Walter Kohn

Walter Kohn
Kohn in 2012
Walter Kohn
Born	9 3, 1923
Birthplace	Vienna, Austria
Died	Template:Death date and age Santa Barbara, California, U.S.
Nationality	American
Occupation	Theoretical physicist, theoretical chemist
Known for	Density functional theory, Hohenberg–Kohn theorems, Kohn–Sham equations, KKR method, Kohn anomaly
Education	Harvard University (PhD)
Spouse(s)	Lois Adams (died 2010) Mara Vishniac Schiff (died 2018)
Awards	Oliver E. Buckley Prize (1961) National Medal of Science (1988) Nobel Prize in Chemistry (1998)

Walter Kohn (Template:IPA-de; March 9, 1923 – April 19, 2016) was an Austrian-born American theoretical physicist and theoretical chemist whose work fundamentally transformed the way scientists understand and compute the electronic properties of materials. Born into a Jewish family in Vienna, Kohn escaped Nazi-occupied Austria as a teenager through the Kindertransport program, an experience that shaped the rest of his life even as he built one of the most consequential careers in twentieth-century science. He shared the 1998 Nobel Prize in Chemistry with John Pople for his development of density functional theory (DFT), a computational framework that replaced unwieldy many-body wavefunctions with equations involving electronic density, enabling far more accurate calculations on complex systems. Density functional theory became an essential tool across materials science, condensed-matter physics, and the chemical physics of atoms and molecules. Over a career spanning more than six decades, Kohn held positions at Carnegie Mellon University, the University of California, San Diego, and the University of California, Santa Barbara, where he served as the founding director of the Institute for Theoretical Physics (later the Kavli Institute for Theoretical Physics). He died at his home in Santa Barbara on April 19, 2016, at the age of 93.

Early Life

Walter Kohn was born on March 9, 1923, in Vienna, Austria, into a Jewish family. He grew up in the Austrian capital during a period of increasing political instability and rising antisemitism. His early years were shaped by the cultural and intellectual life of interwar Vienna, and he attended local schools where he demonstrated academic aptitude, particularly in the sciences.

The annexation of Austria by Nazi Germany in March 1938 — the Anschluss — drastically altered Kohn's life. As a Jewish teenager in an increasingly hostile environment, his educational and personal prospects were destroyed almost overnight. Jewish students were expelled from Austrian schools, and the Kohn family faced escalating persecution.

In 1939, at the age of fifteen or sixteen, Kohn was able to leave Austria through the Kindertransport, the organized rescue effort that transported thousands of Jewish children from Nazi-occupied territories to safety in Great Britain. This was a traumatic separation; his parents, who remained behind in Austria, were later murdered in the Holocaust. Kohn later described the Kindertransport as the event that saved his life, and the loss of his parents remained a defining experience that he spoke about throughout his later years.

After arriving in England, Kohn's time in Britain was short-lived in terms of stability. As a young Austrian national living in wartime Britain, he was classified as an "enemy alien" by British authorities — a common wartime measure applied to nationals of countries with which Britain was at war, regardless of their personal circumstances or the fact that many, like Kohn, were themselves refugees fleeing persecution. He was interned and subsequently sent to Canada as part of the British government's internment and deportation program. It was in Canada that Kohn would begin the next chapter of his life, eventually gaining access to higher education and beginning the academic career that would lead to the Nobel Prize.

Education

Kohn's path to formal higher education began in Canada, where he had been sent as an internee from Britain. Despite the difficult circumstances of his arrival, he was able to enroll at the University of Toronto, where he earned both his Bachelor of Arts and Master of Arts degrees in applied mathematics and physics.

Following his studies in Toronto, Kohn moved to the United States to pursue doctoral research at Harvard University. There, he studied under Julian Schwinger, one of the leading theoretical physicists of the twentieth century and himself a future Nobel laureate. Under Schwinger's supervision, Kohn completed his PhD in physics. The training Kohn received at Harvard, particularly the rigorous approach to quantum mechanics that Schwinger instilled in his students, proved formative for the theoretical work that Kohn would later pursue in electronic structure theory. Schwinger's influence is evident in Kohn's emphasis on variational principles and formal theoretical methods, which became hallmarks of his approach to physics and chemistry.

In 2012, Harvard University awarded Kohn an honorary degree, recognizing his distinguished contributions to science and his connection to the institution where he had received his graduate training.

