
The Enigmatic SubstanceWater 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
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