To ensure the slabs were an accurate representation of the bulk and surfaces, extensive convergence testing to determine the minimal slab size required was conducted. For each of the Miller indices, the positions of atoms in the surface slabs with slab thicknesses ranging 10 to 50 Å were relaxed with PBEsol, while the unit cell shape and size was kept consistent. The average (arithmetic mean) bond distance change with respect to the atom's position in c-direction was used to ensure the bulk-like region was sufficiently large. If the average bond length close to the centre of the slab was close to the bond length of the same character in the bulk, then the slab was deemed to be sufficiently thick. The surface energy was also converged against slab thickness, where a criterion of 0.02 J m -2 was used to check convergence. Lastly, vacuum size was chosen to be 20 Å as this was the minimum thickness at which the planar potential was flat across all Miller indices.