To assess the accuracy of FE estimates obtained from reweighted PGMs we employed the Einstein Crystal method (ECM) based on Ref. , which is representative of the state-of-the-art in the estimate of fully anharmonic FEs from MD data. In our Open MMbased implementation of the ECM we adopted a scalar spring constant (k), leading to the following potential energy expression: 55 are shown using colors corresponding to the three system sizes investigated in this study. The fluctuating coloured lines indicate the raw estimates from the models, and the fluctuating, converging black curves indicate the cumulative weighted averages of these raw estimates, obtained as discussed in section II E). Plots labeled iii display the differences in crystal-free energy between the two forms, with a line of best-fit extrapolating to approximate the same quantities in the thermodynamic limit (y-intercept). All transparent areas represent standard error bars (coloured: PGM models, black: ECM ground truth).