Using the Wannier-Mott effective mass model (Table ), we find large exciton binding energies, particularly for the Ti compounds (due to flat bands and heavy carrier masses), suggesting strong electron-hole interactions in these materials. Moreover, the large Wannier-Mott binding energies, isolated octahedra, low dielectric screening and highly-localized d orbitals suggests that strongly-bound excitons may be formed for Cs 2 TiX 6 . To explore this hypothesis, we extend our model using GW to calculate quasiparticle eigenvalues and include electron-hole interactions via the Bethe-Salpeter equation (BSE). The optical absorption spectrum calculated with this approach, alongside that obtained from hybrid DFT and the experimental data, is shown in Fig. . Figure : Optical absorption spectra of (left to right) Cs 2 TiI 6 , Cs 2 TiBr 6 and Cs 2 TiCl 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).