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1901 2012
Prize category:
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The Nobel Prize in Chemistry 1998
Walter Kohn, John Pople
Walter Kohn
Born: 9 March 1923, Vienna, Austria
Affiliation at the time of the award: University of California, Santa Barbara, CA, USA
Prize motivation: "for his development of the density-functional theory"
Field: Theoretical chemistry
Autobiography
I suppose I
am not the first Nobelist who, on the occasion of receiving this
Prize, wonders how on earth, by what strange alchemy of family
background, teachers, friends, talents and especially accidents
of history and of personal life he or she arrived at this point.
I have browsed in previous volumes of "Les Prix Nobel" and I know
that there are others whose eventual destinies were foreshadowed
early in their lives – mathematical precocity, champion
bird watching, insatiable reading, mechanical genius. Not in my
case, at least not before my late teens. On the contrary: An
early photo of my older sister and myself, taken at a children's
costume party in Vienna – I look about 7 years old –
shows me dressed up in a dark suit and a black top hat, toy
glasses pushed down my nose, and carrying a large sign under my
arm with the inscription "Professor Know-Nothing".
Here then is my attempt to convey to the reader how, at age 75, I
see my life which brought me to the present point: a long-retired
professor of theoretical physics at the University of California,
still loving and doing physics, including chemical physics,
mostly together with young people less than half my age;
moderately involved in the life of my community of Santa Barbara
and in broader political and social issues; with unremarkable
hobbies such as listening to classical music, reading (including
French literature), walking with my wife Mara or alone, a little
cooking (unjustifiably proud of my ratatouille); and a weekly
half hour of relaxed roller blading along the shore, a throwback
to the ice-skating of my Viennese childhood. My three daughters
and three grandchildren all live in California and so we get to
see each other reasonably often.
I was naturalized as an American citizen in 1957 and this has
been my primary self-identity ever since. But, like many other
scientists, I also have a strong sense of global citizenship,
including especially Canada, Denmark, England, France and Israel,
where I have worked and lived with a family for considerable
periods, and where I have some of my closest friends.
My feelings towards Austria, my native land, are – and will
remain – very painful. They are dominated by my vivid
recollections of 1 1/2 years as a Jewish boy under the Austrian
Nazi regime, and by the subsequent murder of my parents, Salomon
and Gittel Kohn, of other relatives and several teachers, during
the holocaust. At the same time I have in recent years been glad
to work with Austrians, one or two generations younger than I:
Physicists, some teachers at my former High School and young
people (Gedenkdiener) who face the dark years of Austria's past
honestly and constructively.
On another level, I want to mention that I have a strong Jewish
identity and – over the years – have been involved in
several Jewish projects, such as the establishment of a strong
program of Judaic Studies at the University of California in San Diego.
My father, who had lost a brother, fighting on the Austrian side
in World War I, was a committed pacifist. However, while the Nazi
barbarians and their collaborators threatened the entire world, I
could not accept his philosophy and, after several earlier
attempts, was finally accepted into the Canadian Infantry Corps
during the last year of World War II. Many decades later I became
active in attempts to bring an end to the US-Soviet nuclear arms
race and became a leader of unsuccessful faculty initiatives to
terminate the role of the University of California as manager of
the nuclear weapons laboratories at Los Alamos and Livermore. I
offered early support to Jeffrey Leiffer, the founder of the
student Pugwash
movement which concerns itself with global issues having a strong
scientific component and in which scientists can play a useful
role. Twenty years after its founding this organization continues
strong and vibrant. My commitment to a humane and peaceful world
continues to this day. I have just joined the Board of the
Population Institute because I am convinced that early
stabilization of the world's population is important for the
attainment of this objective.
After these introductory general reflections from my present vantage
point I would now like to give an idea of my childhood and adolescence.
