September 22, 2022

Understanding the Ways of Water

Molecular-level calculations of water are now faster and easier

Water molecules with an ion

Water molecules interact with ions and each other to give the liquid unique properties.

(Illustration by Stephanie King | Pacific Northwest National Laboratory)

Representing roughly 60 percent of the human body, water is a critical component of life. Nestled within and around our cells, water plays a crucial role in many biological and chemical reactions required to keep us alive.

Despite its importance, scientists have yet to fully understand the molecular-level origin of the numerous anomalous macroscopic properties of water. Special interactions between water molecules give the liquid unique properties that are difficult to model. When additional components, such as ions, are added to water, the calculations to help scientists understand these interactions become even more complex.

Researchers at Pacific Northwest National Laboratory (PNNL), the University of Washington, and the University of Cambridge found a way to simplify these calculations. Their research was published in The Journal of Chemical Physics and selected for the Kudos Research Showcase.

In aqueous solutions, where ions are dissolved in water, the strong nonadditive interactions between molecules and ions present a challenge in accurately partitioning the energy of the system. Describing such a system typically requires tens of thousands of high-level calculations. This takes a lot of time and computational power to get through.

Researchers led by PNNL Lab Fellow Sotiris Xantheas developed a faster and more efficient way to portray the system while still maintaining accuracy. They created a classical representation to evaluate the three-body ion–water–water and water–water–water interactions in aqueous ionic systems.

“Since most of the nonadditive interactions in water are due to polarization, we figured we could save computational time by representing this classically, rather than fitting the model to numerous ab initio calculations,” said Xantheas.

The challenge for these researchers was to make sure that their classical calculations were just as accurate as the ones obtained from ab initio simulations. They benchmarked their method against an accurate data set for 13 different ion systems and found excellent agreement between the two.

This approach provides a fast, efficient, and accurate path toward modeling the three-body term in aqueous ionic systems without the need for tens of thousands of ab initio calculations. It also appears to be fully transferable across systems with different ions.

This work was supported by the Center for Scalable Predictive Methods for Excitations and Correlated Phenomena (SPEC), which is funded by the Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division as part of the Computational Chemical Sciences (CCS) program at PNNL. This research also used computer resources provided by the National Energy Research Scientific Computing Center supported by the Department of Energy, Office of Science. Xantheas also holds a dual appointment with the University of Washington, Department of Chemistry.