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Enhancing the formation of ionic defects to study the ice Ih/XI transition with molecular dynamics simulations

Author(s): Piaggi, Pablo M; Car, Roberto

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Abstract: Ice Ih, the common form of ice in the biosphere, contains proton disorder. Its proton-ordered counterpart, ice XI, is thermodynamically stable below 72 K. However, the formation of ice XI is kinetically hindered, and experimentally it is obtained by doping with KOH. Doping creates ionic defects that promote the migration of protons and the associated change in proton configuration. In this article, we mimic the effect of doping with a bias potential that enhances the formation of ionic defects in molecular dynamics simulations. The recombination of the ions thus formed proceeds through fast migration of the hydroxide along hydrogen bond loops, providing a physical and expedite way to change the proton configuration. A key ingredient of this approach is a machine learning potential trained with density functional theory data and capable of modelling molecular dissociation. We exemplify the usefulness of this idea by studying the order-disorder transition using an appropriate order parameter that distinguishes the proton environments in ice Ih and XI. We calculate the changes in free energy, enthalpy, and entropy associated with the transition. Our estimated entropy agrees with experiment within the error bars of the calculation.
Publication Date: 29-Apr-2021
Electronic Publication Date: 29-Apr-2021
Citation: Piaggi, Pablo M, Car, Roberto. (2021). Enhancing the formation of ionic defects to study the ice Ih/XI transition with molecular dynamics simulations. Molecular Physics, 119 (19-20), 10.1080/00268976.2021.1916634
DOI: doi:10.1080/00268976.2021.1916634
ISSN: 0026-8976
EISSN: 1362-3028
Language: en
Type of Material: Journal Article
Journal/Proceeding Title: Molecular Physics
Version: Author's manuscript

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