I was born in 1923 into a middle class Jewish family in Vienna,
a few years after the end of World War I, which was disastrous
from the Austrian point of view. Both my parents were born in
parts of the former Austro-Hungarian Empire, my father in Hodonin,
Moravia, my mother in Brody, then in Galicia, Poland, now in the
Ukraine. Later they both moved to the capital of Vienna along
with their parents. I have no recollection of my father's parents,
who died relatively young. My maternal grandparents Rappaport
were orthodox Jews who lived a simple life of retirement and,
in the case of my grandfather, of prayer and the study of religious
texts in a small nearby synagogue, a Schul as it was called. My
father carried on a business, Postkartenverlag Brueder Kohn Wien
I, whose main product was high quality art postcards, mostly based
on paintings by contemporary artists which were commissioned by
his firm. The business had flourished in the first two decades
of the century but then, in part due to the death of his brother
Adolf in World War I, to the dismantlement of the Austrian monarchy
and to a worldwide economic depression, it gradually fell on hard
times in the 1920s and 1930s. My father struggled from crisis
to crisis to keep the business going and to support the family.
Left over from the prosperous times was a wonderful summer property
in Heringsdorf at the Baltic Sea, not far from Berlin, where my
mother, sister and I spent our summer vacations until Hitler came
to power in Germany in 1933. My father came for occasional visits
(the firm had a branch in Berlin). My mother was a highly educated
woman with a good knowledge of German, Latin, Polish and French
and some acquaintance with Greek, Hebrew and English. I believe
that she had completed an academically oriented High School in
Galicia. Through her parents we maintained contact with traditional
Judaism. At the same time my parents, especially my father, also
were a part of the secular artistic and intellectual life of Vienna.
After I had completed a public elementary school, my mother
enrolled me in the Akademische Gymnasium, a fine public high
school in Vienna's inner city. There, for almost five years, I
received an excellent education, strongly oriented toward Latin
and Greek, until March 1938, when Hitler Germany annexed Austria.
(This so-called Anschluss was, after a few weeks, supported by
the great majority of the Austrian population). Until that time
my favorite subject had been Latin, whose architecture and
succinctness I loved. By contrast, I had no interest in, nor
apparent talent for, mathematics which was routinely taught and
gave me the only C in high school. During this time it was my
tacit understanding that I would eventually be asked to take over
the family business, a prospect which I faced with resignation
and without the least enthusiasm.
The Anschluss changed everything: The family business was
confiscated but my father was required to continue its management
without any compensation; my sister managed to emigrate rather
promptly to England; and I was expelled from my school.
In the following fall I was able to enter a Jewish school, the
Chajes Gymnasium, where I had two extraordinary teachers: In
physics, Dr. Emil Nohel, and in mathematics Dr. Victor Sabbath.
While outside the school walls arbitrary acts of persecution and
brutality took place, on the inside these two inspired teachers
conveyed to us their own deep understanding and love of their
subjects. I take this occasion to record my profound gratitude
for their inspiration to which I owe my initial interest in
science. (Alas, they both became victims of Nazi
barbarism).
I note with deep gratitude that twice, during the Second World
War, after having been separated from my parents who were unable
to leave Austria, I was taken into the homes of two wonderful
families who had never seen me before: Charles and Eva Hauff in
Sussex, England, who also welcomed my older sister, Minna.
Charles, like my father, was in art publishing and they had a
business relationship. A few years later, Dr. Bruno Mendel and
his wife Hertha of Toronto, Canada, took me and my friend Joseph
Eisinger into their family. (They also supported three other
young Nazi refugees). Both of these families strongly encouraged
me in my studies, the Hauffs at the East Grinstead County School
in Sussex and the Mendels at the University of
Toronto. I cannot imagine how I might have become a scientist
without their help.
My first wife, Lois Kohn, gave me invaluable support during the
early phases of my scientific career; my present wife of over 20
years, Mara, has supported me in the latter phases of my
scientific life. She also created a wonderful home for us, and
gave me an entire new family, including her father Vishniac, a
biologist as well as a noted photographer of pre-war Jewish
communities in Eastern Europe, and her mother Luta. (They both
died rather recently, well into their nineties).
