It is with great sadness that we share with you the passing of our teacher and mentor Reiner Martin Dreizler, who succumbed to a grave illness on 22 November 2023.
Reiner was born on 22 September 1936 in Stuttgart, Germany, grew up in Mannheim, and obtained his physics diploma in 1961 from the University of Freiburg, where he specialized in nuclear physics (Thesis: The spinning top in quantum mechanics). He moved to Canberra to the Australian National University, where he obtained his PhD in 1964 (Thesis: A nuclear core–particle coupling model with applications in the 2s-1d shell). He accepted a postdoctoral position at the University of Pennsylvania and then an assistant professorship there, which he held from 1966 to 1972. He was appointed chair in theoretical physics at Goethe University Frankfurt in 1972, a position he held until 2001.
During his years in Philadelphia, Reiner worked with Abraham Klein and other colleagues on rotational states of deformed nuclei and the theory of spherical nuclei and their excited states, and on collective nucleon motion. Awards during his tenure as theoretical physics chair included the endowed Stefan Lyson Chair (1999–2001). From 2001 to 2004 he also served as director of the Center for Scientific Computing at Goethe University.
In Frankfurt, according to the local tradition, a theoretical physics chairholder was to deliver an entire cycle of theoretical physics courses, starting in the first semester and ending in the sixth, covering classical mechanics, electromagnetism, quantum mechanics, and quantum statistics, basically at the level of what are considered upper-year or even graduate courses in North America. This practice allowed Reiner to train several generations of students. Many of them later joined his group as diploma and PhD students.
Reiner’s research interests remained with the quantum-mechanical many-body problem but gradually moved from nuclear to atomic physics, where, in contrast to nuclear physics, the interactions are known exactly. This work involved gradient expansions of the kinetic energy, semiclassical methods (such as Thomas–Fermi theory, both stationary and time-dependent), eventually moving toward the modern form of density functional theory (DFT). He organized and co-organized important conferences and, in this way, brought together the DFT communities of Europe and North America. In particular, in 1983, together with João da Providência, he organized a NATO Summer School in Alcabideche, Portugal, on DFT. Walter Kohn, John Perdew, Bob Parr, Mel Levy, Elliott Lieb, Ulf von Barth, Carl-Olof Almbladh, Peter Schuck, Matthias Brack, and E. K. U. (Hardy) Gross were among the teachers at this school.
One of the highlights of the work of his students is the well-cited Runge–Gross theorem, which extends the Hohenberg–Kohn theorem to the time-dependent case. Monographs on DFT with Gross, and later with E. Engel, have helped the DFT methodology to gain acceptance in the fields of atomic and molecular physics. Another important application pursued by Reiner and his coworkers is atomic collision theory. Fruitful collaborations emerged with his experimental colleague in Frankfurt, Horst Schmidt-Böcking, and his students.
Reiner’s success was based on creating a welcoming and supporting atmosphere in the institute, for his students and postdoctoral fellows, and also for numerous international visitors. The welcoming spirit extended beyond the confines of the institute: Reiner owned a house in southern France and (together with his wife Margaret) hosted several generations of his group (and guests) there for summer “seminars.” The setting was beautiful and simple, with most of the house turned into a shared space and group activities relegated to the outdoors as much as possible. The physics discussions were held on the terrace (which doubled as dining room), with large sheets of paper laid out on the ground in lieu of a blackboard.
It is this exceptional generosity that Reiner displayed toward his students, research associates, and visitors that will be remembered by all of us. As a first-year lecturer, he invited the 150–200 student class to a day of hiking in the Bergstrasse area; he took his research group on several camping trips (e.g., the Spessart mountain range and the Vosges mountains in Alsace). The organization of workshops to expose the group members to experts in the field (e.g., at the Institute Laue Langevin in Grenoble and in Bordeaux) involved shared driving and finding accommodations and food, which served to bond the group together in unique ways beyond the scientific returns. The weekly group seminars (Friday afternoons) will also be remembered fondly, since they often helped to pave the road for beginning diploma students. Occasionally, senior group members who had coffeemakers in their offices were instructed to bring coffee while Reiner got pastries and cake for all from a nearby bakery.
A number of generations of students, mostly from those trained in the lecture cycles, contributed to the success of the program. Reiner was a pioneer in computational physics, adopting early ideas of accurate numerical solution of equations rather than approximations. These ideas took off in the fields of atomic collisions and laser–atom interactions. These were coupled with creative ideas to deal with the many-electron problem. Based on these contributions, Reiner was elected Fellow of the American Physical Society in 1995.
Reiner was a superior teacher, combining thoughtful lecture preparation with natural charisma. There was a lot to learn in his classes: foremost about theoretical physics, but also about how to organize and present one’s thoughts with apparent ease yet sophistication. When Reiner was approaching retirement, he started (with C. Lüdde) to turn his lecture notes into a textbook series, complemented by interactive problem sets with step-by-step guidance toward their solutions. Four volumes (on classical mechanics, electrodynamics, quantum mechanics, and statistical mechanics), plus a shorter book on quantum scattering theory, were published in German between 2003 and 2018. Two of them were translated into English. Work was in progress to translate the electrodynamics text when Reiner fell ill at the end of September 2023.
A number of Reiner’s students continued in academia, but there is also a significant number whose training in theoretical and computational physics led to careers in industry in Germany. All of us will miss him deeply.
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