A Roaming-like Mechanism for Hydrogen Bond Network Rearrangements in Water Clusters

Unraveling hydrogen bond network rearrangements governing water reactivity is central to the physical and life sciences. Water mediates roaming in other systems, but whether water itself can roam remains unresolved. By mapping the potential energy surfaces of the water dimer and trimer, which together exhaust the elementary rearrangements, we identify five candidate roaming pathways that share reactants and products with the known rearrangements but traverse a flat region near the O–H dissociation asymptote. Each roaming-like saddle point sustains a spin-polarized, broken-symmetry state with near-unity spin population on the migrating hydrogen, consistent with neutral radical character. Interpretable machine learning reaches 93.1% accuracy in buffered within-scan tests and quantifies descriptor contributions, complementing the electronic-structure characterization, although transfer to withheld scans is limited and a distance-and-cluster-size baseline performs comparably. These findings point to roaming-like H atom migration as a candidate elementary channel of hydrogen bond network rearrangement in water clusters, with implications for radical chemistry in extended networks.