Obituary: Walter Kohn (1923–2016)

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OBITUARY Walter Kohn (1923–2016) Walter Kohn is one of the most cited and In 1979 Walter became the founding influential scientists of our times. He died director of the Institute for Theoretical on 19 April 2016 in Santa Barbara. He was Physics at the University of California not only an outstanding scientist, but a Santa Barbara (today’s Kavli Institute for special and extraordinary person — due Theoretical Physics) and moved to the city. to his charisma, and his alert and critical The institute gained a strong international thinking — whose particular life story reflects reputation. For his pioneering and the tragedies of the twentieth century. groundbreaking scientific contributions he Walter was born into a well-situated received multiple honours. For example, he Jewish family in Vienna on 9 March 1923. He was awarded the Nobel Prize in Chemistry in had a pleasant and sheltered childhood until 1998 (together with John Pople) — although, 1938 when Austria joined Nazi Germany — as he emphasized with complaisant irony, as the Anschluss — and Walter was forced to a student he had never taken a single course leave the renowned Academic Gymnasium in chemistry. to continue at a Jewish gymnasium where Walter Kohn’s contributions to modern Emil Nohel, a former assistant of Einstein, solid-state physics are impressively diverse.

elated him about mathematics and SCHEFFLER MATTHIAS In addition to the already mentioned density physics. In 1939, the Kohn parents sent functional theory and the Luttinger–Kohn their 16-year-old son to Great Britain with mostly in close and long-term collaboration model, he provided fundamental insight one of the last children’s transport rescue with Joaquin Luttinger (for example the into defects in semiconductors, into band missions (Kindertransporte). Walter never Luttinger–Kohn model of semiconductor structure theory, superconductivity, van saw his parents again, as they were killed in band structures), Kohn was considering a der Waals interactions in metals, surface Auschwitz. In spring 1940, when there was fundamental challenge of condensed-matter physics, ‘nearsightedness’ of quantum- the risk of a UK invasion by Germany, he physics: how can one simplify the many-body mechanical interactions, and much more. and other men who held a German passport problem of a solid so that its properties can be Walter took a stand in contemporary were considered ‘enemy aliens’ and put in quantitatively evaluated? At the beginning of debates, representing uncompromisingly detention camps: first on the Isle of Man, the 1960s, Kohn had the idea to use the density clear ethical principles. In the 1980s, the and then Walter was shipped to Canada distribution of the ensemble of electrons, Waldheim affair divided Austria, and where he worked in a woodcutter camp. not, as was hitherto usual, the wave function. he stood up for those who refuted the Following his release from internment he The Thomas–Fermi model of 1927 had the belittlement of Waldheim’s Nazi past. studied mathematics and physics in Toronto. same goal, but was a severe approximation. Walter also showed moral courage and In 1946 he moved to Harvard, where In Pierre Hohenberg, who he met during foresight when in January 2003 he publicly the young Julian Schwinger became his a visit at the École Normale Supérieure in took position against a second Iraq war. PhD adviser. After two years he was Paris, Walter found a congenial partner to Although he initiated and organized a able to finish his PhD thesis about the help realize this idea. In 1964, both were able request to Congress that was signed by quantum-mechanical description of the to show that the density of the electrons in 41 US Nobel Prize laureates and numerous collisions of light nuclei. In the following the ground state uniquely determines the organizations, this war started in March years Walter’s focus turned more and more properties of a quantum-mechanical many- 2003 — with disastrous consequences towards solid-state theory. At the time, body system. The corresponding article for the region and the world. Other topics this was not a mainstream decision since was written in only a few weeks and is just that he felt strongly about were the climate the field had only just started to establish six pages long. However, it was clear to problem and alternative forms of energy. itself as a subdiscipline in physics and Walter Kohn that this general proof was just “I am not a one dimensional man.” independent research area. An important the theoretical basis for the solution of the This statement, expressed two years factor that contributed to his choice of problem, and that for practical applications ago in a personal interview with us, research orientation was his consultancy further development was needed. Together aptly characterizes his life and work. All role for the Bell Telephone Laboratories with Lu Sham, postdoc at the University of colleagues will keep him in loving memory, which took place alongside his research California San Diego — Walter Kohn was the due to his charm and his candour. He and teaching at the Pittsburgh Carnegie very first physics professor at this university is deeply missed by his friends. He will Institute of Technology (now Carnegie since 1960 — he derived the famous Kohn– remain a role model for his science and his Mellon University). This brought him into Sham equations, which reduce the many- ethical stand. contact with leading condensed-matter body problem to an effective single-particle and semiconductor physicists such as problem. This approach then became, after MATTHIAS SCHEFFLER AND DIETER HOFFMANN William Shockley and acquainted him with some time, the workhorse of computational Matthias Scheffler is at the Fritz-Haber-Institut the current issues of this research field that materials science, now known as density der Max-Planck-Gesellschaft, Berlin-Dahlem, were rapidly developing in the 1950s. functional theory (DFT). Today, about half of Germany. Dieter Hoffman is at the Max Planck Besides working on a variety of all publications in quantum chemistry refer to Institute for the History of Science, Berlin, Germany. prevailing issues in semiconductor physics, the DFT. e-mail: [email protected]