The effect of the applied trajectory length on the convergence of the self-diffusion coefficient was examined for the SPC/E water model in the NVT ensemble with different system sizes at 293K. Temperature dependence and isotope effects, via using D 2 O instead of H 2 O, were also investigated. A simulation for the polarizable SWM4-DP model was also carried out to compare the effect of different potential models. Radial distribution functions and the neutron weighted structure factor were also calculated; they were found to be insensitive to changing the system size in the range of 216 to 16,000 molecules. On the other hand, the diffusion coefficient is rather sensitive to the applied trajectory length, system size and the method of calculation. The diffusion coefficient is therefore not appropriate for assessing, and distinguishing between, potential models of water, whereas the structure factor could serve as a more stable measure.