Compute difference in conduction band min and valence band max

def _get_bandgap_from_bands(energies, nelec):
        """Compute difference in conduction band min and valence band max"""
        nelec = int(nelec)
        valence = [x[nelec-1] for x in energies]
        conduction = [x[nelec] for x in energies]
        return max(min(conduction) - max(valence), 0.0)