Due to the presence of Cu(d) states in this system, which are poorly modelled by semilocal DFT, hybrid DFT or the GW approximation must be used to accurately calculate the optoelectronic properties. Moreover, the mBJ+U functional has been shown to underestimate the bandgap of Cu-multinary chalcogenides, with a root-mean-square error of 0.24 eV. Using hybrid DFT (HSE06), Cu 2 SiSe 3 is calculated to have a direct bandgap of 1.52 eV at the Γ point, with spin orbit coupling reducing the bandgap to 1.49 eV (∆E g = 0.03 eV). The HSE06 band gap is validated by calculation of the electronic structure using the QSG Ŵ method, which includes additional screening effects from electron-hole interactions. The QSG Ŵ band gap is 1.580 eV, in excellent agreement with HSE06. Solving the dielectric spectrum of Cu 2 SiSe 3 within the Random Phase approximation (RPA) and Bethe-Salpeter equation (BSE) using the QSG Ŵ self-energy directly separates the renormalization of the band structure due to electron-hole screening from direct excitonic effects, Figure . Only one sub-gap exciton is found, at 1.42 eV, giving an exciton binding energy of 0.16 eV. Plotting the contributions of individual bands to the 1.42 eV exciton onto the band structure of Cu 2 SiSe 3 , Figure , it is evident that the exciton has predominantly Wannier-Mott character, with dominant contributions arising from the valence and conduction bands around Γ.