Chapter 7: 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 quantum chemical (QM) data, describing a broad range of intramolecular 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. Good agreement was found between ReaxFF and QM for all these cases. It was 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. It is anticipated 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 for catalysis, crystal growth and crack propagation studies. A specific example of application of this force field to heterogeneous catalysis, showing an oxygen–hydrogen recombination reaction on an iron nanoparticle is provided.