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/DFT hybrid functional type calculation.
Description: {{TAG|AEXX}} specifies the fraction of exact exchange in a Hartree-Fock-type/hybrid-functional calculation.
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----
The sum of the fraction of the exact exchange and LDA exchange is always 1.0, and it is not possible to set the amount of LDA exchange indepently. Examples: if {{TAG|AEXX}}=0.25, 1/4 of the exact exchange is used, and 3/4 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.
{{NB|mind|
*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.
*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.}}


The amount of LDA correlation included in the hybrid functional can be specified through the {{TAG|ALDAC}}-tag.
== Related tags and articles ==
 
{{TAG|ALDAX}},
The amount of gradient correction to the exchange and the correlation contributions can be set indepently, however (some [[specific_hybrid_functionals|popular hybride functionals]] for instance use only 0.8 of the gradient contribition to the exchange). The GGA flags {{TAG|AGGAX}} and {{TAG|AGGAC}} are only used if GGA is already selected (for LDA type calculations no gradient correction will be added regardless of values is used for {{TAG|AGGAX}} and {{TAG|AGGAC}}).
{{TAG|ALDAC}},
 
'''Note''': The defaults are chosen such that the hybrid PBE0 functional is selected for PBE pseudopotentials (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 simple form:
 
<math>
E_{\mathrm{xc}}^{\mathrm{PBE0}}=\frac{1}{4}~E_{\mathrm{x}}~+
~\frac{3}{4}~ E_{\mathrm{x}}^{\mathrm{PBE}}
+~E_{\mathrm{c}}^{\mathrm{PBE}}
</math>
 
Other sensible values are of course {{TAG|AEXX}}=1.0 (full Hartree-Fock type calculations). In this case, VASP also automatically selects {{TAG|ALDAC}}=0.0 and {{TAG|AGGAC}}=0.0, to avoid the addition of a (semi-local) correlation energy.
 
A comprehensive evaluation of the performance of the PBE0 functional, as compared to PBE, can be found in Ref. <ref name="paier:jcp:05"/>.
 
<references>
<ref name="paier:jcp:05">[https://aip.scitation.org/doi/10.1063/1.1926272 J. Paier, R. Hirschl, M. Marsman, and G. Kresse, J. Chem. Phys. 122, 234102 (2005).]</ref>
</references>
 
== Related Tags and Sections ==
{{TAG|AGGAX}},
{{TAG|AGGAX}},
{{TAG|AGGAC}},
{{TAG|AGGAC}},
{{TAG|ALDAC}},
{{TAG|AMGGAX}},
{{TAG|AMGGAC}},
{{TAG|LHFCALC}},
{{TAG|LHFCALC}},
[[Hartree-Fock_and_HF/DFT_hybrid_functionals|hybrid functionals]],
{{TAG|HFSCREEN}},
[[specific_hybrid_functionals|settings for specific hybrid functionals]]
[[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/>


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

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