LTBOUNDLIBXC

LTBOUNDLIBXC = .TRUE. | .FALSE.
Default: LTBOUNDLIBXC = .FALSE. 

Description: LTBOUNDLIBXC specifies whether or not the lower bound for the Kohn-Sham kinetic-energy density ${\tau }_{\sigma }$ (${\tau }_{\sigma }^{\text{W}}<{\tau }_{\sigma }$) is enforced before ${\tau }_{\sigma }$ is used in a METAGGA functional from Libxc.

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The Kohn-Sham kinetic-energy density

${\tau }_{\sigma }=\frac{1}{2}\sum _i\mathrm{\nabla }{\psi }_{i\sigma }^{*}\cdot \mathrm{\nabla }{\psi }_{i\sigma }$

should, in principle, be larger than the von Weizsäcker kinetic-energy density

${\tau }_{\sigma }^{\text{W}}=\frac{{\left|\mathrm{\nabla }{n}_{\sigma }\right|}^2}{8n_{\sigma }}.$

However, for numerical reasons this may not always be the case and can potentially lead to problems depending on the meta-GGA. If LTBOUNDLIBXC=.TRUE. in INCAR, then ${\tau }_{\sigma }=\max ({\tau }_{\sigma },{\tau }_{\sigma }^{\mathrm{W}})$ is applied before ${\tau }_{\sigma }$ is used in a meta-GGA functional from Libxc.

However, according to tests, for some of the most common meta-GGA functionals like SCAN, a violation of the lower bound is technically not a problem and, furthermore, applying ${\tau }_{\sigma }=\max ({\tau }_{\sigma },{\tau }_{\sigma }^{\mathrm{W}})$ leads to very inaccurate forces and stress tensor. Therefore, by default LTBOUNDLIBXC=.FALSE. and Libxc should be compiled with the option --disable-fhc has explained here.

Related tags and articles

LIBXC1, LIBXC2, METAGGA

Examples that use this tag

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