and analogous equations hold for the other rock-salt IV-VI compounds. E Se p is the on-site energy of the Se-p orbital, V Sn s -Se p is the nearest-neighbor Sn-s/Se-p interaction energy, and V Se ppσ is and, respectively, with an additional out-of-plane component. In (d), the ring is warped into six Fermi surface pockets by the R 1 parameter. In (e), the pockets are lifted out of the L-plane by the R 2 parameter. Parameters used to plot these surfaces are listed in Table .