The influence of exchange and correlation is significant in this material, as witnessed by the band gap increase from 1.15 eV to 1.88 eV, when comparing the results from DFT-PBE versus HSE06. UV-vis-NIR spectroscopy was used to measure the band gap using powder of rhombohedral KBiS 2 from replica samples. A Tauc fit to the reflectance data in Figure (c) gives an indirect band gap of is 1.21 eV, shown in Figure . The slight discrepancy of measured band gap compared to the calculated ones is likely due to surface degradation degradation inherent to alkali sulfides in air.