A giant among us

Klaus Ruedenberg honored by American Chemical Society

Klaus Ruedenberg with the display of a localized molecular orbital depicting the distribution of electrons in carbon dioxide.

Klaus Ruedenberg, an Ames Laboratory senior associate and an Iowa State University Distinguished Professor Emeritus, has been chosen to receive the prestigious American Chemical Society Award in Theoretical Chemistry. Nominees for this honor must have accomplished "innovative research in the field of theoretical chemistry characterized by depth, originality and scientific significance." His award states that he has seminally ad-vanced many different, important facets of quantum chemistry, en-compassing fundamental theory, formal mathematical developments, computational methods and software implementations, as well as conceptual interpretations.

Modest about his accomplishments, Ruedenberg said, "What we end up doing is somewhat accidental. We choose among the professional options available to us, those that fascinate us and pose problems we feel we will be able to help solve."

Ruedenberg grew up in Bielefeld, Germany, and earned an M.S. in chemistry at the University of Fribourg, Switzerland. Finding mathematics and theory more attractive than experiments, he switched to theoretical physics at the Univer-sity of Zürich where, in 1950, he obtained a Ph.D. in meson field theory with the renowned Gregor Wentzel, whom he followed to the University of Chicago.

There he became a postdocto-ral fellow with Robert S. Mulliken, who two decades later received the Nobel Prize in theoretical chemistry. "This was then one of the very few places where one could combine chemistry and theoretical physics," Ruedenberg comments, noting that "theoretical chemistry is one of the invasions of physics into chemistry. Still, the field would not have blossomed," he continues, "if it hadn't been for the subsequent development of electronic computers. I was lucky."

The prominent ACS Award in Theoretical Chemistry honors Ruedenberg as one of the few quantum chemists in the world to be recognized as a leader in establishing the field of theoretical chemistry and ensuring its viability during the last 50 years. Many of the methods and concepts he introduced have become widely accepted and used. "Professor Ruedenberg is one of the most highly respected and accomplished theoretical chemists in the world," said Mark Gordon, director of Ames Laboratory's Applied Mathematics and Computational Sciences Program and an ISU Distinguished Professor. Gordon, who nominated Ruedenberg for the ACS award, continued, "Klaus' recognition arises not only because of his outstanding research accomplishments, but also as a result of his many contributions to the scientific community as an articulate and inspiring leader."

Among Ruedenberg's note-worthy contributions is his eluci-dation of the energetic interactions that cause molecule forma-tion. It led to profound insights into the basic origin and physical nature of the chemical bond.

Closely related are his methods for creating a rigorous quantum theoretical foundation for the two-hundred-year-old empirical model of molecules being built from atoms. His approach elucidates how atoms get modified in a molecular environment.

Fundamental theoretical aspects regarding the understanding of chemical reactions have been illuminated by Ruedenberg's studies of molecular potential energy surfaces. His discoveries regarding intersections of energy surfaces are relevant for photochemistry.

In recent work, Ruedenberg has been addressing the problem of electron correlation, a major bottleneck in the quest for accurate predictions of the properties, in particular energies, of ground and excited electronic states of large molecules — a fundamental as well as practical goal of theoretical chemistry.

Ruedenberg is a Fellow of the American Physical Society, the American Institute of Chemists and the American Association for the Advancement of Science. His honors include the Midwest Award of the American Chemical Society, membership in the Inter-national Academy of Quantum Molecular Sciences, and honorary doctorates from the universities of Basel (Switzerland), Bielefeld (Germany) and Siegen (Germany). For many years, he was editor-in-chief of the journal Theoretica Chimica Acta.

A consummate scientist, Ruedenberg has not put aside his classical humanistic upbringing and remains fascinated by his-tory, literature and the human mind and its deep motivations. But he believes that, in our age, basic lessons for these dimensions also flow from the insights of the natural sciences. He observes, for instance, that every stationary state in nature is a dynamic balance between many opposing tendencies — from the motions of electrons to those of the celestial bodies, and from the equilibria in chemical test tubes to those in living organisms. He thinks that to be a responsible human means to acknowledge the many mutually opposing forces in oneself and to affirm the internal tensions and efforts that go with working out a harmonious moral balance.

Ruedenberg and Veronika, his wife of 53 years who came to watch this interview, seem to have achieved such a balance. They appear at home in each other's dedicated company. "I love her," he admitted, smiling at her. Veronika was one of the founders of the Octagon and Farmers Market. She also created and ran the Cheese n' Puppets deli store. The couple shares a love for the arts. He has assisted her in pup-pet performances, and she likes to sing along when he plays German and Swiss songs on the piano.

How fortunate for us that in 1955, when only a few chemistry departments showed any interest in theoretical chemistry, the young professors Robert Rundle and Robert Hansen had the fore-sight to invite Klaus Ruedenberg to Ames Laboratory and Iowa State to delve into the harmonies of electrons in molecules.

by Saren Johnston