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 $\tau_{\sigma}^{\textrm{W}}<\tau_{\sigma}$ may not be fulfilled, which can potentially lead to problems, in particular if the meta-GGA functional is not defined for negative values of $\tau_{\sigma}-\tau_{\sigma}^{\textrm{W}}$. 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. Furthermore, it has been observed that applying $\tau_{\sigma}=\max(\tau_{\sigma},\tau_{\sigma}^{\mathrm{W}})$ may possibly lead 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.

Thus, the recommendation is to set LTBOUNDLIBXC=.TRUE. only in the case convergence shows an erratic behavior. If this choice is made, then the forces and stress tensor should be carefully monitored if a geometry optimization is done.

Related tags and articles

LIBXC1, LIBXC2, METAGGA

Examples that use this tag

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