Figure : Optical absorption spectra of (left to right) Cs 2 SnI 6 , Cs 2 SnBr 6 and Cs 2 SnCl 6 , calculated with both hybrid DFT (dotted violet) and the G 0 W 0 + BSE method (solid blue), alongside the experimental data from UV-Vis spectroscopy (dashed green). To directly compare the spectral shapes, calculated curves have been rigidly shifted to match the experimental absorption onset (unshifted results shown in Section S3). For Cs 2 SnCl 6 , the absorption spectrum recorded by Karim et al. is also shown for comparison (downshifted by 0.5 eV as discussed in Section S3.4).