where µ = (m e × m h )/(m e + m h ) is the reduced mass of the electron-hole pair, Ry is the Rydberg energy (13.6 eV) and m 0 is the electron rest mass. Cs 2 TiCl 6 , 23,37 Cs 2 TiBr 6 , Cs 2 TiI 6 . From Table , we witness the typical trend of larger band gap with smaller and more electronegative halogen anions (E g (Cl)> E g (Br)> E g (I)), as observed across the perovskite(-inspired) family. Typically, the smaller the B-site atom, the smaller the band gap in the A 2 BX 6 family. This is the case experimentally here, with all Ti isomorphs having smaller experimentally-measured band gaps than their Sn counterparts. The opposite trend is found in the computed band gaps, for which hybrid DFT incorrectly predicts larger gaps for the Ti compounds (except for X = Cl). While the direct-allowed gaps computed by hybrid DFT mostly coincide with the lower end of experimental ranges for Cs 2 SnX 6 , neglecting Wannier-Mott predicted exciton binding, the entirely opposite trend is found for each Cs 2 TiX 6 isomorph, with a consistent severe overestimation of the experimental band gap. Notably, screened hybrid DFT (HSE06) tends to slightly underestimate rather than overestimate semiconductor band gaps, with this underestimation typically worsening with larger band gaps. The error in predicted band gaps for Cs 2 TiX 6 results in qualitatively incorrect relative band gap energies for Cs 2 SnX 6 vs Cs 2 TiX 6 (X = I, Br).