Oppenheimer assembles theoretical study group in Berkeley to examine the principles of bomb design. Included are J. Robert Oppenheimer, Hans Bethe, Edward Teller, John Van Vleck, Felix Bloch, Robert Serber, and Emil Konopinski. During the summer the group develops the principles of atomic bomb design, and examines the feasibility of fusion bombs. Oppenheimer emerges as a natural leader. The group estimates the mass of U-235 required for a high yield detonation at 30 kg (estimated at 100 Kt), megaton range fusion bombs are also considered highly likely. During this period Richard C. Tolman and Robert Serber discuss the idea of using explosives to collapse a shell of fissile material in place of the gun assembly method. Serber reports that they co-authored a short paper on the subject, although this paper has not been found. At this time Enrico Fermi and his staff are busy arranging for the materials required for Chicago Pile 1.