Ten Facets, One Force Field: The GAL19 Force Field for Water-Noble Metal Interfaces

Understanding the structure of the water/metal interfaces plays an important role in many areas ranging from surface chemistry to environmental processes. The size, required phase-space sampling, and the slow diffusion of molecules at the water/metal interfaces motivate the development of accurate force fields. We develop and parametrize GAL19, a novel force field, to describe the interaction of water with two facets (111 and 100) of five metals (Pt, Pd, Au, Ag, Cu). To increase transferability compared to its predecessor GAL17, the water-metal interaction is described as a sum of pairwise terms. The interaction energy has three contributions: (i) physisorption is described via a Tang and Toennies potential, (ii) chemisorption and surface corrugation rely on an attractive Gaussian term, and (iii) the angular dependence is explicitly included as a truncated Fourier series. Thirteen parameters are used for each metal surface and were fitted on 250 water adsorption energies computed at the PBE+dDsC level. The performance of GAL19 was evaluated on a set of more than 600 DFT adsorption energies for each surface, leading to an average root-mean-square deviation of only 1 kcal/mol, correctly reproducing the adsorption trends: strong on Pt and Pd but weaker on Ag, Au, and Cu. This force field was then used to simulate the water/metal interface for all ten surfaces for 1 ns. Structural analyses reveal similar tendencies for all surfaces: a first, dense water layer that is mostly adsorbed on the metal top sites and a second layer up to around 6 angstrom, which is less structured. On Pt and Pd, the first layer is strongly organized with water lying flat on the surface. The pairwise additive functional form allows one to simulate the water adsorption on alloys, which is demonstrated at the example of Ag/Cu and Au/Pt alloys. The water/Ag-Cu interface is predicted to be disordered with water mostly adsorbed on Cu which should exacerbate the Ag reactivity. On the contrary, incorporating Pt into Au materials leads to a structuring of the water interface. Our promising results make GAL19 an ideal candidate to get representative sampling of complex metal/water interfaces as a first step toward accurate estimation of free energies of reactions in solution at the metal interface.