We performed density functional theory (DFT) calculations to rationalize the trends in hydrogen and ammonia production rates with varying g-C3N4 concentration, Figure summarized in the Supporting Information. We start with consideration of the pure Cs2AgBiCl6 phase. Notably, the standalone Cs2AgBiCl6 generates a substantial amount of hydrogen, Figure . Recent studies have pointed out halide vacancies as potential defects controlling the hydrogen production in the bromide-based analogue Cs2AgBiBr6. Theoretical studies further showed low formation energies of chloride vacancies, VCl, in Cs2AgBiCl6 bulk phases. We modeled chloride vacancies in different charge states at a (001)-surface of Cs2AgBiCl6 with (Ag0.5Bi0.5)Cl2-termination, see Figure and Table . Our calculations predict deep (+/-) transition levels for a surface VCl at 1.7 eV above the valence band maximum, see Figure .