To fully understand the excitonic properties of Y 2 Ti 2 O 5 S 2 , however, we can explicitly calculate the electronhole pair through the solution of the BSE. In single-shot methods such as G 0 Ŵ0 +BSE, all excitonic effects are combined and so by using QSG Ŵ for the underlying self-energy, any quasiparticle gap renormalisation as a result of electron-hole screening has already been accounted for. Thus, by comparing QSG Ŵ +RPA and QSG Ŵ +BSE spectra the exciton binding energy can be directly extracted. In Figure , the imaginary dielectric spectrum calculated with QSG Ŵ +RPA and QSG Ŵ +BSE are depicted and the resultant absorption coefficients compared with HSE06.