For masses (m tot , m r ) = (0.766, 3.0), (0.696,4.0) and (0.694,4.5), we obtained diffusion coefficients of 27.52 ± 0.01, 28.44 ± 0.02, and 28.31 ± 0.04 nm 2 /ns, respectively. Therefore, for our purpose the optimal value for mass repartitioning is m r = 4.0, as it results in the fastest diffusing water for fixed k crash . Interestingly, the same amount of mass is repartitioned from carbon to methyl hydrogen atoms in force fields employing HMR. The masses of the original TIP3P and final TIP3P-F water models are / ns] Figure 3: Self-diffusion coefficient of neat TIP3P water at T = 310 K and p = 1 bar as a function of the total mass m tot of water molecules for fixed mass repartitioning m r = 4.5 (symbols). The line corresponds to ideal scaling of the diffusion coefficient (line) according to eq 3. Results are not corrected for finite-size effects. Data point are averages over two independent replicas. The differences between the two runs are smaller than the symbols. The inset shows the deviation of the diffusion coefficient from the ideal scaling. presented in Table for reference. While developing the water model required us to use parameters that result in a measurable number of crashes, for production purposes we used a more conservative value of m tot = 1.116 au, hence k crash was on the order of 10 -11 (ns • 1000 mols) -1 . I.e., for a system of 1 million water molecules, we would expect one crash in about 100 milliseconds of MD.