In the ferroelectric literature, thermodynamic potentials are often written in terms of electric polarization P rather than displacement field D. When written in terms of D, the minimization of a thermo potential retains its equivalence to the entropy maximum principle, so there is a clear downside to using P. But aside from this, how does one construct the potentials with P? In the literature that I have found so far, it often looks like P is simply substituted for D, and the point is not explicitly discussed. Secondly, according to the textbook by Lines and Glass, the other downside to using P in the thermodynamic potentials is that constant P conditions are not obtainable in practice. I am having trouble seeing how one obtains constant E or constant D conditions experimentally. The book makes some reference to open circuit conditions being constant D and short circuit being constant E. For short circuit, the field in the ferroelectric should be zero, and thus constant. Is it possible to obtain constant E conditions for E=\=0? For open circuit, I don't see how this corresponds to constant D at all.