LTBOUNDLIBXC

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

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

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The Kohn-Sham kinetic-energy density

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

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

$\tau _{\sigma }^{\textrm {W}}={\frac {\left\vert \nabla n_{\sigma }\right\vert ^{2}}{8n_{\sigma }}}.$

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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