Interestingly, the acceleration in the relaxation times is somewhat less than the roughly three-fold change in solvent self-diffusion or viscosity. For the solutes, the autocorrelation times instead decrease by about a factor of two. To explore this effect, we performed molecular dynamics simulations of the alanine dipeptide by scanning through different total masses of the water molecules, using τ int of the ψ dihedral as a reporter variable. The results in Figure show that autocorrelation times are approximately proportional to the square root of the water mass, as should be expected according to eq 3, and as seen before for peptide. Moreover, a least squares fit in the square root of the mass scale reveals that solute autocorrelation times does not go to zero after extrapolation to zero-mass. The parameters of the fits are reported in Table . A small upturn of τ int as m tot → 0 is not statistically significant, but would be consistent with a transition into the Kramers low-friction regime, where the exchange of energy between solute and solvent degrees of freedom becomes inefficient. However, much shorter time steps would be required for a firmer assessment of this interesting Therefore, as the water mass approaches zero, the dynamics of solute degrees of freedom is dominated by a combination of intramolecular and thermostat friction. This "residual" autocorrelation time is characterized by the a parameter in Table , which quantifies the "internal friction" associated with couplings to intramolecular motions combined with thermostat friction. In practical terms, a represents a limit in the maximum speed up that can be gained by scaling the solvent mass. This theoretical limit is well within the bounds of integration stability of the TIP3P-F model and provides justification for our final water mass. As Figure shows, by using a total mass, m tot = 1.116 au, we are already within error bar of the theoretical limit of the alanine dipeptide system.