.4.0, ALDAX was constrained to be equal to 1.0-AEXX. This constraint is lifted since VASP.6.4.0.
  • For AEXX=1.0, VASP switches off correlation by default (ALDAC=0.0, AGGAC=0.0, and AMGGAC=0.0) and thus runs a full Hartree-Fock calculation.

Related tags and articles

ALDAX, ALDAC, AGGAX, AGGAC, AMGGAX, AMGGAC, LHFCALC, HFSCREEN, List of hybrid functionals, Hybrid functionals: formalism

Examples that use this tag

References