(3) To describe the effect of field enhancement induced by large-sized Au NPs, the mixed FDTD/TDDFT method is used. By dividing the large sized NP into Au 20 + Residue, the requirement of near-field enhancement and hot-electron injection in the water splitting reaction can be simultaneously satisfied. Setting the water near the apex of large-sized tetrahedron, OH bonds cab be disassociated by a very weak laser field.