 In 1922, Niessen’s doctoral thesis, as well as Wolfgang Pauli’s extended doctoral thesis, dealt with the hydrogen molecule ion in the Bohr-Sommerfeld framework.Their work is referred to as the Pauli-Niessen model.Their works helped to show the inadequacy of the old quantum mechanics, which gave physicists the impetus to explore new paths which led to the matrix mechanics formulation of quantum mechanics by Werner Heisenberg and Max Born in 1925  and the wave mechanics formulation by Erwin Schrödinger in 1926, which were shown to be equivalent.