Career

Early Academic Career

After completing his doctorate at Harvard, Kohn embarked on an academic career in theoretical physics. He joined the faculty of Carnegie Mellon University (then known as the Carnegie Institute of Technology) in Pittsburgh, Pennsylvania, where he worked during the 1950s and early 1960s. During this period, Kohn made a number of contributions to solid-state physics and the quantum theory of solids. His early work encompassed problems in the electronic structure of metals and semiconductors, and he developed or co-developed several theoretical tools that became standard in the field.

Among his early contributions was the development of the KKR method (Korringa–Kohn–Rostoker method), a technique for computing the electronic band structure of solids. He also identified what became known as the Kohn anomaly, a phenomenon related to the screening of lattice vibrations by conduction electrons in metals, which produces singularities in the phonon dispersion relations. Another early contribution was the Luttinger–Kohn model, developed with Joaquin Mazdak Luttinger, which provided a framework for describing the electronic band structure of semiconductors near band edges. These contributions established Kohn as a leading figure in theoretical solid-state physics even before his most celebrated work on density functional theory.

In 1961, Kohn received the Oliver E. Buckley Condensed Matter Physics Prize from the American Physical Society, one of the most prestigious awards in condensed-matter physics, in recognition of his contributions to the understanding of the electronic structure of solids.

University of California, San Diego

Kohn subsequently moved to the University of California, San Diego (UCSD), where he continued his work on the quantum mechanics of solids and began to develop the ideas that would become his most lasting contribution to science. It was during this period that the theoretical groundwork for density functional theory was laid.

Development of Density Functional Theory

The work for which Kohn is best known — and for which he received the Nobel Prize — is the development of density functional theory (DFT). This theory emerged in two landmark papers published in the 1960s.

In 1964, Kohn and Pierre Hohenberg published a paper that established what are now known as the Hohenberg–Kohn theorems. These theorems proved that the ground-state properties of a many-electron system are uniquely determined by the electron density, rather than by the many-body wavefunction. This was a profound conceptual shift: rather than needing to solve for a wavefunction that depends on the coordinates of all electrons in a system (a function of 3N variables for N electrons), one could in principle determine all ground-state properties from the electron density, which is a function of only three spatial variables.

In 1965, Kohn and Lu Jeu Sham published a second foundational paper that introduced what are now known as the Kohn–Sham equations. These equations provided a practical computational scheme for applying the Hohenberg–Kohn theorems. The Kohn–Sham approach replaces the intractable many-body problem with a set of single-particle equations for non-interacting electrons moving in an effective potential, which includes the effects of electron-electron interactions through an exchange-correlation functional. The key insight was that an exact theory could be reformulated in terms of single-particle equations, with all the complicated many-body physics folded into a single (in principle exact, but in practice approximate) functional of the density.

The practical impact of DFT was enormous but not immediately apparent. It took several decades for the development of sufficiently accurate approximations to the exchange-correlation functional, along with advances in computing power, to make DFT a broadly practical tool. By the 1990s, however, DFT had become the dominant method for electronic structure calculations in condensed-matter physics, materials science, and increasingly in chemistry. The theory made it possible to calculate the electronic properties of systems containing hundreds or even thousands of atoms — systems that were completely intractable using traditional wavefunction-based methods.

By the time of the Nobel Prize in 1998, DFT-based calculations had become ubiquitous in scientific research, used for everything from designing new materials and understanding catalysis to studying the properties of biological molecules and predicting the behavior of materials under extreme conditions. The Nobel Committee recognized that Kohn's development of DFT had "led to more accurate calculations on complex systems as well as many new insights" and had become "an essential tool for materials science, condensed-phase physics, and the chemical physics of atoms and molecules."

University of California, Santa Barbara

In 1979, Kohn moved to the University of California, Santa Barbara (UCSB), where he would spend the remainder of his career. At UCSB, he served as the founding director of the Institute for Theoretical Physics (ITP), which later became the Kavli Institute for Theoretical Physics (KITP). Under Kohn's leadership, the ITP grew into one of the world's leading centers for theoretical physics research, bringing together physicists from across the globe for extended research programs on topics spanning the breadth of the discipline.

Kohn continued his research at UCSB, extending and refining density functional theory and applying it to new problems. He also worked on other topics in theoretical physics and chemistry, including the theory of surfaces and interfaces, the properties of semiconductors, and the nature of the insulating state.

