Scientists have long realized that water clusters could test the usefulness of water-interaction models for environments other than bulk water. The lack of experimental data and the difficulty of doing accurate quantum chemical computations, however, have hampered progress. Most modeling, points out Jordan, treats the interaction between two water molecules as independent of interactions with other water molecules in the system, even though it is well known that these other interactions have an effect. In a cluster of three water molecules, for example, the interaction between two of them distorts the electron distribution on the third, which in turn modifies the first interaction. How to account for this distortion -- known as "polarization" -- is the main challenge, says Jordan, to developing reliable models, and this is where quantum mechanical calculations come into play.
Jordan is one among a handful of scientists using quantum calculations to study water clusters. It is only recently, he says, that the computer firepower needed to do these computations on small clusters became available. Such calculations give the geometries and binding energies of the clusters and provide other data important for testing water-interaction models. The CRAY C90 at Pittsburgh made it feasible for researchers C. J. Tsai and K. Kim in Jordan's group to carry out these calculations using the GAUSSIAN 92 and Molpro software packages.
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