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John A. Pople Sharing the 1998 Nobel Prize in chemistry are a physicist and a developer of computational codes that made the physicist's concepts more easily applicable to the study of large molecular systems. Walter Kohn, 75, of the University of California at Santa Barbara, and John A. Pople, 72, of Northwestern University, have made pioneering contributions in developing quantum chemistry methods that can be used for theoretical studies of the properties of molecules and the chemical processes in which they are involved. Kohn was cited for the development of the density-functional theory and Pople for the development of computational methods in quantum chemistry. Kohn, a physicist, says of winning the chemistry prize: "You may think they made a mistake, but interdisciplinary acts can be fruitful." Quantum mechanics makes it theoretically possible to understand and calculate how electrons and atomic nuclei interact to build up matter in all its forms. Quantum chemistry--the application of quantum mechanics to chemical problems--exploits this knowledge to describe the molecular system. In the 1960s, two events became decisive: the development of an entirely new theory for describing the spatial distribution of electrons and the use of the increasing potential offered by the computer. Kohn's theoretical work has formed the basis for simplifying the mathematics in descriptions of the bonding of atoms, a prerequisite for many of today's calculations. Pople developed the entire quantum-chemical methodology now used in various branches of chemistry. In 1964 Kohn showed that the total energy for a system described by the laws of quantum mechanics can be theoretically calculated if the electrons' spatial distribution (electron density) is known. The question is only how the energy depends on the density in a system with free electrons. Kohn demonstrated that it suffices to know the average number of electrons located at any one point in space without considering the motion of each individual electron. This has led to a computationally simpler method, the density-functional theory, which makes it possible to study very large molecules. Today, calculations can explain how enzymatic reactions occur, from calculating the geometrical structure of molecules to mapping chemical reactions. "Physicists dealing with solids liked the idea right away, but chemists rejected it for 20 or 25 years," Kohn says. "When Professor Pople changed his mind, the tide turned, and it changed the mind of the whole chemistry community." As Kohn explains, traditional methods could work well for systems containing molecules with up to 15 atoms. Today, density functional theory makes it possible to conceptualize the structure and function of molecules such as DNA with hundreds or thousands of atoms. Pople's approach allows scientists to create computer models of chemical reactions that are difficult or impossible to re-create in the laboratory, making possible the theoretical study of molecules, their properties, and their interface in chemical reactions. The particulars of a molecule or a chemical reaction are fed into the computer, and the output describes the properties of that molecule or how a chemical reaction occurs. Pople realized that if theoretical methods were to gain any significance within chemistry, they had to be "simple enough to be used by the average chemist, but accurate enough to be useful." Through significant improvements in the theoretical methodology at the end of the 1960s, Pople designed a computer program "that met the challenge of developing sufficient computations and providing them to the general chemist to study individual molecules." GAUSSIAN-70, as the program was called, very soon became widely used. Pople continued during the 1970s and 1980s to refine the methodology, while building up a well-documented model chemistry. New developments in computer hardware and software have made the program accessible to chemists in universities and commercial companies worldwide. In the 1990s Pople incorporated Kohn's density-functional theory into his methodology, opening up new possibilities for analyzing ever-more complex molecules. "In the early days many practical chemists were skeptical, saying, 'All you're doing is calculating what we already know,'" Pople says. "Now the theory is catching up to the research. Experimental chemists can use the techniques to understand molecular processes that cannot be obtained from experiments alone without being mathematicians or programmers." Ilene Schneider is an Irvine, Calif.-based freelance writer specializing in health care and biotechnology enterprises. E-mail: ISchnei440@aol.com; Web site: www.cpinckney.com/ilene

Quantum Achievements Win Nobel Prize in Chemistry

Nov 22, 1998

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