To further aid in the characterisation of Y 2 Ti 2 O 5 S 2 as a photocatalyst, we explored its excitonic properties theoretically using many-body perturbation theory. First, we calculated the quasiparticle band structure using the QSGW and QSG Ŵ methods, to compare with our hybrid DFT calculations, using the HSE06 relaxed geometry. The band structures calculated with QSG Ŵ is depicted in Figure , while the QSGW electronic structure can be found in the Supplementary Information Figure .