Generic copyright boilerplates
At the moment the sources have personal copyright boilerplates, which are somewhat random: the year entries are all over the place due to copy-paste, and the boilerplates don't portray an accurate reflection of the authorship.
In line with many other projects, I would propose a unified license boilerplate:
/* * Libxc: a library of exchange, correlation, and kinetic energy functionals * * Copyright (c) 2006-2020 The Libxc developers. * * This Source Code Form is subject to the terms of the Mozilla Public * License, v. 2.0. If a copy of the MPL was not distributed with this * file, You can obtain one at http://mozilla.org/MPL/2.0/. * */
which would go into every file and would be updated every year to have the year up to date.