For FLiNaK, a deep neural network-based forcefield was developed in our previously published study, and simulation details can be found in the literature. The forcefield can be found on Github. For FLiNaK with 5 mol% CsF, AIMD simulations were performed using Vienna Ab initio Simulation Package (VASP), version 5.4.4. The simulation box contained 106 atoms with 53 F, 23 Li, 6 Na, 21 K pv and 3 Cs sv (true names in the VASP repository). We used the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional and the Projector Augmented Wave (PAW) pseudopotential. We also included the dispersion force using the vdW-DF method with the optB88 functional because it predicted the density of similar salts (FLiNaK with Cr impurities) very well. A 1×1×1 k-point mesh was chosen. Gaussian smearing was used with a width of 0.1 eV. The planewave cuto↵ energy was set to 520 eV, which is approximately 1.3 times the maximum ENMAX of all the elements. The time step was set to 1 fs with NVT ensemble using Nosé-Hoover thermostat. The initial configuration was constructed by randomly assigning the positions of the atoms, and the minimum initial distance between atoms was set to 1.8 Å. We first simulated the system with multiple volumes at T = 800, 900, and 1200 K to obtain the density at 0 atm as a function of temperature. We fitted the pressure-volume dependence to the thirdorder Birch-Murnaghan equation of state. The correlation between the temperature and the density was found to be ⇢(kg/m 3 ) = 2759.6 0.587 • T (K). There are no measurements of the density of FLiNaK-CsF but this density is similar to that measured in FLiNaK. Finally, we yielded a total of 35 ps of production trajectories at 800 K. The number of time steps were set to 7000 for each independent run at equilibration and production phases. The X-ray weighted structure factors were computed using LiquidLib.