Molecular dynamics simulations are carried out to compute the intradiffusion coefficients of H₂ and O₂ in H₂O for temperatures ranging from 275.15 to 975.15 K and pressures ranging from 0.1 to 200 MPa. These conditions span vapor, liquid, and supercritical conditions. For the vast majority of the state points examined, experimental data are not available. The accuracy of six H₂ and six O₂ force fields is tested in reproducing the available experimentally measured densities, self-diffusivities, and shear viscosities of the pure gas and the intradiffusivity of the gas in H₂O. Namely, we screen the H₂ force fields developed by Buch, Vrabec and co-workers, Hirschfelder et al., Cracknell, a modified Silvera–Goldman, and Marx and Nielaba. For O₂, the force fields by Bohn et al., Miyano, Coon et al., Hansen et al., Vrabec et al., and Watanabe are tested. Overall, the force fields by Buch and Bohn for H₂ and O₂, respectively, were found to perform the best, and combined with the TIP4P/2005 H₂O force field are used to compute the intradiffusivities in the entire temperature and pressure range. The new data are used to develop an engineering model that can predict the H₂ and O₂ intradiffusivity in vapor, liquid, and supercritical H₂O. The new model uses 11 parameters and has an accuracy of 4–11%. The model is validated with other available experimental and simulation data for H₂ and O₂ in H₂O and pure H₂O. Aside from the extensive collection of new data for the intradiffusivities of H₂ and O₂ in H₂O, we present new data for the densities, shear viscosities, and self-diffusivities of pure TIP4P/2005 H₂O in the same wide temperature and pressure range. The new data and the engineering model presented here can be used for the design and optimization of chemical processes, for which the knowledge of H₂ and O₂ diffusivities in H₂O is important.

The translational self-diffusion coefficient and the shear viscosity of water are related by the fractional Stokes–Einstein relation. We report extensive novel molecular dynamics simulations for the self-diffusion coefficient and the shear viscosity of water. The SPC/E and TIP4P/2005 water models are used in the temperature range 220–560 K and at 1 or 1,000 bar. We compute the fractional exponents t, and s that correspond to the two forms of the fractional Stokes–Einstein relation $(D/T)\sim {\eta }^t$ and $D\sim (T/\eta {)}^s$ respectively. We analyse other available experimental and numerical simulation data. In the current analysis two temperature ranges are considered (above or below 274 K) and in both cases deviations from the Stokes–Einstein relation are observed with different values for the fractional exponents obtained for each temperature range. For temperatures above 274 K, both water models perform comparably, while for temperatures below 274 K TIP4P/2005 outperforms SPC/E. This is a direct result of the ability of TIP4P/2005 to predict water densities more accurately and thus predict more accurately the water self-diffusion coefficient and the shear viscosity. GRAPHICAL ABSTRACT