Band properties from the eigenvalues as a tuple,
        (band gap, cbm, vbm, is_band_gap_direct).

def eigenvalue_band_properties(self):
        """
        Band properties from the eigenvalues as a tuple,
        (band gap, cbm, vbm, is_band_gap_direct).
        """
        vbm = -float("inf")
        vbm_kpoint = None
        cbm = float("inf")
        cbm_kpoint = None
        for spin, d in self.eigenvalues.items():
            for k, val in enumerate(d):
                for (eigenval, occu) in val:
                    if occu > self.occu_tol and eigenval > vbm:
                        vbm = eigenval
                        vbm_kpoint = k
                    elif occu <= self.occu_tol and eigenval < cbm:
                        cbm = eigenval
                        cbm_kpoint = k
        return max(cbm - vbm, 0), cbm, vbm, vbm_kpoint == cbm_kpoint