Exp PBE0 PBE0 lit. PBEsol 𝑎 / Å Band gap / eV 2.95 2.99 2.97 0.95 SOC splitting energy / eV 0.64 As shown in Table , a direct band gap of 2.99 eV at Γ is calculated with the PBE0 functional including spin-orbit coupling effects (PBE0+SOC, Figure ), which is in good agreement with experimental values obtained from a variety of measurements: pressure-dependent optical absorption from Ves et al., 2.95 eV; transmission spectra from Storm et al., 3.11 eV. This also matches well previous hybrid DFT calculations, with Yu et al. most recently reporting a value of 2.97 eV. On the other hand, the direct band gap computed with PBEsol is severely underestimated as is often the case in semi-local DFT. The hole conductivity effective mass (or the transport mass) determined from PBE0 calculations is slightly lower than that from PBEsol. Semi-local DFT functionals often struggle to accurately describe localised 𝑑 states (regardless of electronic occupation) without the inclusion of the Hubbard U parameter, so a mismatch in the transport effective mass is not unexpected. In CuI, the inclusion of SOC has a direct effect on the band edges. Figure illustrates that without SOC, the valence band maximum (VBM) of CuI has three degenerate bands. The addition of SOC results in the lifting of the degeneracy, with one of the bands (the split-off band) dropping in energy, a common feature of zincblende semiconductors. As for the band gap, PBEsol tends to undervalue the spin-orbit splitting at the Γ-point of the VBM which is around 470 meV whereas the value computed with PBE0 is around 630 meV, in excellent agreement the value of 640 meV determined experimentally by Blacha et al. using hydrostatic pressure-dependent thin film absorption.