A Viennese childhood
Walter Kohn was born on March 9, 1923, into a middle-class Jewish family in Vienna, a few years after the First World War had broken up the Austro-Hungarian Empire. His father, Salomon Kohn, was born in Hodonín in Moravia; his mother, Gittel, née Rappaport, in Brody in Galicia. His mother’s parents were Orthodox Jews, and his grandfather spent his retirement in prayer and the study of religious texts in a small nearby synagogue. Through them, Kohn wrote, the family kept in touch with traditional Judaism, while his parents, especially his father, were also part of the secular artistic and intellectual life of Vienna.
His father ran a publishing business, Postkartenverlag Brüder Kohn, which produced high-quality art postcards based on paintings it commissioned from contemporary artists. It had flourished before the war but struggled through the 1920s and 1930s. His mother was a highly educated woman who knew German, Latin, Polish and French. Walter attended the Akademisches Gymnasium in Vienna’s inner city, where he loved Latin and showed no talent at all for mathematics, which earned him his only C. He assumed he would one day take over the family business, a prospect he faced, he admitted, without the least enthusiasm.
The Anschluss and escape
In March 1938 Nazi Germany annexed Austria, and everything changed. The family business was confiscated, though his father was forced to keep managing it without pay. His older sister, Minna, managed to emigrate to England, and Walter was expelled from his school. That autumn he entered the Chajes Gymnasium, a Jewish school, where two remarkable teachers, Emil Nohel in physics and Victor Sabbath in mathematics, gave him his first love of science while acts of persecution and brutality took place outside the walls. Both teachers later became victims of the Nazis.
In August 1939, three weeks before the Second World War began, Kohn reached England on the Kindertransport, the rescue operation that brought some ten thousand mostly Jewish children out of Nazi-controlled Europe. His parents could not leave Austria. He was taken in by Charles and Eva Hauff in Sussex, a family who had never met him but had business ties to his father. He started training as a farmer, fell seriously ill with meningitis, and then enrolled at the county school in East Grinstead, where he concentrated on mathematics, physics and chemistry. His parents, Salomon and Gittel Kohn, were later murdered in the Holocaust, as were other relatives and several of his teachers.
Interned as an “enemy alien”
In May 1940, shortly after he turned 17, Britain interned most men holding enemy passports, and the Jewish refugee Walter Kohn was counted among them. After about two months in British camps, including on the Isle of Man, he was shipped in July 1940 in a convoy through U-boat-infested waters to Quebec and then to a camp at Trois-Rivières that held both German civilians and refugees. In the camps he studied whatever he could: a course in set theory taught by an interned Viennese mathematician, Fritz Rothberger, and classes in a camp school organised by an interned art historian. He passed the McGill University matriculation exams there, and with the 20 cents a day he earned as a lumberjack he saved up to buy textbooks on pure mathematics and chemical physics that stayed on his shelves for the rest of his life.
Released in January 1942, he was welcomed in Toronto by the family of Bruno and Hertha Mendel, who also supported other young refugees. The physicist Leopold Infeld, who had worked with Albert Einstein, persuaded him to study mathematics and physics at the University of Toronto rather than engineering. Because he now counted as a German national he was barred from the chemistry building, where war work was going on, and a sympathetic dean found a way to admit him as a special student. Kohn later wrote that he could not imagine how he might have become a scientist without the families who took him in.
Soldier, student and physicist
Rejecting his father’s pacifism because the Nazis threatened the whole world, Kohn joined the Canadian army after several attempts and served in the Canadian Infantry Corps in the last year of the war. In his spare time he wrote a mathematics paper on spinning tops. He received a wartime bachelor’s degree in applied mathematics in 1945 and a master’s degree in 1946, then won a fellowship to Harvard, where he studied under the brilliant young theorist Julian Schwinger and earned his PhD in physics in 1948 with work on the scattering of three particles that produced what is still called Kohn’s variational principle.
He became a solid-state physicist almost by accident, after agreeing to teach one of the first broad courses on the subject in the United States. From 1950 to 1960 he worked at the Carnegie Institute of Technology in Pittsburgh, with long stays in Copenhagen at Niels Bohr’s institute and summers at Bell Labs, then the world’s leading centre for solid-state research. There, often with his close collaborator J. M. Luttinger, he did important work on the impurities that control the behaviour of the semiconductor materials used in transistors. With Norman Rostoker he developed a method for calculating the electronic structure of crystals now known as the KKR method. In 1960 he moved to the new University of California, San Diego, where he helped build the physics department.
