LTHOMAS

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

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

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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 (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

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.

Related tags and articles

LHFCALC, HFSCREEN, LRHFCALC, Thomas-Fermi screening, List of hybrid functionals, Hybrid functionals: formalism

Examples that use this tag

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