Solvation occurs when molecules of a solvent surround and stabilize those of a solute. The solvent molecules are always in random thermal motion--the higher the temperature, the faster they move around. As the solvent molecules move, the solute sees a constantly changing environment--sometimes surrounded by six solvent molecules, sometimes seven, sometimes turned this way, sometimes that. All these different environments yield slightly different solute energies such that the solute energy fluctuates rapidly.

These solvent-driven energy fluctuations have a crucial effect on the outcome of a chemical reaction. The problem, however, is that there are limits to physical methods used to study chemical interactions, such as optical spectroscopy, a technique that uses light to examine molecular interactions. "Spectroscopy can give you a pretty detailed picture of the timescales of motions in the system, but it can't actually tell you what's moving around in the system," explains Matt Zwier, a 2004 Hope College graduate who has been a student of Krueger's. However, says Zwier, simulating the interaction between solvent and solute provides a way to study these movements that spectroscopy doesn't. "A simulation will allow you to see what's actually moving."

Largely using NCSA's Platinum and Titan Linux clusters, Krueger and Zwier are working on perfecting a computational approach that uses a combination of molecular dynamics and quantum mechanics to identify and calculate the movements of solvent molecules and their effect on the excitation energy of solute molecules. Their method is based on an earlier method developed by Ian Mercer, Ian Gould, and David Klug of Imperial College in London. It combines classical mechanics--specifically molecular dynamics (MD)--with quantum mechanics (QM) to calculate the optical properties of a solute-solvent system.

Krueger emphasizes that while there exist a number both of computational and experimental methods for studying solvation, "there's not a strong connection between computational and experimental research. So one of the things we're trying to achieve is to connect our computational method very directly with experimental results." The experimental and computational parts of their work are complementary; each allows them to examine details of the interaction that the other might leave obscure.

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The MD/QM simulation.