The calculated (HSE06+SOC) bandgap of the P 2 1 /c athermal ground-state crystal is 1.78 eV, as shown in Figure , which is significantly larger than the experimentally determined optical bandgap of 1.4-1.5 eV. However, our CE model and MD simulations demonstrate significant cation disorder in these materials, which has been shown to markedly impact the optical properties of photovoltaic absorbers. To investigate the relationship between bandgap and atomic disorder, the electronic bandgap of the training structures was calculating using the optB86b-vdW functional on which a CE model was then trained. A scissor operator of +0.45 eV was applied to the results from the MC sampling of the CE model due to the tendency for GGA functionals to underestimate the bandgap. This scaling was validated by calculating the DFT bandgaps of 1024 atom supercells corresponding to MC cluster vectors from T = 200K to 700K, showing close matches to the scaled results, ∆E g = 0.02 eV, with further discussion in the Supplementary Information. In Figure (b), we see the typical trend of increasing temperature and therefore increasing atomic disorder resulting in a reduction of the bandgap energies of Sn 2 SbS 2 I 3 . We find a dramatic reduction in the bandgap from 1.8 eV at T < 300 K to below 1.45 eV at T = 700 K.