Density functional theory
In the autumn of 1963, on a sabbatical at the École Normale Supérieure in Paris, Kohn was thinking about alloys and asked himself a simple question: is everything about a system of electrons determined by the density of those electrons, that is, by how many are found on average at each point in space? The standard approach to quantum chemistry tracked the motion of every single electron, which made the mathematics explode in complexity as molecules grew. Using a short argument based on a classic variational principle, Kohn convinced himself that the density alone determines all of a system’s properties. It seemed so remarkable that he did not trust himself, and he asked a young American physicist sharing the office, Pierre Hohenberg, to work through it with him. Their paper, published in 1964, became the Hohenberg–Kohn theorem.
Back in San Diego, Kohn and his postdoctoral fellow Lu Jeu Sham turned the idea into a practical tool. The Kohn–Sham equations of 1965 replaced the impossible problem of many interacting electrons with a solvable problem of single electrons moving in an effective field. Together, the two papers founded density functional theory, usually called DFT. A later study of the Physical Review found that Kohn was an author of five of the hundred most-cited papers the journal had published between 1893 and 2003, including the first two.
How it changed science
It took decades of work by many researchers, and much faster computers, before DFT became accurate enough for everyday use, but by the 1990s it was transforming chemistry and materials science. The Nobel committee noted that its simplicity makes it possible to study very large molecules, for example to explain how enzymatic reactions occur, and that it had become one of the most widely used methods in quantum chemistry. The Royal Swedish Academy of Sciences awarded Kohn the 1998 Nobel Prize in Chemistry for his development of density functional theory, sharing it with John Pople, who built the computational methods that made such calculations widely available.
Today DFT calculations run on computers in universities and companies around the world. Scientists use them to predict the structure of molecules, map how chemical reactions proceed, and screen new materials before anyone makes them in a laboratory: catalysts for cleaner industrial processes, electrode materials for batteries, semiconductors, solar cell materials and candidate drug molecules. In effect, Kohn gave chemists and engineers a way to test ideas about matter on a computer, saving years of trial and error.
A Jewish refugee’s conscience
Kohn wrote that his feelings toward Austria, his native land, were and would remain very painful, dominated by his memories of a year and a half as a Jewish boy under the Nazi regime and by the murder of his parents. In later life he was nonetheless glad to work with younger Austrians who faced the country’s past honestly, including physicists, teachers at his old school and young volunteers doing Holocaust memorial service. He described himself as having a strong Jewish identity and was involved in several Jewish projects, among them establishing a strong programme of Judaic Studies at the University of California, San Diego. On matters of belief he described himself as a deist.
His sense of responsibility reached beyond physics. He took part in efforts to end the American–Soviet nuclear arms race, led faculty initiatives to end the University of California’s role in managing the nuclear weapons laboratories at Los Alamos and Livermore, supported the student Pugwash movement on science and global issues, and joined the board of the Population Institute. Naturalised as an American in 1957, he also felt a strong sense of global citizenship, with close ties to Canada, Denmark, England, France and Israel.
Later years and legacy
In 1979 Kohn moved to the University of California, Santa Barbara, as founding director of the National Science Foundation’s Institute for Theoretical Physics, which he led until 1984, and he remained at Santa Barbara for the rest of his life, working with students and postdoctoral fellows on DFT into his eighties. His first wife, Lois, supported his early career; his second wife, Mara, was the daughter of Roman Vishniac, the biologist and photographer of pre-war Jewish life in Eastern Europe. He had three daughters. In his last years he turned his energy to clean energy, serving as executive producer of The Power of the Sun, a 2005 documentary narrated by John Cleese about the history and science of solar power, with a companion film for students explaining how a silicon solar cell works.
Walter Kohn died of cancer at his home in Santa Barbara on April 19, 2016, aged 93. The boy who was expelled from his school in Vienna, interned as an enemy alien and orphaned by the Holocaust gave the world one of the most widely used tools in all of science, the method that lets researchers calculate the behaviour of matter from first principles.
Sources & further reading
Walter Kohn, Biographical, NobelPrize.org · Press release: The Nobel Prize in Chemistry 1998, NobelPrize.org · Public Invited to Free Screening of New Documentary on Solar Power Produced by Nobel Laureate, UC Santa Barbara · Walter Kohn, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.