Following statement copied from https://www.tddft.org/programs/libxc/ (28.11.2019):
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Libxc is written in C and has Fortran and Python bindings. It is released under the MPL license (v. 2.0). Contributions are welcome.

To cite Libxc, the current 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)

The previous reference to the library was * 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).

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https://gitlab.com/libxc/libxc