It is useful to recognize that the only states within ~ ± k B T of the Fermi level contribute significantly to the entropy. Other states are either fully occupied, f = 1, or completely unoccupied, f = 0. In either case, these states do not contribute to the entropy. If one assumes that the density of states is constant within ± k B T of the Fermi level, one can derive that the electron heat capacity, equal to: