# [Octopus-users] using MGGA from libxc

Sat Jan 22 12:37:28 WET 2011

Hello All,

I have a (probably naive)  question about libxc library:

I want to know the xc potential within MGGA.
For this, I call xc_mgga_vxc. But how to construct v_xc(r)  from the
output of
xc_mgga_vxc: We evidently need  \delta tau / \delta \rho, which is not
among the output parameters of    xc_mgga_vxc ?
(\tau is the kinetic energy density, and \rho is the density).

Thank you very much in advance.