Water Structure and Energetics. For neat water (Table ), the combined effect of repartitioning the masses (m r ) and decreasing the total mass (m tot ) by a factor of ≈ 26 does not substantially change the average potential energy per molecule, ⟨E pot ⟩ /N , or the average volume, ⟨V ⟩, while substantially increasing the diffusion coefficient, D, of the water molecules. Even though the change in the potential energy is minor, it is statistically significant. Similar systematic tendencies in the potential energy as a function of the time step have been reported previously. Additionally, as Figure shows, our approach leaves the radial distribution functions unchanged. time step, we expect similar effects for the TIP3P-F model presented here. We performed additional simulations using the protocol and temperature (298.15 K) reported by Asthagiri and Beck , and calculated the temperatures associated with the translational (T trs ) and rotational (T rot ) degrees of freedom, using the original TIP3P model, the model with hydrogen mass repartition-only (TIP3P-R), and the fast water model (TIP3P-F). Asthagiri and Beck tested equipartition with a velocity Verlet integrator combined with either the velocity rescaling or Langevin thermostat. Here, we used the velocity Verlet integrator with the velocity rescaling thermostat (VV-VR), and the Leap Frog integrator with velocity rescaling (LF-VR) or Langevin thermostats (LF-L). Supporting Figure shows our results for all three integrator combinations and all three water models. While increasing the time step leads to an increase in T trs and a decrease in T rot in case of VV-VR, the combinations LF-VR and LF-L showed the opposite tendency. Systems with only repartitioned hydrogen masses (without rescaling) are basically unaffected by the integration time step, yielding T trs and T rot close to the target value. By contrast, deviations between the two temperatures are amplified with the TIP3P-F model as the integration time step increases. In their work, Asthagiri and Beck 34 attribute this behavior to the short relaxation times of the rotational velocity, comparable to the time scale of vibrational modes. Therefore, such deviations in the energy repartition are inherent to the rigid body description of water and further work is required to elucidate its effect on dynamics and thermodynamics of biological systems. For algorithms requiring the sampling of canonical partition functions, such as replica exchange, small deviations from the Boltzmann distribution can be amplified. To demonstrate the increase in sampling efficiency and the preservation of equilibrium properties by the fast water model, we performed molecular dynamics simulations of alanine polypeptides. In particular, the alanine dipeptide serves as a minimal system to mimic the backbone dynamics of a protein. Additionally, the alanine penta-and decapeptides can exhibit transitions between secondary structure elements commonly found in folded proteins.