CHAPTER 6: A Reaxff Reactive Force-field for Proton Transfer Reactions in Bulk Water and its Applications to Heterogeneous Catalysis

We have developed a ReaxFF reactive force-field description for bulk water and for proton transfer in the aqueous phase. This ReaxFF potential was parameterized exclusively against quantumchemical (QM) data, describing a broad range of intra- and intermolecular water interactions, including H–H, HO–OH and O=O bond dissociation energies, charge distributions, angle bending energies, binding energies for [H₂O]_2–35-clusters, H-transfer reactions pathways in H₃O⁺/H₂O, [H₂O]_n and HO⁻/H₂O-clusters and ice densities and cohesive energies. We find good agreement between ReaxFF and QM for all these cases. We found that this QM-based ReaxFF potential gives a good description of bulk water phases and proton migration, reproducing experimental density, radial distribution and diffusion data, while it overestimates proton diffusion in the OH⁻/water system. We anticipate that this ReaxFF water description, in combination with ongoing ReaxFF-work on amines, carboxylic acids, phosphates and inorganic materials, should be highly suitable for simulating biochemical reactions involving enzymes and DNA. Furthermore, this potential can be used to simulate water reactions and proton diffusion on metal and metal oxide surfaces, opening up applications to catalysis, crystal growth and crack propagation studies. We have provided a specific example of this force field to heterogeneous catalysis, showing an oxygen/hydrogen recombination reaction in an iron nanoparticle.