We model GBs in a periodic supercell containing two grains of finite width brought into contact to produce two symmetrically-equivalent GBs. We ensure that the grains in each model are at least 15 Å thick in order to minimize interactions between the grains. To determine low-energy GB structures, we performed a systematic scan over all possible rigidbody translations between the grains, with the translation steps taken to be as close to 0.5 Å as possible whilst allowing the resulting grid of translations to be commensurate with the lattice vectors parallel to the GB plane. The structure with the lowest energy is determined to be a stable structure, where the formation energy, γ, in a periodic supercell is given by