LNOAUGXC
LNOAUGXC = .TRUE. | .FALSE.
Default: LNOAUGXC = .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 meta-GGA functional from Libxc.
The KS kinetic-energy density [math]\displaystyle{ \tau_{\sigma} }[/math] should in principle be larger than the von Weizsäcker kinetic-energy density [math]\displaystyle{ \tau_{\sigma}^{\textrm{W}}=\left\vert\nabla\rho_{\sigma}\right\vert^{2}/\left(8\rho_{\sigma}\right) }[/math]. 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 [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[1], a violation of the lower bound is technically not a problem and, furthermore, applying [math]\displaystyle{ \tau_{\sigma}=\max(\tau_{\sigma},\tau_{\sigma}^{\mathrm{W}}) }[/math] 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.