AEXX: Difference between revisions

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.

References

@@ Line 1: / Line 1: @@
 {{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.
 ----
-{{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>
-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>
-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 articles ==
+{{TAG|ALDAX}},
+{{TAG|ALDAC}},
 {{TAG|AGGAX}},
 {{TAG|AGGAC}},
-{{TAG|ALDAX}},
+{{TAG|AMGGAX}},
-{{TAG|ALDAC}},
+{{TAG|AMGGAC}},
 {{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}}