further augmented in intensity. While there is no strong excitonic peak below the 2.99 eV quasiparticle gap, the absorption is non-zero as a result of a series of weakly allowed excitonic transitions: 2.59, 2.72, 2.87, 2.91 and 2.94 eV. Each of these transitions is two-fold degenerate, with one component being forbidden (dark) and the other being very weakly allowed along x/y -this behaviour would partly align with the above finding in the HSE06 optical calculations that the fundamental gap at Γ is weakly allowed, and is parity forbidden across the flat bands between Γ and Z. The theoretical exciton binding energy is thus 0.4 eV, significantly higher than that predicted from the Wannier-Mott model -even when including the possibility of a 2D exciton due to the anisotropic electronic structure. It may be noted that although Y 2 Ti 2 O 5 S 2 is predicted to have a high static dielectric constant, the high frequency component (seen in Supplementary Figure ) is relatively small (∼4.6). Using the high-frequency dielectric constant alone, the 2D Wannier-Mott model would predict an order of magnitude increase in the exciton binding energy to 28 meV, though this would still be an order of magnitude lower than that predicted by the BSE calculations.