A dielectric-dependent hybrid functional approach was also tested, which can improve the description of dielectric screening from hybrid DFT with fixed exchange (e.g. HSE06) and give reduced band gap prediction errors, though this only slightly reduced the hybrid DFT gap for Cs 2 TiI 6 by 0.04 eV (α SCF = 24 %), still giving a significantly over-estimated band gap with a relative error ∆E g =∼70 %. Even using the computationally-intensive GW approximation, typically a gold standard for predicting band gaps, the calculated quasiparticle gaps in fact show far worse overestimation (both for Cs 2 TiX 6 -as previously noted by Cucco et al., and Cs 2 SnX 6 ; Section S3). These observations suggest the presence of physical interactions in Cs 2 TiX 6 which are not captured in these single-particle electronic models.