Finally, the bulk effective charge carrier masses, dielectric constant and optical absorption spectrum discussed here were calculated as a part of a previous study. To assess the character of the exciton behaviour in Y 2 Ti 2 O 5 S 2 , many-body quasiparticle electronic structures and optical spectra were calculated within the Questaal package through the quasiparticle self-consistent GW (QSGW ) method. Questaal utilises a full-potential linear muffin-tin orbital basis: augmentation sphere radii (in atomic units) of 2.80, 2.10, 2.49 and 1.52 were used for Y, Ti, S and O atoms respectively, with automatically generated interstitial smooth Hankel functions and an l-cutoff of 4 used for all atoms. Local orbitals were necessary to include the Ti 3p and Y 4p within the valence electrons. A GW k-mesh of 5×5×5 and interstitial G-vector cutoff of 10 Ry 1 2 were found to converge the quasiparticle gap to within 10 meV, with a 7×7×7 k-mesh used to obtain the initial PBE exchange-correlation potential. QSGW, which tends to demonstrate excellent agreement with experimental band gaps for s-p dominated semiconductors, overestimates band gaps systematically due to the usage of the Random Phase Approximation (RPA) -this can be improved by including ladder diagrams, which can describe the additional contribution by explicit two-particle electron-hole interaction to the screened Coulomb W ; 32 these can be obtained via the Bethe-Salpeter equation (BSE), and their additional to QSGW has lead to exceptional accuracy with respect to experimental band gaps in previous cases. The inclusion of these effects within a self-consistent calculation of the self-energy is here denoted QSG Ŵ, and within the explicit solution of optical transitions, to obtain excitonic features, QSG Ŵ +BSE (without, QSG Ŵ +RPA).