From Fig. , we note that the vibrational free energies of the end-members, particularly for NaCl, are predicted to be too low compared to the DFT results, thus resulting in an overstabilisation of the pure phases, which should lead to a less miscible solid-solution. Constraining the end-members free energies to those calculated from DFT-based lattice dynamics, thus leads to a further stabilisation of the solid-solution, and with a maximum temperature of the miscibility gap of 850 K, which is less than 100 K above the experimental observation. Furthermore, For the ones with vibrational entropy included, the red curve marked by an asterisk (*) is the one where the vibrational free energies of the end-members are calculated with DFT. Experimental results from Nacken (N) , Bunk and Tichelaar (BT) , and Barrett and Wallace (BW) tabulated by Thompson and Waldbaum are given for comparison.