Extension to Ag 1 -x Pd x . For the Ag 1 -x Pd x solid solution, an MLFF is trained in a similar way to that of Na 1 -x K x Cl. Fig. shows the training and validation errors as a function of training set size. The final training error is 4.8 meV Å-1 (all data) and the validation error evaluated on the last 31 rattled structures is 17.2 meV Å-1 (when 600 relaxations are included). A similar trend to that of Na 1 -x K x Cl with decreasing validation error with training set size, and a flattening of the curve after ≃150 trajectories is observed, but we note that the validation error is significantly larger here.