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    Switching requires a metallic contact, raising fundamental issues about the interface between the FE and the electrode. Polarization leads to fixed charge of opposite sign at the two metal-FE interfaces. Electrode free charge acts to screen the polarization charge creating dipoles of the same sign at the two interfaces, however, the screening is always imperfect and the residual depolarizing field alters the electrostatic potential inside the FE. The partially covalent nature of the bonds in the FE changes the band structure with respect to that of a perfectly ionic compound which, from consideration of dynamical charge tensors, depends on the atomic distortion. Reduction/oxidation conditions at the surface can control the film polarization.