Science - Written by on June 1, 2009

Supercomputing Tests the Waters

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This is a typical configuration of liquid water. Reds are oxygen, white hydrogen and grey bonds within a molecule. The blue walls show the electron density when a molecule is on the surface of the cube. Image credit: David Ceperley and John Gergely (NCSA)

This is a typical configuration of liquid water. Reds are oxygen, white hydrogen and grey bonds within a molecule. The blue walls show the electron density when a molecule is on the surface of the cube.
Image credit: David Ceperley and John Gergely (NCSA)

Simulations explore mysterious properties of Earth’s most abundant molecule

The mysterious properties of water molecules keep our planet warm and full of life. Although water is one of our most common and vital resources, many of its characteristics are still a mystery.

David Ceperley and John Gergely, physicists at the Department of Physics and at the National Center for Supercomputing Applications at the University of Illinois, Urbana-Champaign, will use the Cray XT Jaguar supercomputer at Oak Ridge National Laboratory to create one of the most accurate descriptions of water’s microscopic properties.

“Our project is to calculate the energies and forces of water molecules. These types of calculations are part of a long-term goal of being able to design new materials,” Ceperley said. “If you’re looking for materials with certain properties, then you search through lots of different compounds and get just a few likely candidates. Computers should help the search for viable new compounds.”

Water’s presence in all biological matter and its molecular properties are of interest in many different fields. One area of interest is trying to understand its existence and behavior in extreme conditions such as those found on other planets. Another is to shed light on how proteins surrounded by water in our bodies are affected by the forces and movement of the water molecules. Understanding the nature of water in highly confined spaces, such as nanotubes, could lead to the development of technology to supply clean water. The possibilities are almost endless.

Water, a versatile substance, is complicated to study. Many factors can alter it at the atomic scale, and these tiny alterations are reflected in properties on a much larger scale. For example, in liquid water, molecules cluster in a tetrahedral, or pyramid-shaped, structure, but water molecules in ice group as a hexagonal structure like a snowflake. The rearrangement of hydrogen bonds between molecules is the difference between solid and liquid water.

Water’s complex nature has slowed down water research. Many models are accurate only under specific conditions and cannot be transferred to other phases of water or environments. Ceperley will address the melting and freezing points of water, which in some theoretical models are different from those found experimentally. This means that other predictions coming from computer calculations will not necessarily be correct.

Ceperley’s group will use Quantum Monte Carlo methods to compute how the electrons arrange themselves in a small sample of water.

“The idea of Quantum Monte Carlo is that you move the electrons around randomly,” Ceperley said. “Monte Carlo is named after the game of chance or roulette, but with rules. Quantum just means you’re using the algorithms to address the quantum mechanical properties of the electrons. This is the first time Quantum Monte Carlo methods have been used to do a problem like this, meaning a liquid.”

This method used on Jaguar, the world’s fastest supercomputer for open scientific research, may lead to more accurate results than obtained by other methods.

Ceperley and Gergely will be able to see where the discrepancies lie between the theoretical models and experimental results of freezing and melting points. This more accurate model of electron distribution and movement could, unlike current models, be used for ice, water, steam, and other environments. Other researchers will use these models to make their own models and experiments more accurate.

Another little-understood property of water is the quantum effects of protons in it. Funded by the Department of Energy’s Scientific Discovery through Advanced Computing program, Ceperley’s team will use 32 million processor hours on Jaguar in 2009 to try to figure out how the protons affect water. This is a challenge because the effects of protons are very similar to those caused by electrons.

This project will provide more accurate information than has previously been available and bring theoretical models closer to what is seen in experiments.

“The goal is to go beyond pure water and do more complex things,” Ceperley said. “In the long term we hope to learn how to do the electronic calculations for any type of system made out of ions and electrons. Because that’s what everything is made out of.” — Elizabeth Storey

Oak Ridge National Laboratory is supported by the US Department of Energy’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.