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Characterizing structural and phase transformations of water at the molecular level is key to understanding a variety of multiphase processes ranging from ice nucleation in the atmosphere to hydration of biomolecules and wetting of solid surfaces. In this study, state-of-the-art quantum simulations with a many-body water potential energy surface, which exhibits chemical and spectroscopic accuracy, are carried out to monitor the microscopic melting of the water hexamer through the analysis of vibrational spectra and appropriate structural order parameters as a function of temperature. The water hexamer is specifically chosen as a case study due to the central role of this cluster in the molecular-level understanding of hydrogen bonding in water. Besides being in agreement with the experimental data available for selected isomers at very low temperature, the present results provide quantitative insights into the interplay between energetic, entropic, and nuclear quantum effects on the evolution of water clusters from "solid-like" to "liquid-like" structures. This study thus demonstrates that computer simulations can now bridge the gap between measurements currently possible for individual isomers at very low temperature and observations of isomer mixtures at ambient conditions.

The water dimer cation, (H2O)2+, has long served as a prototypical reference system for water oxidation chemistry. In spite of this status, a definitive explanation for the anomalous—and dominant—features in the experimental vibrational spectrum [Gardenier, G. H.; Johnson, M. A.; McCoy, A. B. J. Phys. Chem. A, 2009, 113, 4772–4779] has not been determined, and harmonic analyses qualitatively fail to reproduce these features. In this computational study, accurate quantum chemistry methods are combined with a fully coupled, six-dimensional anharmonic model to show that the unassigned bands are the result of resonant mode interactions and strong anharmonic coupling. Such coupling is fundamentally due to the unique electronic structure of this open-shell ion and the manner in which auxiliary modes affect the natural charge-transfer properties of the shared-proton stretch. These unique vibrational signatures provide a key reference point for modern spectroscopic and mechanistic analyses of water-oxidation cata...