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Groundbreaking Simulations of Water's Phase Diagram Advance Study of Complex Molecular Systems

Published June 13, 2023

Comparison between the experimental (left) and simulated (right) phase diagram of water over a wide range of temperature and pressure conditions. The areas colored in green, orange, gray, brown, pink, and blue correspond to the regions of stability for ice Ih, II, III, V, and VI and liquid water, respectively.  Credit: Paesani Lab, UC San Diego

By Kimberly Mann Bruch and Cynthia Dillon

Water, despite its simple molecular structure, behaves in ways that have long puzzled scientists. While it is universally recognized as essential to life, water's anomalous properties present unique challenges for computational modeling. But in a groundbreaking study published in Nature Communications, scientists from UC San Diego have made a major stroke in simulating water's phase diagram with remarkable precision using Expanse at the San Diego Supercomputer Center (SDSC).

As a refresher, a phase diagram is a visual map that illustrates the phase changes of a given substance under varying pressure and temperature conditions. For water, these phases could be ice, liquid and vapor.

The UC San Diego research team, led by Professor Francesco Paesani (Department of Chemistry and Biochemistry, Materials Science and Engineering and Halıcıoğlu Data Science Institute), have achieved a feat considered unattainable by providing a comprehensive computational representation of water's phase diagram that aligns with experimental measurements. According to Paesani, his team’s work has culminated in a model that achieves what has been an aspirational goal for the scientific community since 1971, when scientists first began working to develop a molecular model capable of accurately predicting the properties of water across various phases.

How’d they do it? With an innovative approach that uses a data-driven, many-body model called MB-pol, marking a milestone in better understanding fundamental science about water and setting a solid foundation for future research in a wide range of scientific fields.

“Water's simple formula belies its complex behavior,” Paesani said. "Using MB-pol, we've been able to model water across a wide range of temperatures and pressures, providing insights into how factors such as enthalpic, entropic and nuclear quantum effects shape its free-energy landscape. This work illustrates how recent advancements in first-principles, data-driven simulations have opened the door to realistic computational studies of complex molecular systems.”

The team's use of the MB-pol potential model, combined with advanced enhanced-sampling algorithms, represents a novel approach to the computational study of water.

“Our success with the MB-pol model is built on decades of collective scientific exploration and curiosity—we are standing on the shoulders of giants,” Paesani said. “MB-pol is the first model capable of accurately predicting the properties of water in all its phases—from isolated molecules in the gas phase to liquid water and various forms of ice. This represents a quantum leap in our understanding of one of the most fundamental and complex molecules.”

Sigbjørn Bore, a postdoctoral researcher worked with Paesani on this study. “Our results indicate that it will soon be possible to perform realistic molecular simulations of complex systems, bridging the gap between computer modeling and experiments,” he said. “Our work provides a more accurate model for studying phenomena such as supercooled water and ice nucleation. It's a game-changer.”

According to the UC San Diego scientists, not only does this research have profound implications for the study of water, but it also sets the stage for potential applications of the MB-pol model to other complex molecular systems. This development could lead to significant breakthroughs in diverse fields like drug design, climate modeling, developing efficient materials for water harvesting and water desalination, and understanding biological processes on a molecular level.

“The successful application of the MB-pol model to water's phase diagram signals a new era of advanced molecular modeling, marking a milestone in a journey that started over fifty years ago,” Paesani said. “This research has the potential to revolutionize our understanding of the molecular world and offer invaluable insights for a multitude of scientific disciplines.”

In addition to Expanse at SDSC, located on the UC San Diego campus, Paesani and Bore used the Delta supercomputer at the National Center for Supercomputing Applications (NCSA), as well as computational resources from the Department of Defense High Performance Computing Modernization Program and the Scientific Computing Core at the Flatiron Institute, which is a division of the Simons Foundation.

“Given the computational demands of our extensive simulations, it is important to underscore that our comprehensive explorations of water's phase diagram, spanning wide temperature and pressure ranges over long timescales, would not have been possible without the availability of high-performance computing resources,” Bore noted.

Paesani also said that the Air Force Office of Scientific Research (AFOSR) support for the team’s work enabled this pivotal step forward in computational molecular science and could potentially revolutionize our understanding of the molecular world around us.

The research was funded by the AFOSR through the Multidisciplinary University Research Initiative (MURI) “Unraveling the Mechanisms of Ice Nucleation and Anti-Icing Through an Integrated Multiscale Approach” (grant no. FA9550-20-1-0351). Computational work on Expanse was funded by the National Science Foundation Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS, allocation no. CHE110009).