Volume 4, Issue 29 July 2007
Semiclassical Chemistry
The world as we know it is a soup of chemicals. Air has a complex recipe of oxygen, nitrogen, smog, and a dozen other substances; the oceans, their briny broth of salts and minerals and water. Even our own cells are seething cauldrons of ions and proteins and DNA.
These chemicals bump and mingle, swapping atoms and electrons to transmute themselves into new substances. These, in turn, react with still other molecules in an endless chain of chemical reactions. Light and heat are needed to spark some reactions; others occur in an instant, releasing small spurts of energy along the way.
Theoretical chemist William Miller has won the prestigious 2007 Welch Award in Chemistry for his work developing reaction kinetics theory. photo courtesy of William Miller
UC Berkeley Professor of Chemistry William Miller has developed the theory to help understand these molecular transformations molecule by molecule. As a theoretical chemist, he uses mathematics to work out the principles that control reaction kinetics.
"Chemical reaction rates have always been recognized as a central problem of chemistry. But for many years, it was considered a very messy problem, because chemists didn't have the tools to get in and look at reaction rates at the molecular level," Miller says.
These days, modeling the fate of thousands of atoms and molecules within a given system is a multimillion-dollar business. Such molecular dynamics simulations are used in drug design, biological reaction simulations, atmospheric models—virtually any area where chemical reactions occur in complex mixtures of molecules. Understanding what happens at the molecular level as these reactions proceed is critical to a wide range of chemical problems.
For example, Miller is currently working with colleagues in the Combustion Research Program at the Lawrence Berkeley National Laboratory to better understand how gasoline burns. "It's really a problem in chemical reaction dynamics. All sorts of reactions are going on," Miller says. "One can minimize pollutants, and control emissions by understanding how they're generated. If you can make these reactions just one percent more efficient, that's billions of dollars in savings."
At present, most molecular dynamics modeling programs use plain old Newtonian mechanics—the kind they teach in high school—to calculate the paths of thousands of atomic nuclei as they ricochet off some atoms and combine with others. But this can miss some important effects, Miller says.
"You can go a long way simply with classical mechanics, as in many molecular dynamics simulations for protein motion, folding and drug design. But it fails in some situations," Miller says.
The tricky thing about molecular dynamics is the properties of the molecules themselves. If they were as tiny as quarks and neutrinos, the complex equations of quantum mechanics would be required to predict their movements. If they were as heavy as golf balls or baseballs, the approximations of classical Newtonian mechanics would be sufficient. Atoms and molecules, however, fall somewhere in-between.
Miller realized that with certain mathematical approximations to quantum mechanics, he could combine the best of both worlds. His semiclassical theory makes it possible to calculate the motions of atomic nuclei using classical mechanics equations, but then add quantum effects to it. "The approach is basically doing classical mechanics calculations with a wrinkle thrown in. It makes the calculations harder, but not so much harder," Miller says. "It has all of classical mechanics in it, plus an approximate treatment of the quantum effects. So it doesn't miss anything qualitatively," Miller says.
Miller has now adapted a branch of his semiclassical theory called the 'initial value representation' to make molecular dynamics modeling even easier for large molecular systems. "We calculate the nuclear motions on the computer in the same way, using classical mechanics, but use them as input to our semiclassical theory," he says. "It's emerging as a very practical way to implement semiclassical theory for these large molecular systems."
Already several theoretical groups are using the simplest version of these equations. "My goal is to convince this body of people who do classical simulations to convert," Miller says.
One body he doesn't need to convince is other chemists. In May, it was announced that Miller will receive the 2007 Welch Award in Chemistry. He shares the $300,000 prize with fellow theoretical chemist Noel Hush of the University of Sydney. Miller and Hush, among the first chemists to specialize in theory, are the first in their field to win the award. Today, theoreticians make up ten to fifteen percent of the chemistry faculty at most major universities. "It's a recognition that theoretical chemistry is increasingly making significant contributions to the field," Miller says.