After these rather personal reminiscences I now turn to a brief
description of my life as a scientist.
When I arrived in England in August 1939, three weeks before the
outbreak of World War II, I had my mind set on becoming a farmer
(I had seen too many unemployed intellectuals during the 1930s),
and I started out on a training arm in Kent. However, I became
seriously ill and physically weak with meningitis, and so in January
1940 my "acting parents", the Hauffs, arranged for me to attend
the above-mentioned county school, where – after a period
of uncertainty – I concentrated on mathematics, physics
and chemistry.
However, in May 1940, shortly after I had turned 17, and while
the German army swept through Western Europe and Britain girded
for a possible German air-assault, Churchill ordered most male
"enemy aliens" (i.e., holders of enemy passports, like myself) to
be interned ("Collar the lot" was his crisp order). I spent about
two months in various British camps, including the Isle of Man,
where my school sent me the books I needed to study. There I also
audited, with little comprehension, some lectures on mathematics
and physics, offered by mature interned scientists.
In July 1940, I was shipped on, as part of a British convoy
moving through U-boat-infested waters, to Quebec City in Canada;
and from there, by train, to a camp in Trois Rivieres, which
housed both German civilian internees and refugees like myself.
Again various internee-taught courses were offered. The one which
interested me most was a course on set-theory given by the
mathematician Dr. Fritz Rothberger and attended by two students.
Dr. Rothberger, from Vienna, a most kind and unassuming man, had
been an advanced private scholar in Cambridge, England,
when the internment order was issued. His love for the intrinsic
depth and beauty of mathematics was gradually absorbed by his
students.
Later I was moved around among various other camps in Quebec and
New Brunswick. Another fellow internee, Dr. A. Heckscher, an art
historian, organized a fine camp school for young people like
myself, whose education had been interrupted and who prepared to
take official Canadian High School exams. In this way I passed
the McGill
University junior Matriculation exam and exams in
mathematics, physics and chemistry on the senior matriculation
level. At this point, at age 18, I was pretty firmly looking
forward to a career in physics, with a strong secondary interest
in mathematics.
I mention with gratitude that camp educational programs received
support from the Canadian Red Cross and
Jewish Canadian philanthropic sources. I also mention that in
most camps we had the opportunity to work as lumberjacks and earn
20 cents per day. With this princely sum, carefully saved up, I
was able to buy Hardy's Pure Mathematics and Slater's Chemical
Physics, books which are still on my shelves. In January 1942,
having been cleared by Scotland Yard of being a potential spy, I
was released from internment and welcomed by the family of
Professor Bruno Mendel in Toronto. At this point I planned to
take up engineering rather than physics, in order to be able to
support my parents after the war. The Mendels introduced me to
Professor Leopold Infeld who had come to Toronto after several
years with Einstein. Infeld, after
talking with me (in a kind of drawing room oral exam), concluded
that my real love was physics and advised me to major in an
excellent, very stiff program, then called mathematics and
physics, at the University of Toronto. He argued that this
program would enable me to earn a decent living at least as well
as an engineering program.
However, because of my now German nationality, I was not allowed
into the chemistry building, where war work was in progress, and
hence I could not enroll in any chemistry courses. (In fact, the
last time I attended a chemistry class was in my English school
at the age of 17.) Since chemistry was required, this seemed to
sink any hope of enrolling. Here I express my deep appreciation
to Dean and head of mathematics, Samuel Beatty, who helped me,
and several others, nevertheless to enter mathematics and physics
as special students, whose status was regularized one or two
years later.
I was fortunate to find an extraordinary mathematics and applied
mathematics program in Toronto. Luminous members whom I recall
with special vividness were the algebraist Richard Brauer, the
non-Euclidean geometer, H.S.M. Coxeter, the aforementioned
Leopold Infeld, and the classical applied mathematicians John
Lighton Synge and Alexander Weinstein. This group had been
largely assembled by Dean Beatty. In those years the University
of Toronto team of mathematics students, competing with teams
from the leading North-American Institutions, consistently won
the annual Putman competition. (For the record I remark that I
never participated). Physics too had many distinguished faculty
members, largely recruited by John C. McLennan, one of the
earliest low temperature physicists, who had died before I
arrived. They included the Raman specialist H.L.
Welsh, M.F. Crawford in optics and the low-temperature physicists
H.G. Smith and A.D. Misener. Among my fellow students was
Arthur Schawlow,
who later was to share the Nobel Prize for the development of the
laser.
During one or two summers, as well as part-time during the school
year, I worked for a small Canadian company which developed
electrical instruments for military planes. A little later I
spent two summers, working for a geophysicist, looking for (and
finding!) gold deposits in northern Ontario and Quebec.
After my junior year I joined the Canadian Army. An excellent
upper division course in mechanics by A. Weinstein had introduced
me to the dynamics of tops and gyroscopes. While in the army I
used my spare time to develop new strict bounds on the precession
of heavy, symmetrical tops. This paper, "Contour Integration in
the Theory of the Spherical Pendulum and the Heavy Symmetrical
Top" was published in the Transactions of American Mathematical
Society. At the end of one year's army service, having completed
only 2 1/2 out of the 4-year undergraduate program, I received a
war-time bachelor's degree "on – active – service" in
applied mathematics.
In the year 1945-6, after my discharge from the army, I took an
excellent crash master's program, including some of the senior
courses which I had missed, graduate courses, a master's thesis
consisting of my paper on tops and a paper on scaling of atomic
wave-functions.
My teachers wisely insisted that I do not stay on in Toronto for
a Ph.D, but financial support for further study was very hard to
come by. Eventually I was thrilled to receive a fine Lehman
fellowship at Harvard. Leopold Infeld recommended that I should
try to be accepted by Julian Schwinger, whom
he knew and who, still in his 20s, was already one of the most
exciting theoretical physicists in the world.
Arriving from the relatively isolated University of Toronto and
finding myself at the illustrious Harvard, where many faculty and
graduate students had just come back from doing brilliant
war-related work at Los Alamos, the MIT Radiation Laboratory, etc., I felt very
insecure and set as my goal survival for at least one year. The
Department Chair, J.H. Van Vleck, was very
kind and referred to me as the Toronto-Kohn to distinguish me
from another person who, I gathered, had caused some trouble.
Once Van Vleck told me of an idea in the band-theory of solids,
later known as the quantum defect method, and asked me if I would
like to work on it. I asked for time to consider. When I returned
a few days later, without in the least grasping his idea, I
thanked him for the opportunity but explained that, while I did
not yet know in what subfield of physics I wanted to do my
thesis, I was sure it would not be in solid state physics. This
problem then became the thesis of Thomas Kuhn, (later a renowned
philosopher of science), and was further developed by myself and
others. In spite of my original disconnect with Van Vleck, solid
state physics soon became the center of my professional life and
Van Vleck and I became lifelong friends.
After my encounter with Van Vleck I presented myself to Julian
Schwinger requesting to be accepted as one of his thesis
students. His evident brilliance as a researcher and as a
lecturer in advanced graduate courses (such as waveguides and
nuclear physics) attracted large numbers of students, including
many who had returned to their studies after spending "time out"
on various war-related projects.
