def get_under_hollow(self):
        """ Return HCP if an atom is present below the adsorbate in the 
        subsurface layer and FCC if not"""
        C0 = self.B[-1:] * (3, 3, 1)
        ads_pos = C0.positions[4]

        C = self.get_subsurface_layer() * (3, 3, 1)

        ret = 'FCC'
        if np.any([np.linalg.norm(ads_pos[:2] - ele.position[:2]) < 0.5 *
                   cradii[ele.number] for ele in C]):
            ret = 'HCP'

        return ret
Return HCP if an atom is present below the adsorbate in the 
        subsurface layer and FCC if not