The Enigmatic Substance

Water is ubiquitous. It covers two-thirds of the globe, and it accounts for 70 percent of our body's weight. Life on Earth -- animal and plant -- would cease to exist without it. You might think that by now scientists would know all there is to know about water, but many questions remain about this essential and, in many ways, mysterious substance.

No one understands, for instance, why liquid water is able to carry so much heat. "If the heat capacity of water were half of its actual value," says chemist Ken Jordan of the University of Pittsburgh, "the temperature fluctuations in our environment would be more extreme, and this would have important implications for life itself."

Another unanswered question involves its structure on the molecular level -- specifically in the top one or two layers of liquid water where it meets water vapor in the air. "No calculation carried out to date," Jordan says, "is able to account quantitatively for the surface tension of liquid water."

Insight into these questions can be provided by computer modeling of water clusters, a research interest Jordan is pursuing at the Pittsburgh Supercomputing Center. Water clusters are small groupings of water molecules that differ in many ways from "bulk" water, such as you find in a glass of water. The biggest difference is that all or most of the molecules in a small cluster are on the surface, where they have fewer chemical interactions with other water molecules than do the molecules in bulk water.

Jordan is one among a handful of scientists applying quantum mechanical calculations to the study of water clusters, work which can test the usefulness of the water-interaction models used in simulating proteins and DNA. His calculations aim at accurately mapping the potential-energy surfaces for different-sized water clusters. His calculations have also explored how the phase transition from solid to liquid water differs in water clusters from the behavior of bulk water.

Three representative low-energy structures of a cluster of six water molecules. Bonds (purple lines) connect hydrogen atoms (gray) to oxygen (red).

Researcher: Ken D. Jordan, University of Pittsburgh.
Hardware: CRAY Y-MP C90
Software: GAUSSIAN 92, Molpro
Keywords: water clusters, water, potential energy, quantum chemistry, GAUSSIAN, phase transitions.

Related Material on the Web:
Information including animation from Ken Jordan's theoretical chemistry group.
Projects in Scientific Computing

References, Acknowledgements & Credits