I told Schwinger briefly of my very modest efforts using
variational principles. He himself had developed brilliant new
Green's function variational principles during the war for
wave-guides, optics and nuclear physics (Soon afterwards Green's
functions played an important role in his Nobel-Prize-winning
work on quantum electrodynamics). He accepted me within minutes
as one of his approximately 10 thesis students. He suggested that
I should try to develop a Green's function variational method for
three-body scattering problems, like low-energy
neutron-deuteron scattering, while warning me ominously, that he
himself had tried and failed. Some six months later, when I had
obtained some partial, very unsatisfactory results, I looked for
alternative approaches and soon found a rather elementary
formulation, later known as Kohn's variational principle for
scattering, and useful for nuclear, atomic and molecular
problems. Since I had circumvented Schwinger's beloved Green's
functions, I felt that he was very disappointed. Nevertheless he
accepted this work as my thesis in 1948. (Much later L. Fadeev
offered his celebrated solution of the three-body scattering
problem).
My Harvard friends, close and not so close, included P.W. Anderson, N. Bloembergen, H.
Broida (a little later), K. Case, F. De Hoffman, J. Eisenstein,
R. Glauber, T. Kuhn, R. Landauer, B. Mottelson, G. Pake,
F. Rohrlich, and C. Slichter. Schwinger's brilliant lectures on
nuclear physics also attracted many students and Postdocs from
MIT, including J. Blatt, M. Goldberger, and J.M. Luttinger. Quite
a number of this remarkable group would become lifelong friends,
and one – J.M. "Quin" Luttinger – also my closest
collaborators for 13 years, 1954-66. Almost all went on to
outstanding careers of one sort or another.
I was totally surprised and thrilled when in the spring of 1948
Schwinger offered to keep me at Harvard for up to three years. I
had the choice of being a regular post-doctoral fellow or
dividing my time equally between research and teaching. Wisely
– as it turned out – I chose the latter. For the next
two years I shared an office with Sidney Borowitz, later
Chancellor of New
York University, who had a similar appointment. We were to
assist Schwinger in his work on quantum electrodynamics and the
emerging field theory of strong interactions between nucleons and
mesons. In view of Schwinger's deep physical insights and
celebrated mathematical power, I soon felt almost completely
useless. Borowitz and I did make some very minor contributions,
while the greats, especially Schwinger and Feynman, seemed to be on
their way to unplumbed, perhaps ultimate depths.
For the summer of 1949, I got a job in the Polaroid laboratory
in Cambridge, Mass., just before the Polaroid camera made its
public appearance. My task was to bring some understanding to
the mechanism by which charged particles falling on a photographic
plate lead to a photographic image. (This technique had just been
introduced to study cosmic rays). I therefore needed to learn
something about solid state physics and occasionally, when I encountered
things I didn't understand, I consulted Van Vleck.
It seems that these meetings gave him the erroneous impression
that I knew something about the subject. For one day he explained
to me that he was about to take a leave of absence and, "since
you are familiar with solid state physics", he asked me if I
could teach a course on this subject, which he had planned to
offer. This time, frustrated with my work on quantum field
theory, I agreed. I had a family, jobs were scarce, and I thought
that broadening my competence into a new, more practical, area
might give me more opportunities.
So, relying largely on the excellent, relatively recent monograph
by F. Seitz, "Modern Theory Of Solids", I taught one of the first
broad courses on Solid State Physics in the United States. My
"students" included several of my friends, N. Bloembergen, C.
Slichter and G. Pake who conducted experiments (later considered
as classics) in the brand-new area of nuclear magnetic resonance
which had just been opened up by E. Purcell at Harvard
and F. Bloch at
Stanford. Some of my students often understood much more than I,
they were charitable towards their teacher.
At about the same time I did some calculations suggested by
Bloembergen, on the recently discovered, so-called Knight shift
of nuclear magnetic resonance, and, in this connection, returning
to my old love of variational methods, developed a new
variational approach to the study of wavefunctions in periodic
crystals.
