In 1952 he was invited to Harvard University as a Fulbright scholar. There he worked together with Donald Howard Menzel and Hari Kesab Singh in the field of non-linear gases. His work with the Boltzmann equation led to his well-known BGK collision model in 1954 together with Eugene P. Gross and Max Krook. It was at first extensively developed for ionized gases with many applications. These days, the BGK collision operator is essential for the recent development of lattice Boltzmann automata methods.