(a) Band gaps at L point for PbX calculated by different methods. Red circles, green up triangles, purple squares, and blue down triangles represent the band gaps got from GGA no SOC, GGA SOC, HSE03 no SOC and mBJ no SOC calculations respectively. (b) SOC TB band gap E_G for PbX as a function of (red circles). If we decrease C₁ only, the band gap decreases about 0.2 eV as shown as the green up triangle. If we decrease C₁ and increase D₃ at the same time, the final band gap is shown as the purple square. Available experimental data at 4.2 K (blue pentagons) are plotted for comparison. (c) and (d) Outermost s-electron binding energy E_s and atomic radius for S, Se, Te, and Po as a function of the atomic number Z.