Electronic band structures and independent-particle optical absorption spectra were calculated with the HSE06 hybrid DFT functional, including spin-orbit coupling effects (HSE06+SOC) due to the presence of heavy-atom elements -see Kavanagh et al. and Cucco et al. for discussions on the impact of SOC on the electronic structure of these materials. For optical calculations, the number of empty virtual states was increased using vaspup2.0 until the high-frequency dielectric constant ε ∞ was converged to a precision of 0.01, and tetrahedron k-point smearing was used. Electronic densities of states (Figures &) and unfolded band structures (Figures S6 & S7) were plotted using sumo and easyunfold 50 respectively.