Pathways for the formation of ice polymorphs from water predicted by a metadynamics method

Area of Science:

• Computational Physics
• Materials Science
• Physical Chemistry

Background:

• Understanding ice crystal formation mechanisms is crucial but challenging due to limitations in traditional simulation timescales.
• Molecular dynamics (MD) simulations offer molecular-level insights but are often restricted to microsecond timescales, insufficient for observing crystal nucleation and growth.
• Existing research highlights the need for advanced computational methods to bridge the gap between simulation capabilities and real-world ice formation phenomena.

Purpose of the Study:

• To overcome the timescale limitations of conventional MD simulations for studying ice crystal formation.
• To develop and apply a computational method capable of reproducing the formation of diverse ice polymorphs.
• To investigate the free-energy landscape governing the formation pathways of different ice structures.

Main Methods:

• Employed a metadynamics (MTD) enhanced molecular dynamics (MD) simulation approach.
• Utilized two discrete oxygen-oxygen radial distribution functions, represented by Gaussian window functions, as collective variables.
• Systematically varied Gaussian window function parameters to explore different ice crystal structures.

Main Results:

• Successfully reproduced the formation of various ice polymorphs, including cubic ice, stacking disordered ice, hexagonal ice, and high-pressure ice VII.
• Observed the formation of layered ice structures with both known (ice VII) and previously unknown structures.
• The generated free-energy landscape indicated that ice crystal formation proceeds through high-density water intermediates structurally similar to the resulting ice.

Conclusions:

• The MTD-enhanced MD method effectively overcomes timescale limitations, enabling the study of ice crystal formation mechanisms.
• This computational approach can successfully predict the formation of known ice polymorphs and potentially discover novel crystalline structures.
• The findings provide valuable insights into the molecular-level processes governing water's phase transitions and crystal nucleation.