Force field construction and performance. We first demonstrate the methodology for the Na 1 -x K x Cl solid solution. MLFFs are constructed from the relaxation trajectories using a flavour of the Gaussian Approximation Potential (GAP) method with the Smooth Overlap of Atomic Positions (SOAP) radial and angular descriptor (see Computational Methods). The training error of the MLFF on forces from DFT is 4.8 meV Å-1 (all data) and the validation error evaluated on the last 31 rattled structures is 6.5 meV Å-1 (when 600 relaxation trajectories are included). The learning curve of the MLFF trained on relaxations included in the cluster expansion is shown in Fig. in the Supporting Information (SI). After the inclusion of 150-250 relaxation trajectories in the training data, the validation error is already relatively low, though with an overall non-smooth decrease.