For our cluster expansion model training set, we performed full geometry optimisations, allowing changes to the cell shape and volume as well as internal atomic coordinates. Each geometry optimisation was deemed converged when all atomic forces were smaller than 0.01 eV Å-1 . Our cluster expansion model was fitted us-ing the MAPS component of the ATAT code , which produced a model with 10 non-zero ECIs and a cross-validation score of 0.013 eV per structure . Additional information about the cluster expansion fitting and resulting model are provided in the Supporting Information.