Despite the successes, the physical theories based on the principle faced increasing challenges the early 1920s. Theoretical calculations by Van Vleck and by Kramers of the ionization potential of Helium disagreed significantly with experimental values. Bohr, Kramers, and John C. Slater responded with a new theoretical approach now called the BKS theory based on the correspondence principle but disavowing conservation of energy. Einstein and Wolfgang Pauli criticized the new approach and the Bothe–Geiger coincidence experiment showed that energy was conserved in quantum collisions.