A renormalization of the HamiltonianH, previously devised by Silverman and Joseph to calculate the temperature‐dependent ferroelectric‐mode frequency in perovskites exhibiting a second‐order transition, is extended to the case of a first‐order phase change. The extension involves inclusion inHof all anharmonic terms to sixth order which give rise, after a thermal average is taken, to polarization and temperature dependences in the internal energy, which are similar to those appearing in the phenomenological theory of Devonshire. Validity of the latter theory in high electric fields can now be understood as a consequence of field independence of the acoustic modes, which make the dominant contribution to the temperature dependence of the internal energy. Since the optical frequencies must depend on field under these circumstances, so that validity of the Devonshire theory requires that they make little contribution to thermal properties, the optical branch should lie above the acoustic, except in (001) planes where the unstable modes occur which are responsible for the transition. From domain‐wall calculations in the tetragonal phase of barium titanate, it is concluded that dispersion is weak in the (001) planes, so that all frequencies withkin such a plane will be low. This suggests that (001) planes should be surfaces of discontinuity in frequency, which leads in turn to the prediction that the electric susceptibility and switching rate in tetragonal barium titanate will exhibit thickness dependence of purely lattice‐dynamical origin, with the thickness dependence of the latter exceeding that of the former. The theory thus provides a means of correlating the field and thickness dependence of switching and polarization with the structure and temperature dependence of the lowest transverse optic branch.