Returns the list of faces of this coordination geometry. Each face is given as a
        list of its vertices coordinates.

def faces(self, sites, permutation=None):
        """
        Returns the list of faces of this coordination geometry. Each face is given as a
        list of its vertices coordinates.
        """
        if permutation is None:
            coords = [site.coords for site in sites]
        else:
            coords = [sites[ii].coords for ii in permutation]
        return [[coords[ii] for ii in f] for f in self._faces]