(427f) Free Energies Calculations of Methanol-Pt(111) Under Liquid Solvent: Development and Assessment of an Interfacial Force Field

Accounting for solvent effects on the thermochemistry of liquid/metal interfaces—e.g., as employed in heterogeneous catalysis, electrochemistry, and biomedicine—is a grand challenge; however, characterization of solvent effects is challenging because of configurational disorder in the solvent structure. Our group previously developed a multiscale sampling strategy for simulating configurational influences to the free energies of species adsorbed to solid surfaces under liquid solvent and showed that this method can reproduce exactly the free energies calculated with empirically-based implicit solvation for systems that do not interact strongly with the liquid environment. For systems that do interact strongly with the liquid environment, our method serves as an improvement. However, our method does not account for thermal fluctuations in the internal geometry of the surface species, which could influence the free energies via conformational changes in the adsorbed species, which impact the local structure of liquid molecules. Herein, we investigate how conformational changes of adsorbate species influence their free energies. To do this, we develop a force field (FF) that can capture chemical bonding between the catalytic species and the catalyst surface as well as thermal fluctuations of the catalytic species. Specifically, we derive the MePt-FF, which can be used to model CH₃OH adsorption to Pt(111). To map the CH₃OH-Pt potential energy surface, we construct a set of 600 frames by dragging the CH₃OH to (out-of-plane, Figure 1a) and over (in-plane, Figure 1b) the Pt(111) surface. Partial optimizations of the CH₃OH are performed using the VASP code. Figure 1c shows a parity plot comparing the CH₃OH-Pt binding energy computed with DFT to that generated from MePt-FF. We demonstrate the use of MePt-FF in calculating the free energy of CH₃OH under liquid solvent and compare with our prior method to elucidate the influence of conformational changes on the free energy of the CH₃OH/Pt(111) system.