 R. de L. Kronig

R. de L. Kronig, with W. G. Penney, constructed the Kronig-Penney model for electron motion through a periodic crystal potential. The model turns the general periodicity argument behind Bloch's theorem into an explicit calculation of allowed energies.^[ref]

A periodic potential can exclude intervals of energy even when an electron moves freely inside each simplified region. This links wave matching to a solid's band gaps, while retaining an approximation to the many-electron crystal problem.