Although my appointment was good for another year and a half, I
began actively looking for a more long-term position. I was a
naturalized Canadian citizen, with the warmest feelings towards
Canada, and explored every Canadian university known to me. No
opportunities presented themselves. Neither did the very meager
US market for young theorists yield an academic offer. At this
point a promising possibility appeared for a position in a new
Westinghouse nuclear reactor laboratory outside of Pittsburgh.
But during a visit it turned out that US citizenship was required
and so this possibility too vanished. At that moment I was
unbelievably lucky. While in Pittsburgh, I stayed with my
Canadian friend Alfred Schild, who taught in the mathematics
department at the Carnegie Institute of Technology (now Carnegie Mellon
University). He remarked that F. Seitz and several of his
colleagus had just left the physics department and moved to
Illinois, so that – he thought – there might be an
opening for me there. It turned out that the Department Chair, Ed
Creutz was looking rather desperately for somebody who could
teach a course in solid state physics and also keep an eye on the
graduate students who had lost their "doctor-fathers". Within 48
hours I had a telegram offering me a job!
A few weeks later a happy complication arose. I had earlier
applied for a National Research Council fellowship for 1950-51
and now it came through. A request for a short postponement was
firmly denied. Fortunately, Ed Creutz agreed to give me a
one-year leave of absence, provided I first taught a compressed
course in solid state physics. So on December 31, 1950 (to
satisfy the terms of my fellowship) I arrived in
Copenhagen.
Originally I had planned to revert to nuclear physics there, in
particular the structure of the deuteron. But in the meantime
I had become a solid state physicist. Unfortunately no one in
Copenhagen, including Niels Bohr, had even heard the expression
"Solid State Physics". For a while I worked on old projects. Then,
with an Indian visitor named Vachaspati (no initial), I published
a criticism of Froehlich's pre-BCS theory of superconductivity,
and also did some work on scattering theory.
In the spring of 1951, I was told that an expected visitor for
the coming year had dropped out and that the Bohr Institute could
provide me with an Oersted fellowship to remain there until the
fall of 1952. Very exciting work was going on in Copenhagen,
which eventually led to the great "Collective Model of the
Nucleus" of A. Bohr and B. Mottelson, both of whom had become
close friends. Furthermore my family and I had fallen in love
with Denmark and the Danish people. A letter from Niels Bohr to
my department chair at Carnegie quickly resulted in the extension
of my leave of absence till the fall of 1952.
In the summer of 1951, I became a substitute teacher, replacing
an ill lecturer at the first summer school at Les Houches, near
Chamonix in France, conceived and organized by a dynamic young
French woman, Cécile Morette De Witt. As an "expert" in
solid state physics, I offered a few lectures on that subject.
Wolfgang Pauli,
who visited, when he learned of my meager knowledge of solids,
mostly metallic sodium, asked me, true to form, if I was a
professor of physics or of sodium. He was equally acerbic about
himself. Some 50 years old at the time, he described himself as
"a child-wonder in menopause" ("ein Wunderkind in den
Wechseljahren"). But my most important encounter was with Res
Jost, an assistant of Pauli at the ETH in Zurich, with whom I
shared an interest in the so-called inverse scattering problem:
given asymptotic information, (such as phase-shifts as function
of energy), of a particle scattered by a potential V(r), what
quantitative information can be inferred about this potential?
Later that year, we both found ourselves in Copenhagen and
addressed this problem in earnest. Jost, at the time a senior
fellow at the Institute for Advanced Study in Princeton, had to
return there before we had finished our work. A few months later,
in the spring of 1952, I received an invitation from Robert
Oppenheimer, to come to Princeton for a few weeks to finish our
project. In an intensive and most enjoyable collaboration, we
succeeded in obtaining a complete solution for S-wave scattering
by a spherical potential. At about the same time I.M. Gel'fand in
the Soviet Union published his celebrated work on the inverse
problem. Jost and I remained close lifelong friends until his
death in 1989.
