To cite Libxc, the up-to-date reference is

Susi Lehtola, Conrad Steigemann, Micael J.T. Oliveira, and Miguel
A.L. Marques, Recent developments in Libxc - A comprehensive library
of functionals for density functional theory, Software X 7, 1 (2018).
doi: 10.1016/j.softx.2017.11.002

The reference for older versions of libxc, before the switch to Maple
in version 4 (released in 2017), is

Miguel A. L. Marques, Micael J. T. Oliveira, and Tobias Burnus, Libxc:
a library of exchange and correlation functionals for density
functional theory, Comput. Phys. Commun. 183, 2272 (2012).
doi: 10.1016/j.cpc.2012.05.007
