The monolayer was cut along the (001) direction, and an additional 15 Å of vacuum was added perpendicular to the monolayer to prevent interactions with periodic replicas. Similarly, we optimize a slab model of the double perovskite Cs2AgBiCl6 using a 2×2 in-plane supercell, with (001) direction, and added 15 Å of vacuum to maintain separation from periodic images. Two different surface terminations were investigated for Cs2AgBiCl6: (Ag0.5Bi0.5)Cl2 and CsCl, see Figure .