November 1, 1998
It's not a Nobel for Tulane," says John Perdew, "but it's as close as I've been able to come." Perdew, professor of physics, is referring to the 1998 Nobel Prize in Chemistry, which was awarded in October to University of California- Santa Barbara physicist Walter Kohn for his development of the density- functional theory, which allows scientists to compute the properties of atoms and molecules that were previously too complicated to tackle.
In announcing this year's winners, the Royal Swedish Academy of Sciences cited Perdew as one of a handful of scientists who refined Kohn's theory to the point that it could be applied not only to solid-state physics but to quantum chemistry as well. In order to understand how atoms stick together to form molecules--and in order to understand what atoms, molecules and solids can exist--scientists need to be able to calculate the energy of electrons.
According to quantum theory, however, that energy is controlled by a very complex wave. So complex, in fact, that even using modern computers, it is impossible for scientists to calculate the energy. That was the situation Kohn faced when he began his groundbreaking work at the University of California- San Diego 35 years ago.
Instead of engaging in the Sisyphean task of attempting to solve the equation, however, Kohn opted for a new tack: He simplified the equation to the point that it could be solved on computer. While Kohn's solution was good enough for solid-state physics, when it came to the more exacting world of chemistry, his local density approximation simply wasn't accurate enough. Beginning in the early 1970s, Perdew and his mentor, Rutgers University physicist David Langreth, worked to develop a more accurate approximation for the exchange-correlation energy.
During the last 25 years, Perdew, drawing on the work of colleagues, including Tulane chemistry professor Mel Levy, developed what he calls the "generalized gradient approximation," a more accurate calculation that adds information to the local density approximation. The generalized gradient approximation improved on the accuracy of the initial local density approximation by 10 times, which finally enabled quantum chemists to use the density-functional theory to calculate the atomic structure of much more complex molecules.
"This function was refined to the point where, starting about 10 years ago, chemists got interested in it, and it now has become the standard method of electronic structure calculation in chemistry, too," Perdew says. "That's why Kohn is winning the Nobel Prize in chemistry and not physics, because it's a very hot topic right now."
The generalized gradient theory has been used to make realistic computer calculations for systems of several hundred atoms. In the next 10 years, Perdew expects that calculations for systems of several thousand atoms will be possible. Such knowledge of how this atomic "glue" works could ultimately lead to a better understanding of biological molecules. While he might not have a Nobel Prize on his mantel, Perdew does have the satisfaction of knowing the scientific community is paying attention.
According to the Institute for Scientific Information in Philadelphia, Perdew ranked as the 59th most-cited physicist between 1981 and 1997. The ISI also ranked the Tulane physics department third in citations per paper between 1981 and 1994, ranking just behind Princeton and Harvard. Perdew hopes to continue to improve the accuracy of the density-functional theory, ideally by a factor of five. "Then it would be possible to design chemicals, medicines and materials on the computer," Perdew says.
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