AB/F is a composition variable that describes the ratio of cation-cation pairs. Further details of MQM can be found elsewhere. Binary interaction parameters of molten LiF-NaF, LiF-KF, NaF-KF, LiF-CsF, NaF-CsF, and KF-CsF salt systems are taken from the literature. The Kohler/Topp approximation was used to extrapolate to ternary and quaternary solid and liquid phase equilibria. Ternary interaction parameters for LiF-NaF-KF and LiF-KF-CsF liquid solutions are also taken from the literature. Tables S1-S4 (Supporting Information) present parameters for both pure compounds and models that are selected for each phase. Phase distributions of FLiNaK-CsF (5 mol%) at di↵erent temperatures are computed using FactSage 8.1 Equilib module. The phase equilibria results have been compared with our MD and scattering results; however, we have not optimized any of the CALPHAD parameters. The temperature sampling rate was chosen at 1 C interval to locate the phase changes.