The tragic story of Hans Hellmann

In 1933, quantum chemist Hans Hellmann was one of the hundreds of German professors who lost their posts when all non-Aryans were deemed unfit for civil service. A year later, he fled Nazi Germany with his family to seek refuge and employment in the USSR, where he received a prestigious appointment from the state and became a Soviet citizen.

Then the Soviet government began targeting foreigners as part of Joseph Stalin’s ideological purges. Within three years of his first forced migration, Hellmann had once again become an enemy in his home country. “I am really afraid to engage in too much correspondence with other countries,” he wrote to his mother from Moscow in late 1937. “In fact, the wall between us gets higher every day.” A few months after Hellmann wrote that letter, Soviet secret police broke into his apartment, arrested him for espionage, and took him to Moscow’s Taganka prison. He was executed in May 1938 at the age of 34.

Hellmann was one of hundreds of thousands of victims of the Soviet Union’s Great Purge, yet his story stands out because of his scientific achievements and the tragic path that led to his death. He was a pioneering physical chemist who used quantum mechanics to examine the forces at work in generating and destroying chemical bonds. Like other displaced German physicists (see the related article), he had to make a new home after fleeing Nazi Germany—but his status as a foreigner sealed his fate in Stalin’s purge. And if not for the determined efforts of his son, Hellmann’s story might have never been fully known.

Early life and career

Hans Gustav Adolf Hellmann was born on 14 October 1903 in the small harbor town of Wilhelmshaven in northwestern Germany. By all accounts he was a thrill seeker, says his niece Petra Netter, who is now a retired psychologist at the University of Giessen. At least once he suffered a severe injury in a motorbike accident.

When it was time for college, Hellmann enrolled at the Institute of Technology in Stuttgart to pursue electrical engineering. Within a year, he had switched his focus to physics. It was an exciting time to study physical science in Germany. Hellmann’s mentors included crystallographer Paul Peter Ewald as well as Otto Hahn and Lise Meitner, who supervised his master’s project on preparing radioactive samples. Lectures by Walther Kossel on the electronic theory of valency gave Hellmann a taste of the recent work on various types of bonding.

Hellmann’s doctoral thesis adviser, the experimental physicist Erich Regener, proved an especially important figure in his life. Along with guiding his student through research on the decomposition of atmospheric ozone, Regener introduced Hellmann to the woman who became his wife. Viktoria Bernstein was the daughter of Jewish parents in Ukraine and the niece of Regener’s wife; she had been adopted into the Regener household following the deaths of her parents. In the very busy year of 1929, Hellmann earned his doctorate, began an assistantship in Hanover, married Bernstein, and had a son, Hans Jr, who was born on Hellmann’s 26th birthday. His work and his family life were taking off.

Influenced by Kossel’s lectures and surrounded by a talented group of chemists in Hanover, Hellmann turned his attention to the field that would soon be called quantum chemistry. The topic appealed to him because it provided the opportunity to use the tools of quantum mechanics to decode the physical nature of chemical bonding.

In the spring and summer of 1933, Hellmann published in Zeitschrift für Physik two papers that clarified the factors at work in the formation of covalent bonds. His molecular virial theorem enables the determination of both the potential and the kinetic energy of a molecule, which is necessary to understand the energy needed for bonding. He then demonstrated that to calculate the force between atoms in a molecule, performing a relatively simple classical calculation is equivalent to undertaking a complex quantum one. Other researchers independently came up with a similar idea around the same time, but only two names would stick—it’s now called the Hellmann–Feynman theorem.

Hellmann’s body of work before age 30 should have led to a prime position in the German university system. But it was 1933, and newly appointed chancellor Adolf Hitler was asserting his control over Germany. A law passed in June disqualified Hellmann from the civil service because of his wife’s Jewish heritage. It didn’t help that Hellmann was a vocal leftist; Netter reports that her uncle discussed socialist ideas whenever he visited her parents’ house. On Christmas Eve, Hellmann received a letter that officially dismissed him from the German civil service, effective 31 March 1934.

Anticipating the arrival of that letter, Hellmann had already been looking for jobs outside Germany. He likely received multiple offers, including one for a position in the US. He accepted a post at the Karpov Institute of Physical Chemistry in Moscow. The choice made sense at the time, considering his wife’s birthplace, his support of socialism, and an impressive salary. But the decision would ultimately cost him his life.

Success and tragedy in the USSR

In May 1934 Hellmann joined Karpov as head of the theory group in a department dedicated to the structure of matter. Just after his arrival, Hellmann devised another technique for simplifying convoluted chemistry. He considered atoms of two elements in the same group of the periodic table, such as lithium and potassium. It is far more difficult to quantum-mechanically determine the chemical properties of potassium than of lithium, because the complexity of the calculation scales with the number of total electrons. But Hellmann simplified the process by consolidating the effects of the atoms’ nucleus and inner electrons into a parameter called a pseudopotential. After accounting for each atom’s pseudopotential, a quantum chemist need only focus on the atoms’ valence electrons.

