Fill up the atomic orbitals with available electrons.
        Return HOMO, LUMO, and whether it's a metal.

def obtain_band_edges(self):
        '''
        Fill up the atomic orbitals with available electrons.
        Return HOMO, LUMO, and whether it's a metal.
        '''
        orbitals = self.aos_as_list()
        electrons = Composition(self.composition).total_electrons
        partial_filled = []
        for orbital in orbitals:
            if electrons <= 0:
                break
            if 's' in orbital[1]:
                electrons += -2
            elif 'p' in orbital[1]:
                electrons += -6
            elif 'd' in orbital[1]:
                electrons += -10
            elif 'f' in orbital[1]:
                electrons += -14
            partial_filled.append(orbital)

        if electrons != 0:
            homo = partial_filled[-1]
            lumo = partial_filled[-1]
        else:
            homo = partial_filled[-1]
            try:
                lumo = orbitals[len(partial_filled)]
            except:
                lumo = None

        if homo == lumo:
            metal = True
        else:
            metal = False

        return {'HOMO': homo, 'LUMO': lumo, 'metal': metal}