In conclusion, we have developed a methodology for including vibrational free energies in solid solution cluster expansions at very low additional computational cost. This is achieved by using force fields trained on the relaxation trajectories already available for producing the data for the cluster expansion in the first place. We demonstrate the viability of the methodology by calculating compositional phase diagrams for two structurally and compositionally simple (pseudo)binaries, Na 1 -x K x Cl and Ag 1 -x Pd x , as archetypical examples of ionic and metallic solid solutions, respectively. In both cases, particularly for Na 1 -x K x Cl, the agreement between calculated and experimentally observed phase diagrams is significantly improved.