LTHOMAS: Difference between revisions

Latest revision as of 21:16, 21 October 2025

LTHOMAS = .TRUE. | .FALSE.
Default: LTHOMAS = .FALSE.

Description: LTHOMAS selects a decomposition of the exchange functional based on Thomas-Fermi exponential screening.

If LTHOMAS=.TRUE. the decomposition of the exchange operator (in a range-separated hybrid functional) into a short range and a long range part will be based on Thomas-Fermi exponential screening. The Thomas-Fermi screening length k_TF is specified by means of the HFSCREEN tag.

For typical semiconductors, a Thomas-Fermi screening length [math]\displaystyle{ k_{\rm TF} }[/math] of about 1.8 Å^-1 yields reasonable band gaps. In principle, however, the Thomas-Fermi screening length depends on the valence-electron density. VASP determines [math]\displaystyle{ k_{\rm TF} }[/math] from the number of valence electrons (read from the POTCAR file) and the volume (leading to an average density [math]\displaystyle{ \bar{n} }[/math]) and writes the corresponding value of [math]\displaystyle{ k_{\rm TF}=\sqrt{4k_{\rm F}/\pi} }[/math], where [math]\displaystyle{ k_{\rm F}=(3\pi^2\bar{n})^{1/3} }[/math] to the OUTCAR file (note that this value is only printed for information; it is not used during the calculation):

 Thomas-Fermi vector in A             =   2.00000

The setting of the sX-LDA functional is shown on the page listing the hybrid functionals.

Mind:

If LTHOMAS=.TRUE., then LHFCALC=.TRUE. is automatically set.
If LTHOMAS=.TRUE., then AEXX=1 is automatically set, but AEXX can be set to another value.

Important: When AEXX=1 (the default for LTHOMAS=.TRUE.), the correlation [math]\displaystyle{ E_{\mathrm{c}}^{\mathrm{SL}} }[/math] is not included. However, it can be included by setting ALDAC=1.0 and AGGAC=1.0.

Since VASP counts the semi-core states and d-states as valence electrons, although these states do not contribute to the screening, the values reported by VASP are often not recommended.

@@ Line 1: / Line 1: @@
 {{TAGDEF|LTHOMAS|.TRUE. {{!}} .FALSE.|.FALSE.}}
-Description: {{TAG|LTHOMAS}} selects a decomposition of the exchange functional based on Thomas-Fermi screening.
+Description: {{TAG|LTHOMAS}} selects a decomposition of the exchange functional based on Thomas-Fermi exponential screening.
 ----
-If {{TAG|LTHOMAS}}=.TRUE. the decomposition of the exchange operator (in a range separated hybrid functional) into a short range and a long range part will be based on Thomas-Fermi screening.
+If {{TAG|LTHOMAS}}=.TRUE. the decomposition of the exchange operator (in a [[Hybrid functionals: formalism|range-separated hybrid functional]]) into a short range and a long range part will be based on Thomas-Fermi exponential screening.
 The Thomas-Fermi screening length ''k''<sub>TF</sub> is specified by means of the {{TAG|HFSCREEN}} tag.
-For typical semiconductors, a Thomas-Fermi screening length of about 1.8 &Aring;<sup>-1</sup> yields reasonable band gaps. In principle, however, the Thomas-Fermi screening length depends on the valence electron density; VASP determines this parameter from the number of valence electrons (read from the {{FILE|POTCAR}} file) and the volume and writes the corresponding value to the {{FILE|OUTCAR}} file:
+For typical semiconductors, a Thomas-Fermi screening length <math>k_{\rm TF}</math> of about 1.8 &Aring;<sup>-1</sup> yields reasonable band gaps. In principle, however, the Thomas-Fermi screening length depends on the valence-electron density. VASP determines <math>k_{\rm TF}</math> from the number of valence electrons (read from the {{FILE|POTCAR}} file) and the volume (leading to an average density <math>\bar{n}</math>) and writes the corresponding value of <math>k_{\rm TF}=\sqrt{4k_{\rm F}/\pi}</math>, where <math>k_{\rm F}=(3\pi^2\bar{n})^{1/3}</math> to the {{FILE|OUTCAR}} file (note that this value is only printed for information; it is not used during the calculation):
    Thomas-Fermi vector in A             =   2.00000
-Since, VASP counts the semi-core states and ''d''-states as valence electrons, although these states do not contribute to the screening, the values reported by VASP are often incorrect.
+The setting of the sX-LDA functional is shown on the [[list_of_hybrid_functionals|page listing the hybrid functionals]].
+{{NB|mind|
+*If {{TAG|LTHOMAS}}{{=}}.TRUE., then {{TAG|LHFCALC}}{{=}}.TRUE. is automatically set.
+*If {{TAG|LTHOMAS}}{{=}}.TRUE., then {{TAG|AEXX}}{{=}}1 is automatically set, but {{TAG|AEXX}} can be set to another value.}}
+{{NB|important|When {{TAG|AEXX}}{{=}}1 (the default for {{TAG|LTHOMAS}}{{=}}.TRUE.), the correlation <math>E_{\mathrm{c}}^{\mathrm{SL}}</math> is not included. However, it can be included by setting {{TAG|ALDAC}}{{=}}1.0 and {{TAG|AGGAC}}{{=}}1.0.}}
+Since VASP counts the semi-core states and ''d''-states as valence electrons, although these states do not contribute to the screening, the values reported by VASP are often not recommended.
 == Related tags and articles ==
 {{TAG|LHFCALC}},
 {{TAG|HFSCREEN}},
-[[:Category:Hybrid functionals: formalism#Thomas_Fermi|Thomas-Fermi screening]],
+{{TAG|AEXX}},
-[[list_of_hybrid_functionals|settings for specific hybrid functionals]]
+{{TAG|LMODELHF}},
+{{TAG|LRHFCALC}},
+[[list_of_hybrid_functionals|List of hybrid functionals]],
+[[Hybrid_functionals:_formalism|Hybrid functionals: formalism]]
 {{sc|LTHOMAS|Examples|Examples that use this tag}}