def get_cbm_vbm(self, tol=0.001, abs_tol=False, spin=None):
        """
        Expects a DOS object and finds the cbm and vbm.

        Args:
            tol: tolerance in occupations for determining the gap
            abs_tol: An absolute tolerance (True) and a relative one (False)
            spin: Possible values are None - finds the gap in the summed
                densities, Up - finds the gap in the up spin channel,
                Down - finds the gap in the down spin channel.

        Returns:
            (cbm, vbm): float in eV corresponding to the gap
        """
        # determine tolerance
        tdos = self.get_densities(spin)
        if not abs_tol:
            tol = tol * tdos.sum() / tdos.shape[0]

        # find index of fermi energy
        i_fermi = 0
        while self.energies[i_fermi] <= self.efermi:
            i_fermi += 1

        # work backwards until tolerance is reached
        i_gap_start = i_fermi
        while i_gap_start - 1 >= 0 and tdos[i_gap_start - 1] <= tol:
            i_gap_start -= 1

        # work forwards until tolerance is reached
        i_gap_end = i_gap_start
        while i_gap_end < tdos.shape[0] and tdos[i_gap_end] <= tol:
            i_gap_end += 1
        i_gap_end -= 1
        return self.energies[i_gap_end], self.energies[i_gap_start]