Although he had only limited fluency in Russian, Hellmann thrived in his new position, and his superiors noticed. He received the Soviet equivalent of a full professorship in 1935, a considerable raise in 1936, and the title of leading scientist in 1937. At the urging of government officials, Hellmann became a Soviet citizen.

Hellmann’s position also gave him the chance to achieve a long-standing goal: publishing a quantum chemistry textbook. He had written most of it in Germany but couldn’t get it published after Hitler’s rise to power. Hellmann revised the manuscript with the help of colleagues and students at Karpov. His Russian-language textbook was published in 1937; remarkably, a publisher in Leipzig agreed to distribute a German version. “It was very progressive for its time,” says Roald Hoffmann, a Cornell theoretical chemist and Nobel laureate who owns an original copy of the 1937 German printing. Though the book sold poorly in Germany, it would serve as an important reminder of Hellmann’s achievements during the many decades that his fate remained a mystery.

As Hellmann’s career was soaring, Stalin was beginning his purge of “anti-Soviet” elements within the USSR. Foreigners—even those who had fled persecution in other nations—were regarded as having questionable loyalty. Despite his success, or perhaps partly because of the jealousy his success may have wrought, Hellmann seemed to recognize that he was under suspicion. In one letter to relatives, he expressed a desire to leave Karpov for another institution that was considered more tolerant. In another letter he described how foreign-born people in the Soviet Union “would vanish.” He clearly knew that he was in danger.

Late on the night of 9 March 1938, Soviet secret police broke into the family apartment and dragged Hellmann away. Hans Jr, who was 8 years old at the time, says that he still remembers men in dark coats waking him up and searching for material to use as evidence against his father. A few days later, Viktoria went to Karpov in an unsuccessful effort to collect her husband’s salary. She spotted a notice on the wall denouncing her husband, which was signed by two of his colleagues. One of them took over Hellmann’s research group after the arrest.

Hellmann’s wife and son wouldn’t definitively learn of his fate for another 51 years.

Getting the story out

Kicked out of their Karpov-provided apartment, Viktoria and Hans Jr scraped together a living near Moscow as war broke out. But even Viktoria’s Slavic background couldn’t protect her from the Soviets’ aggressive campaign to quash dissension. She was arrested in 1941 and charged with conspiring to translate for invading German soldiers—soldiers who would have thrown her into a concentration camp because of her Jewish heritage.

For the second time, Hans Jr saw a parent hauled away. Police took the 11-year-old to an orphanage, but he ran away to his aunt’s house in Moscow. For the next half century, he would go by the name of Genadij Minchin. Gradually he pieced together information about his family. In 1956 he got to see his mother, who despite being “rehabilitated” was forced to live near the camp in Kazakhstan where she had been imprisoned. Two years later, through a Red Cross initiative, he received contact information for the family of his father’s sister, who lived in Hamburg, but he was not permitted to leave the USSR to meet them. During the final months of the Soviet Union’s existence, in 1991, his forced confinement in that country finally came to an end. He emigrated to Germany with his wife and one of his sons, changed his name back to Hans Hellmann Jr, and obtained German citizenship.

Still, he didn’t know the full story of his father’s life. In 1989 he and his mother finally received the death certificate that confirmed their fears—Hans Hellmann Sr had been executed on 29 May 1938. The turning point in Hans Jr’s quest came in 1992, when a history professor put him in touch with Sabine Arnold, a historian and journalist who was based in Moscow. With Arnold’s help, Hans Jr wrote a request to receive copies of a file on his father that had been compiled by the People’s Commissariat for Internal Affairs (NKVD), the precursor to the KGB. The file included a confession letter, handwritten but not by Hellmann, that authorities had almost certainly forced him to sign. “His hand was trembling when he wrote his signature,” Arnold says. Another document in the file admitted that the NKVD had no proof that Hellmann had committed the crime of espionage of which he was accused.

Over the next several years, Hans Jr collaborated with Arnold, chemist Eugen Schwarz, and others to write a comprehensive personal and scientific biography of his father. Schwarz, from the University of Siegen, had been fascinated by Hellmann since reading his papers as a graduate student in the 1960s. The biography, along with a reissued edition of Hellmann’s textbook, has helped put a story behind a name that often appears, without context, next to Feynman’s. “He truly was an outstanding scientist,” says Nobel laureate Hoffmann. Beyond his limited but impressive research, Hellmann’s legacy lies in his rare ability to simplify, whether in proposing pseudopotentials or explaining them to a broad audience in a textbook.

Viktoria lived into her 90s in Russia. Hans Jr, now 88, lives in Siegen. Every two years he takes an hour-long ride to the University of Marburg, where the theoretical chemistry group hosts a Hans Hellmann Award Lecture given by a leading European quantum chemist. He plans to make the trip again for the next lecture in December 2019.