To calculate defect formation energies in Li 3 OCl, we have performed a series of hybrid-DFT calculations using the plane-wave DFT code vasp using the HSE06 hybrid functional . Hybrid DFT typically gives more accurate descriptions of electronic structure than conventional pure-LDA or GGA functionals while also approximately correcting for the self-interaction error exhibited by local or semi-local functionals; the selfinteraction error can lead to unphysical delocalisation of charge for defects in wide-gap oxides giving a qualitatively incorrect description of different defect charge states . All calculations used a plane-wave basis cutoff of 520 eV and defect calculations were performed using a 3 × 3 × 3 supercell of the Li 3 OCl unit cell. Interactions between core and valence electrons were described using the projector augmented wave method