The Bonding of Quantum Physics with Quantum Chemistry
The bond that developed between quantum physics and quantum chemistry, that led to the award of a big chemistry prize to the physicist Walter Kohn in 1998, developed not without trial. Here I give an account of it. An element in this bond has been a frien
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The bond that developed between quantum physics and quantum chemistry, that led to the award of a big chemistry prize to the physicist Walter Kohn in 1998, developed not without trial. Here I give an account of it. An element in this bond has been a ffriendship between Walter Kohn and me. My having reached 80 first, he has already kindly spoken of this1 . Now it is my turn. In the 20s and early 30s there was a flush of successes in establishing the ability of quantum mechanics to describe the simplest molecules accurately: the Born-Oppenheimer approximation, the nature of chemical bonding, and the fundamentals of molecular spectroscopy. But then the quantitative theory of molecular structure, which we call quantum chemistry, was stymied, by the difficulty of solving the Schrodinger ¨ equation for molecules. The senior chemical physicists of the 30s pronounced the problem unsolvable. But the younger theoreticians in the period coming out of WWII thought otherwise. Clearly one could make substantial progress toward the goal of complete solution, because the equation to solve was known and had a simple universal structure. The boundary conditions too were known. It would not be as easy as handling an infinite periodic solid, but a number of us set to work. The special demand of chemistry was to quantify f very small molecular changes. Successes came slowly, but with the development of computers and a lot of careful, f clever work, by the 90s the quantitative problem was essentially solved. The emergent hero of the chemical community was John Pople, whose systematic strategy and timely method developments were decisive. The methods of what is termed “ab initio” quantum chemistry became available and used everywhere. Over the years the quantum chemists did a lot more than gradually improve their ability to calculate wavefunctions and energies from Schr¨ o¨dinger’s equation. All the while they have served molecular spectroscopy, physical inorganic chemistry, and physical organic chemistry. Relevant for the present story was the development by Per-Olov Lowdin ¨ in 1955 of the density matrix 1
W. Kohn. In: Reviews of Modern Quantum Chemistry. (Ed.) K.D. Sen. Vol. I, II, World Scientific, Singapore 2002, pp. v-vii
187 M. Scheffler et al. (eds.), Walter Kohn © Springer-Verlag Berlin Heidelberg 2003
Robert G. Parr
reduction of the Schrodinger ¨ equation, especially the identification and mathematical physics of natural spin orbitals and their occupation numbers. The hope was, although hope floundered, that the Schr¨ o¨dinger problem could be resolved in terms of the first- and second-order density matrices. Foundering came because of the difficulty of incorporating the Pauli principle. Beginning way a back in the 20s, Thomas and Fermi had put forward a theory using just the diagonal element of the first-order density matrix, the electron density itself. This so-called statistical theory totally failed f ffor chemistry because it could not account for the existence of molecules. Nevertheless, in 1968, after years of doing wonders wit
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