All supercells-including cell parameters-are relaxed with DFT to a convergence threshold of 0.01 eV Å-1 . The Vienna ab initio simulation (VASP) package with the projector augmented wave approximation is used for all simulations. The PBEsol functional is used, the plane wave cutoff is set to 700 eV, and a 2×2×2 Γ-centered k-point grid is used for the base cell, and scaled accordingly for larger supercells.