May 13, 2026
Julius Jellinek, a Senior Fellow in the University of Chicago’s Computation Institute from 2006 to 2018 and Senior Scientist Emeritus at Argonne National Laboratory, passed away on April 6. He was 80.
The following in memoriam was written by Ilya A. Shkrob, Argonne National Laboratory chemist and longtime colleague and friend.
Julius Jellinek, one of the most influential theorists in cluster chemistry of his generation, was born in 1946 in Uzhhorod, on the western edge of the Soviet sphere, into a Hungarian Jewish family of modest means. Both of his parents were Holocaust survivors. The city stood at a cultural and linguistic crossroads—near the borders of Czechoslovakia, Hungary, Poland, and Romania—and Jellinek grew up surrounded by multiple languages and perspectives. This early cosmopolitan environment left a lasting imprint on his intellectual outlook.
Coming from a border region and carrying a recognizably Jewish surname offered limited prospects in the Soviet system. Yet he benefited from a formative intellectual influence at the local university: Yuri Lomsadze, a gifted teacher whose emphasis on first principles and deep physical reasoning profoundly shaped his students. Lomsadze instilled in Jellinek a lifelong fascination with fundamental theoretical questions.
In 1976, Jellinek’s family emigrated to Israel, where he completed his PhD at the Weizmann Institute of Science. There he shifted from high-energy physics toward chemical physics, working with Eli Pollak, Michael Baer, and Donal Kouri on reaction dynamics. He later received a fellowship that enabled further study abroad and joined the group of Stephen Berry at the University of Chicago. Nearly forty years old, Jellinek arrived as a mature and extensively trained physicist, but without the defining breakthrough that would establish his independent scientific identity. That transformation occurred in Hyde Park, within Berry’s unusually broad intellectual environment.
Stephen Berry was a scientist of wide-ranging curiosity and bold intuition, often pursuing ideas that initially appeared speculative but later proved prescient. Among his long-standing interests was the extension of thermodynamic reasoning to finite systems—few-atom molecules and clusters. His fascination with structural flux in molecules, exemplified by his work on pseudorotation, strongly shaped the direction of his group.
Beginning in the mid-1970s, Berry repeatedly reformulated this problem and passed it on to successive students and postdocs. Among them were Gregory Natanson and Jellinek, who together engaged with what was at the time an intractable challenge: describing phase-like behavior in finite atomic systems.
What followed was a sustained period of conceptual and technical development that culminated nearly a decade later. Drawing on Landau’s theory of phase transitions—mathematically rigorous yet physically transparent—they formulated criteria for solid-like and liquid-like behavior in atomic clusters. This provided, for the first time, a coherent language for discussing phase transitions in systems now understood as nanoscale.
At the time, these ideas were met with skepticism. Their initial manuscript was rejected by Physical Review Letters with unusually sharp reviews, even by the standards of the field. Critics argued that the framework lacked direct computational or experimental support.
A turning point came from a referee’s suggestion that a 13-atom icosahedral cluster with a central atom could serve as a minimal test case. Berry’s response to the referee included a schematic potential-energy landscape that later became iconic in the field. By late 1984, the conceptual framework was largely in place, but direct computational validation was still missing.
That step came in 1985, when Berry recruited the computationally skilled Thomas Beck to work with Jellinek. In their 1986 study, long molecular dynamics trajectories revealed distinct regimes of solid-like behavior, liquid-like motion, and coexistence depending on cluster energy. These results confirmed the theoretical framework and opened a systematic path forward. The work became highly cited and helped define the modern computational study of clusters.
“My group in the mid-1980s was exploring many-body interactions in adsorbed two-dimensional systems, sharing much common intellectual ground with the pioneering theoretical contributions by Julius into the complex dynamics of finite, atomic-scale systems,” said Steven Sibener, Carl William Eisendrath Distinguished Service Professor in Chemistry. “His seminal discoveries with Stephen Berry on the phase behavior of such clusters have proven to be highly impactful and enduring to this day. Julius was a true gentleman scholar, always delighted to share his latest insights and their consequences for cluster science and beyond.”
For Jellinek, this was a defining scientific breakthrough. It established his international reputation and enabled his appointment in 1986 at Argonne National Laboratory, where a growing cluster research program was taking shape around Eric Parks, Stephen Riley, and later Mark Knickelbein.
The emergence of that program was itself the result of an unusual sequence of developments. In the late 1970s, a molecular beam group was seeking new directions. One researcher, Charles Young, submitted a proposal to the Atomic Energy Commission suggesting isotope fractionation using molecular beams. After an extended silence, the agency unexpectedly endorsed the idea and provided substantial funding for a dedicated apparatus. A large and sophisticated instrument was constructed. The original isotope-fractionation concept was ultimately disproven, and funding interest rapidly declined. The group was left with a powerful but underutilized machine, which was then repurposed for the production of mass-selected metal clusters. This shift created an immediate need for theoretical guidance in an unfamiliar experimental domain.
The convergence of experimental capability and theoretical insight gave rise to what became known as the Chicago–Argonne school of cluster chemistry, whose influence persists today. Over the following decades, Jellinek and his collaborators explored an exceptionally broad range of problems in finite-system physics and chemistry. Throughout this period, Berry remained a central intellectual reference point—a benchmark for both scientific style and ambition.
Jellinek’s relationship to science was deeply personal and intensely focused. Although he received numerous honors and traveled widely, his primary reward remained the intellectual experience of discovery. His final major paper in 2025, with Darya Aleinikava, returned to 13-atom clusters and reported a new phenomenon he termed the “chameleon effect,” again revealing unexpected behavior in a system he had helped make foundational. In many ways, it was a fitting conclusion to a scientific journey defined by returning to simple systems and finding them inexhaustibly rich.