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

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

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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