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Since Walter Kohn’s talk in this year’s meeting is about a topic completely unrelated to his Nobel Prize winning research, it is worth contemplating briefly on the great impact of his major contribution to chemistry and physics.
Kohn was originally trained as a physicist under the tutelage of Julian Schwinger at Harvard University. Schwinger had been a child prodigy and was known in the world of physics in the same way that a supremely talented virtuoso violinist might be known in the world of music. He was always impeccably dressed and drove a fancy Cadillac. His talks and papers were densely mathematical and used to leave listeners and readers flabbergasted. There were few people who could keep up with him when he gave a talk, frequently lasting for three or fours hours. However, as brilliant as he was, one physicist noted that his talks were like a virtuoso violinist’s highly polished performances, more technique than physical insight. Nevertheless Schwinger essentially fathered the field of quantum electrodynamics along with Richard Feynman, Sin-Itiro Tomonaga and Freeman Dyson. Schwinger, Tomonaga and Feynman were awarded the Nobel Prize for their efforts in 1965.
Compared to Schwinger, Kohn’s work provided great physical insight applicable across broad fields of physics and chemistry. Until Kohn wrote his papers, there was essentially one way to find the energy of a system of subatomic particles. For doing this you wrote down the relevant Schrödinger equation and obtained the wavefunction. The wavefunction after proper manipulation gave you the energy. The problem was that as valuable as this technique was, it was complicated and technically intensive. In the 1960s, Kohn along with his colleagues Sham and Hohenberg published two key papers that expounded on a concept developed by Llewellyn Thomas and Enrico Fermi in the 1930s. The concept involved calculating the energy of a system of particles as a function of the electron density and not the wavefunction. In the jargon of physics, the energy of the system was a functional of the density, and the underlying framework came to be called Density Functional Theory (DFT). This trivial-sounding difference made the whole process much more practical and technically feasible.
The value of the concept was not apparent right away, but the ensuing three decades saw enormous progress in the application of quantum mechanics to chemistry using efficient computer programs. The man who was singularly responsible for this development was English mathematician-turned-chemist John Pople at Carnegie Mellon University in the United States. Pople developed extremely facile techniques for calculating atomic and molecular orbitals and wavefunctions and incorporated them into readily available and easy to use computer programs. His work made a half-century’s development of quantum chemistry accessible to thousands of scientists in physics, chemistry, biology and engineering. But as Pople’s wavefunction-based approaches became dominant, so did Kohn’s original formulation of density functional theory. DFT was incorporated into similar computer programs and several benchmarking studies and key developments in the 80s and 90s proved that DFT techniques could get you the same information as wavefunction-based techniques but frequently using much lesser time and resources.
In the years since its original formulation DFT has become so prevalent in the calculation of atomic and molecular properties that many scientists use it as a black box in their work. It has become so widely used that it’s easy to forget that someone had to invent it. Today solid-state physicists use it extensively to calculate the properties of solids, engineers use it to calculate the properties of polymers and engineering materials, organic chemists use it to understand and improve chemical reactions, and biochemists use it to understand the interaction of drugs with proteins. A concept which originated in quantum physics is now ubiquitously used in fields far beyond those in which it was originally applicable. The original idea has sprouted wings and has flown to places which its creator himself would not have dreamt of. And that creator is Walter Kohn.
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