Using the ground truth method described above (ECM), a total of 1.84 µs of data was sampled, with 0.32 µs of this data (17.4%) sampling the physical ensembles (i.e., λ = 1). This percentage is > 5% because each physical simulation was four times longer than any other simulation of the same ECM run to collect sufficient MD data for later use in the PGM models. Collecting this data in the context of ECM ensured that the average potential energy in each system was the same across all FE methods being compared in this work. Notably, one major methodological discrepancy between the ground truth method and the PGMs can be linked to how the center of mass was removed. In EC calculations, the center of mass of the supercells was removed based on the position of all atoms (including the hydrogens).