For each structure, the k-point mesh and cutoff energy for the plane wave basis set were converged using a criterion of 1 meV per atom on the total energy. A cutoff energy of 500 eV was found to be sufficient for all three systems, and k-meshes of 2×2×3, 3×2×2 and 3×1×1 were found to be sufficient for the standardized primitives of the iodide, bromide and chloride compounds respectively. All structures were optimized using GGA DFT initially until the forces on each atom did not exceed 0.01 eV/Å.