The error for the end-members, especially NaCl is significantly larger than for the mixed systems, Fig. . We attribute this to a poor cancellation of errors for systems with few atoms, and thus few phonon branches, despite the seemingly similar agreement in terms of phonon dispersion and DOS. For this reason, we will construct two temperature dependent cluster expansions; one with all vibrational free energies calculated with the MLFF, and one with the vibrational free energies of the end-members calculated with DFT. Since calculating the end-member phonon dispersions with DFT is relatively cheap and should always be part of the validation of the MLFF, this comes at no additional computational cost. Finally, we observe that the overall agreement with DFT is already good for the MLFF trained on a smaller data set, though with some significant outliersespecially for pure NaCl and the structure depicted in Fig. .