After my return to Carnegie Tech in 1952, I began a major
collaboration with N. Rostoker, then an assistant of an
experimentalist, later a distinguished plasma theorist. We
developed a theory for the energy band structure of electrons for
periodic potentials, harking back to my earlier experience with
scattering, Green's functions and variational methods. We showed
how to determine the bandstructure from a knowledge of purely
geometric structure constants and a small number (~ 3) of
scattering phase-shifts of the potential in a single
sphericalized cell. By a different approach this theory was also
obtained by J. Korringa. It continues to be used under the
acronym KKR. Other work during my Carnegie years, 1950-59,
includes the image of the metallic Fermi Surface in the
phonon spectrum (Kohn anomaly); exponential localization of
Wannier functions; and the nature of the insulating state.
My most distinguished colleague and good friend at Carnegie was
G.C. Wick, and my first PhD's were D. Schechter and V.
Ambegaokar. I also greatly benefitted from my interaction with T.
Holstein at Westinghouse.
In 1953, with support from Van Vleck, I obtained a summerjob at
Bell Labs as assistant of W. Shockley, the
co-inventor of the transistor. My project was radiation damage of
Si and Ge by energetic electrons, critical for the use of the
recently developed semiconductor devices for applications in
outer space. In particular, I established a reasonably accurate
energy threshold for permanent displacement of a nucleus from its
regular lattice position, substantially smaller than had been
previously presumed. Bell Labs at that time was without question
the world's outstanding center for research in solid state
physics and for the first time, gave me a perspective over this
fascinating, rich field. Bardeen, Brattain and Shockley ,
after their invention of the transistor, were the great heroes.
Other world class theorists were C. Herring, G. Wannier and my
brilliant friend from Harvard, P.W. Anderson. With a few
interruptions I was to return to Bell Labs every year until 1966.
I owe this institution my growing up from amateur to
professional.
In the summer of 1954 both Quin Luttinger and I were at Bell Labs
and began our 13-year long collaborations, along with other work
outside our professional "marriage". (Our close friendship lasted
till his death in 1997). The all-important impurity states in
the transistor materials Si and Ge, which govern their electrical
and many of their optical properties, were under intense experimental
study, which we complemented by theoretical work using so-called
effective mass theory. In 1957, I wrote a comprehensive review
on this subject. We (mostly Luttinger) also developed an effective
Hamiltonian in the presence of magnetic fields, for the complex
holes in these elements. A little later we obtained the first
non-heuristic derivation of the Boltzman transport equation for
quantum mechanical particles. There followed several years
of studies of many-body theories, including Luttinger's famous
one-dimensional "Luttinger liquid" and the "Luttinger's theorem"
about the conservation of the volume enclosed by a metallic Fermi
surface, in the presence of electron electron interaction. Finally,
in 1966, we showed that superconductivity occurs even with purely
repulsive interactions – contrary to conventional wisdom
and possibly relevant to the much later discovery of high-Tc superconductors.
In 1960, when I moved to the University of California San Diego,
California, my scientific interactions with Luttinger, then at
Columbia
University, and with Bell Labs gradually diminished. I did
some consulting at the nearby General Atomic Laboratory,
interacting primarily with J. Appel. My university colleagues
included G. Feher, B. Maple, B. Matthias, S. Schultz, H. Suhl and
J. Wheatley, – a wonderful environment. During my 19-year
stay there I typically worked with two postdocs and four graduate
students. A high water mark period were the late 1960s, early
1970s, including N. Lang, D. Mermin, M. Rice, L.J. Sham, D.
Sherrington, and J. Smith.
I now come to the development of density functional theory (DFT).
