The simulation that forms the basis of Krueger and Zwier's work involves a single solute molecule, in this case a dye called oxazine-4. The oxazine-4 molecule is surrounded by about 12,000 methanol molecules, which constitute the solvent. "Basically…we're looking at the fluctuations that occur as the system just sits there at room temperature," says Krueger. "We're not doing anything to the system; it just sits there, with all the oxazine and methanol molecules moving around…all those little fluctuations tell us about how the solute and solvent are interacting."

The molecular dynamics component of the simulation, which is applied classical mechanics, involves sampling all the configurations that take place as the system fluctuates and taking periodic snapshots that show the positions of all the atoms at a given point in time. "The classical mechanics part [of the simulation] is valuable to us because it's a very simple treatment, so that we can afford to have a very large system with 12,000 methanol molecules," Krueger explains. "We can treat it for a fairly long time and get millions of different snapshots that show how all the atoms are arranged."

However, classical mechanics does not work for elementary particles--in this case, electrons. Therefore, a quantum mechanics component, which helps to predict the behavior of electrons, is used to calculate the excitation energy of the solute for each of the millions of snapshots. "The oxazine molecule is bathed in all these tiny charges from the methanol molecules, and in each snapshot the 80,000 charges from the methanol molecules are going to be a little bit different and so, therefore, is the oxazine molecule," Krueger explains. "When we do the quantum mechanics, it registers both the changes in the oxazine structure and the effects of the methanol solvent through all those little charges."

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The absorbance lineshape dependence on quantum-mechanical model chemistry.