John Hasbrouck Van Vleck (March 13, 1899 – October 27, 1980) was an American physicist. Born in Middletown, Connecticut the son of mathematician Edward Burr Van Vleck and grandson of astronomer John Monroe Van Vleck, he grew up in Madison, Wisconsin, and went to Harvard for undergraduate and graduate studies. He joined the University of Minnesota as an assistant professor in 1923, then moved to the University of Wisconsin-Madison before settling at Harvard. Van Vleck developed fundamental theories of the quantum mechanics of magnetism and the bonding in metal complexes (crystal field theory).
Van Vleck participated in the Manhattan Project by serving on the Los Alamos Review committee in 1943. The committee, established by General Leslie Groves, also consisted of W.K. Lewis of MIT, Chairman; E.L. Rose, of Jones & Lamson; E.B. Wilson of Harvard; and Richard C. Tolman, Vice Chairman of NDRC. The committee's important contribution (originating with Rose) was a reduction in the size of the firing gun for the Little Boy bomb. This concept eliminated additional design-weight and sped up production of the bomb for its eventual release over Hiroshima.
In the year 1961-62 he was George Eastman Visiting Professor at University of Oxford and Professorial Fellow of Balliol College. He was awarded the Lorentz Medal in 1974. For his contributions to the understanding of electrons in magnetic solids, van Vleck was awarded the 1977 Nobel Prize in Physics, along with Philip W. Anderson and Sir Nevill Mott. Van Vleck transformations are also named after him.