It is also possible to draw comparison to recent findings with G 0 Ŵ0 +BSE on anatase TiO 2 , which despite low effective masses, high dielectric behaviour and a 3D-connected crystal structure has recently predicted to have a strongly bound Frenkel-type exciton with 2D character. Baldini et al. rationalise the existence of such a localised exciton (with many states contributing to the transition) through the parallel curvature of valence and conduction bands in the electronic structure of anatase meaning electron and hole states have similar group velocities across a section of the Brillouin Zone. In Y 2 Ti 2 O 5 S 2 not only are the valence and conduction bands parallel across Γ to Z, but 'VB-1' and the CBM are also parallel along Γ to X and along Γ to N, potentially leading to further localization in other Cartesian directions as well, and a high exciton binding energy. The chosen path across the Brillouin Zone uses notation from Bradley and Cracknell. Excitonic contributions from the valence bands are depicted in blue and conduction bands in orange, and was plotted with software included within the Questaal package. bi-excitons or association of excitons with a defect centre. It is clear that further detailed study of the excitons in Y 2 Ti 2 O 5 S 2 using both theoretical and experimental methods is necessary to properly characterise its optical properties and fully understand its capability as a photocatalyst.