Ab-initio calculations were performed using density functional theory (DFT) implemented within the plane-wave periodic code, VASP. The projector-augmented wave method (PAW) was used to describe the interaction between the core electrons and the valence electrons. All electrons were treated explicitly for Li, while for O, Cl, Br and H the standard pseudopotentials supplied with VASP were used. The hybrid functional HSE06 (Heyd-Scuseria-Ernzerhof) was used in order to address the self-interaction error thus allowing for an accurate description of the band gap and electronic properties of Li 3 OCl. Hybrid functionals have consistently displayed improved geometry and electronic properties of semiconductors. The band gap calculated herein is ∼ 6.6 eV which aligns well with previous HSE06 studies (∼ 6.4-6.6 eV) . To date, however, there is no experimental band gap with which to compare. Determination of the bulk electronic and structural properties were performed on the cubic form (5 atom primitive cell) of Li 3 OCl and Li 3 OBr (space group: Pm 3m). To this end, a full geometric relaxation using a Γ-centred k-point grid of 6 × 6 × 6 and a plane wave energy cutoff of 500 eV was carried until the largest force on any atom was below the force criterion of 0.01 eV Å -1 . The intrinsic defects were simulated using a 3 × 3 × 3 supercell expansion of the conventional cell containing 135 atoms. Spin-polarised geometry relaxations (to the same convergence criteria as the conventional cell) of each defect cell and its respective charge states were performed using a Γ-centred 2×2×2 k-point mesh and 500 eV plane wave energy cutoff. A Γ-centred k-point 2×2×2 mesh was chosen in order to capture the M -point in the 1 st Brillouin zone, which is the valence band maximum (VBM) and conduction band minimum (CBM) (SI Figure ), which was not captured in a Monkhorst-Pack 2×2×2 grid. The bulk electronic and structural properties are given in the supporting information (Secions SI 2, and SI 1).