These findings have important implications for optoelectronic applications. Calculations were performed using both Density Functional Theory (DFT) and quasiparticle Green's function (GW ) approaches within periodic boundary conditions, through the Vienna Ab Initio Simulation Package (VASP). Scalar-relativistic pseudo-potentials were employed to describe the interaction between core and valence electrons, via the projectoraugmented wave (PAW) method. Specifically, the Cs sv, Sn d, Ti pv, I, Br and Cl VASP PAW potentials were used. The effect of pseudopotential choice and DFT starting point (semi-local vs hybrid) on the GW results was tested and found to give qualitatively similar results, with the same large overestimation of bandgaps relative to experiment (details in Section S3).