Diffuse reflectance UV-Vis spectrometry was measured on CuBiSeCl2 powder to ascertain the optical band gap. The band gap was determined from a Tauc plot, shown in Figure (b), and is found to be 1.40 (3) eV, in good agreement with the value derived from band structure calculations (1.5 eV). Complete agreement between experiment and calculation is not expected since the DFT calculates the band structure at 0 K, whereas UV-Vis diffuse reflectance was measured at 300 K. However, as the temperature is increased, the calculated band gap will decrease, indicating even better agreement between calculation and experiment.