The crystal lattice is not explicitly taken into account in the free electron model, but a quantum-mechanical justification was given a year later (1928) by Bloch's theorem : an unbound electron moves in a periodic potential as a free electron in vacuum, except for the electron mass m e becoming an effective mass m* which may deviate considerably from m e (one can even use negative effective mass to describe conduction by electron holes). Effective masses can be derived from band structure computations that were not originally taken into account in the free electron model.