LTBOUNDLIBXC
LTBOUNDLIBXC = .TRUE. | .FALSE.
Default: LTBOUNDLIBXC = .FALSE.
Description: LTBOUNDLIBXC specifies whether or not the lower bound for the Kohn-Sham kinetic-energy density [math]\displaystyle{ \tau_{\sigma} }[/math] ([math]\displaystyle{ \tau_{\sigma}^{\textrm{W}}\lt \tau_{\sigma} }[/math]) is enforced before [math]\displaystyle{ \tau_{\sigma} }[/math] is used in a METAGGA functional from Libxc.
The Kohn-Sham kinetic-energy density
- [math]\displaystyle{ \tau_{\sigma}=\frac{1}{2}\sum_{i}\nabla\psi_{i\sigma}^{*}\cdot\nabla\psi_{i\sigma} }[/math]
should, in principle, be larger than the von Weizsäcker kinetic-energy density[1]
- [math]\displaystyle{ \tau_{\sigma}^{\textrm{W}}=\frac{\left\vert\nabla n_{\sigma}\right\vert^{2}}{8 n_{\sigma}}. }[/math]
However, for numerical reasons [math]\displaystyle{ \tau_{\sigma}^{\textrm{W}}\lt \tau_{\sigma} }[/math] may not be fulfilled, which can potentially lead to problems, in particular if the meta-GGA functional is not defined for negative values of [math]\displaystyle{ \tau_{\sigma}-\tau_{\sigma}^{\textrm{W}} }[/math]. If LTBOUNDLIBXC=.TRUE. in INCAR, then [math]\displaystyle{ \tau_{\sigma}=\max(\tau_{\sigma},\tau_{\sigma}^{\mathrm{W}}) }[/math] is applied before [math]\displaystyle{ \tau_{\sigma} }[/math] is used in a meta-GGA functional from Libxc.
However, according to tests, for some of the most common meta-GGA functionals like SCAN[2], a violation of the lower bound is technically not a problem. Furthermore, it has been observed that applying [math]\displaystyle{ \tau_{\sigma}=\max(\tau_{\sigma},\tau_{\sigma}^{\mathrm{W}}) }[/math] 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.