A Hamiltonian approach to fermion Casimir energy

A derivative expansion of the one-fermion-loop effective action for the O(4) σ model can be computed in a variety of ways. This can then be used to investigate the effect of fermions on skyrmion stability. Here a method of calculating the energy density itself as a derivative expansion is outlined. The method would be applicable to a wide class of models. It is illustrated by computing the four-derivative terms in the fermion Casimir energy density in the O(4) σ model. The result agrees with previous calculations. (orig.)