In the fall of 1963, I spent a sabbatical semester at the
École Normale Supérieure in Paris, as guest and in the
spacious office of my friend Philippe Nozières. Since my
Carnegie days I had been interested in the electronic structure
of alloys, a subject of intense experimental interest in both the
physics and metallurgy departments. In Paris I read some of the
metallurgical literature, in which the concept of the effective
charge e* of an atom in an alloy was prominent, which
characterized in a rough way the transfer of charge between
atomic cells. It was a local point of view in
coordinate space, in contrast to the emphasis on
delocalized waves in momentum space, such as
Bloch-waves in an average periodic crystal, used for the rough
description of substitutional alloys. At this point the question
occurred to me whether, in general, an alloy is completely
or only partially characterized by its electronic density
distribution n(r): In the back of my mind I knew that this was
the case in the Thomas-Fermi approximation of interacting
electron systems; also, from the "rigid band model" of
substitutional alloys of neighboring elements, I knew that there
was a 1-to-1 correspondence between a weak perturbing potential
v(r) and the corresponding small change n(r) of the
density distribution. Finally it occurred to me that for a single
particle there is an explicit elementary relation between the
potential v(r) and the density, n(r), of the groundstate. Taken
together, these provided strong support for the conjective that
the density n(r) completely determines the external potential
v(r). This would imply that n(r) which integrates to N, the total
number of electrons, also determines the total Hamilton H and
hence all properties derivable from H and N, e.g. the
wavefunction of the 17th excited state, 
17
(r1,...,rN)! Could this be true? And how
could it be decided? Could two different potentials,
v1(r) and v2(r), with associated different
groundstates 1 (r1,...,rN) and
2 (r1,...,rN) give rise to the
same density distribution? It turned out that a simple
3-line argument, using my beloved Rayleigh Ritz variational
principle, confirmed the conjecture. It seemed such a remarkable
result that I did not trust myself.
By this time I had become friends with another inhabitant of
Nozière's office, Pierre Hohenberg, a lively young American,
recently arrived in Paris after a one-year fellowship in the
Soviet Union. Having completed some work there he seemed to be
"between" problems and I asked if he would be interested in
joining me. He was. The first task was a literature search to see
if this simple result was already known; apparently not. In short
order we had recast the Rayleigh-Ritz variational theorem for the
groundstate energy in terms of the density n (r) instead of the
many electron wave function , leading to what is now called
the Hohenberg Kohn (HK) variational principle. We fleshed out
this work with various approximations and published it.
Shortly afterwards I returned to San Diego where my new
postdoctoral fellow, Lu J. Sham had already arrived. Together we
derived from the HK variational principle what are now known as
the Kohn-Sham (KS) equations, which have found extensive use by
physicists and chemists, including members of my group.
Since the 1970s I have also been working on the theory of
surfaces, mostly electronic structure. The work with Lang in the
early 1970s, using DFT, picked up and carried forward where J.
Bardeen's thesis had left off in the 1930s.
In 1979, I moved to the University of California, Santa Barbara
to become the initial director of the National Science
Foundation's Institute for Theoretical Physics (1979-84). I have
continued to work with postdoctoral fellows and students on DFT
and other problems that I had put aside in previous years. Since
the middle 1980s, I have also had increasing, fruitful
interactions with theoretical chemists. I mention especially
Robert Parr, the first major theoretical chemist to believe in
the potential promise of DFT for chemistry who, together with his
young co-workers, has made major contributions, both conceptual
and computational.
Since beginning this autobiographical sketch I have turned 76. I
enormously enjoy the continuing progress by my younger DFT
colleagues and my own collaboration with some of them. Looking
back I feel very fortunate to have had a small part in the great
drama of scientific progress, and most thankful to all those,
including family, kindly "acting parents", teachers, colleagues,
students, and collaborators of all ages, who made it
possible.
From Les Prix Nobel. The Nobel Prizes 1998, Editor Tore Frängsmyr, [Nobel Foundation], Stockholm, 1999
This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/Nobel Lectures. The information is sometimes updated with an addendum submitted by the Laureate.
Copyright © The Nobel Foundation 1998
MLA style: "Walter Kohn - Autobiography". Nobelprize.org. 22 May 2013 http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1998/kohn.html