He eventually became an emeritus professor of physics at UCSB, a position he held until his death. The university's chancellor issued a memorandum upon Kohn's death, noting the loss to the UCSB community and the scientific world.

Later Interests

In addition to his scientific work, Kohn became involved in issues related to energy and the environment later in his career. He also remained engaged with questions about the history of science and the experiences of refugee scientists. Throughout his later years, Kohn spoke publicly about his experiences as a refugee from Nazism and the importance of the Kindertransport in saving his life. He was recognized not only as a scientist but as a Holocaust survivor whose personal history carried broader significance.

Personal Life

Walter Kohn was married twice. His first wife was Lois Adams, who died in 2010. He later married Mara Vishniac Schiff, who was the daughter of the photographer Roman Vishniac, known for documenting Jewish life in Eastern Europe before the Holocaust. Mara Vishniac Schiff died in 2018.

Kohn's personal history was deeply marked by the Holocaust. His parents, who remained in Austria after he escaped through the Kindertransport, were killed by the Nazis. Kohn rarely spoke about this loss in his early career but became more open about his experiences in later decades, viewing it as important to bear witness to the history of the period.

Despite having received the Nobel Prize in Chemistry, Kohn noted that he had last formally studied chemistry as a teenager in Vienna, before his education was interrupted by the Nazi persecution of Jews. This biographical detail became a frequently cited aspect of his story, highlighting both the disruption caused by the Holocaust and the interdisciplinary nature of his later scientific contributions.

Kohn became a United States citizen and lived in Santa Barbara, California, for the final decades of his life. He died at his home in Santa Barbara on April 19, 2016, at the age of 93.

Recognition

Kohn received numerous awards and honors throughout his career, reflecting the broad impact of his work across physics and chemistry.

His most significant awards include:

• Oliver E. Buckley Condensed Matter Physics Prize (1961) — awarded by the American Physical Society for his contributions to the electronic theory of solids.
• National Medal of Science (1988) — the highest scientific honor bestowed by the United States government, awarded by the President.
• Nobel Prize in Chemistry (1998) — shared with John Pople, for the development of density functional theory and computational methods in quantum chemistry, respectively.

Kohn was elected a Fellow of the Royal Society (ForMemRS), one of the highest honors in the international scientific community.

In 2012, Harvard University conferred an honorary degree upon Kohn, recognizing his distinguished contributions to science and his status as one of the university's most notable alumni.

In January 2022, several years after his death, Kohn's Nobel Prize medal was put up for auction. The auction attracted attention from both the scientific community and collectors of scientific memorabilia.

Legacy

Walter Kohn's legacy rests primarily on the transformative impact of density functional theory on modern science. DFT is one of the most widely used theoretical tools in physics, chemistry, and materials science. By the early twenty-first century, a substantial fraction of all computational studies of the electronic properties of molecules and materials employed DFT or methods derived from it. The theory has been applied to problems ranging from drug design and catalysis to the study of planetary interiors and the development of new electronic materials.

The conceptual contribution of the Hohenberg–Kohn theorems — the demonstration that the electron density contains all the information needed to determine the ground-state properties of a system — represented a paradigm shift in quantum mechanics. Before Kohn's work, the standard approach to quantum mechanical calculations involved solving for the many-body wavefunction, an object of such complexity that exact solutions were possible only for the simplest systems. By establishing the electron density as the fundamental variable, Kohn opened an entirely new avenue for computational physics and chemistry.

The practical impact of the Kohn–Sham equations has been even more far-reaching. The equations provided a computational scheme that could be implemented on computers and systematically improved through better approximations to the exchange-correlation functional. Generations of scientists have built on this framework, developing new functionals and applying DFT to an ever-wider range of problems.

Beyond his specific scientific contributions, Kohn's life story — from child refugee to Nobel laureate — has been cited as a powerful example of the human consequences of persecution and the contributions that refugees can make when given the opportunity to rebuild their lives. His experience as a Kindertransport survivor who lost his parents to the Holocaust, yet went on to make foundational contributions to science, has been the subject of numerous retrospectives and educational programs.

At UCSB, Kohn's legacy endures through the Kavli Institute for Theoretical Physics, which he founded and which continues to serve as a major international center for theoretical physics research. His contributions to the university and to the broader scientific community in Santa Barbara helped establish the region as an important center for physics research.

Pierre Hohenberg and James Langer, writing in Physics Today, described Kohn as "a giant of theoretical physics," a characterization that reflects his standing within the scientific community at the time of his death and in the years since.