Structural and dynamical properties predicted by reactive force fields simulations for four common pure fluids at liquid and gaseous non-reactive conditions

Four common pure fluids were chosen to elucidate the reliability of reactive force fields in estimating bulk properties of selected molecular systems: CH4, H2O, CO2 and H-2. The pure fluids are not expected to undergo chemical reactions at the conditions chosen for these simulations. The combustion' ReaxFF was chosen as reactive force field. In the case of water, we also considered the aqueous' ReaxFF model. The results were compared to data obtained implementing popular classic force fields. In the gas phase, it was found that simulations conducted using the combustion' ReaxFF formalism yield structural properties in reasonable good agreement with classic simulations for CO2 and H-2, but not for CH4 and H2O. In the liquid phase, combustion' ReaxFF simulations reproduce reasonably well the structure obtained from classic simulations for CH4, degrade for CO2 and H-2, and are rather poor for H2O. In the gas phase, the simulation results are compared to experimental second virial coefficient data. The combustion' ReaxFF simulations yield second virial coefficients that are not sufficiently negative for CH4 and CO2, and slightly too negative for H-2. The combustion' ReaxFF parameterisation induces too strong an effective attraction between water molecules, while the aqueous' ReaxFF yields a second virial coefficient that is in reasonable agreement with experiments. The combustion' ReaxFF parameterisation yields acceptable self-diffusion coefficients for gas-phase properties of CH4, CO2 and H-2. In the liquid phase, the results are good for CO2, while the self-diffusion coefficient predicted for liquid CH4 is slower, and that predicted for liquid H-2 is about nine times faster than those expected based on classic simulations. The aqueous' ReaxFF parameterisation yields good results for both the structure and the diffusion of both liquid and vapour water.