Self-diffusion coefficient. As with the other properties examined in this work, significant improvement in the reproduction of the experimental self-diffusion constant occurred from about 2005 onwards (Figure ). Notable exceptions occurred where structuring was of higher priority, such as TIP4P/Ice (for better reproduction of the phase diagram), SPC/ε and FBA/ε (for better reproduction of the dielectric constant), and TIP3P-ST (for better reproduction of the surface tension). Conversely, the TIP5P-2018 model deliberately sacrificed good agreement with the experimental dielectric constant in order to better reproduce the experimental self-diffusion coefficient. With the exception of TIP3P-FB, the TIP3P-based models consistently failed to reproduce the experimental self-diffusion coefficient of water. This is perhaps not surprising, as the original TIP3P model was developed for Monte Carlo, not MD, simulations, and the self-diffusion coefficient was not part of the parameterization process or reported in the original paper. The CVFF, PCFF, and COMPASS models also failed to reproduce the experimental self-diffusion coefficient. These models were parameterized to be used with the commercial simulation package Discover, 138 and indeed a selfdiffusion coefficient of 2.45 × 10 -5 cm 2 /s was reported for CVFF water when a quintic spline switching function (not currently available in LAMMPS) was used for the non-bonded interactions. While an anomalously high diffusion coefficient is not generally desirable, it can be used in some cases to deliberately speed up conformational sampling, as an alternative to other advanced sampling methods. Self-diffusion coefficients depend on the size of the simulation box, and calculated values increase as the box size increases. 145