Systematic Bottom-Up Coarse-Graining of Hydrated Excess Proton Transport Across Scales

Proton transport is a multiscale phenomenon critical to chemistry, physics, biology, and materials science, yet development of a high-fidelity multiscale model bridging quantum to mesoscale dynamics remains a major challenge due to reactive bonding rearrangements in the Grotthuss proton shuttling. Here, we present HUMID, a Hydronium Ultra-coarse-grained Model with Improved Dynamics, which enables the investigation of hydrated protons across much larger spatiotemporal scales via bottom-up coarse-graining that recapitulates structure and dynamics at a reduced resolution. The unique structural correlations and dynamics arising from the hydronium cation are captured by introducing internal states to the coarse-grained sites, as derived from quantum mechanics, with a systematic design principle for determining these internal states. HUMID accurately reproduces diffusion and hydronium time correlation functions with a three-order-of-magnitude computational speedup. This model lays a foundation for studying mesoscale phenomena governed by proton transport and establishes design principles for reactive CG models.