The origin of the high piezoelectric response observed in complex solid solution perovskites like PbZn_1/3Nb_2/3O3-PbTiO₃ (PZN-PT) or PbMg_1/3Nb_2/3O₃-PbTiO₃ (PMN-PT) is still not well understood. PMN is a relaxor with fascinating and controversial behavior, and is also the relaxor end member of the new high strain piezoelectric PMN-PT. Atomistic simulations will be important in gaining insight into these important materials. It is not practical to model these chemically disordered systems with first-principles and therefore an accurate potential model that is transferable across composition is required. We have developed a shell potential model for PMN-PT by fitting to extensive first-principles data of the end members. We will report on molecular dynamics (MD) results of the polarization behavior of PMN-xPT as a function of chemical ordering, temperature, applied electric field, and PT concentration. Our MD results reproduce the complex compositional phase diagram of PMN-xPT including the existence of a monoclinic phase in a small composition range. We will discuss preliminary work on examining pyrochlores using a similar approach. Our ongoing efforts to improve the accuracy of potential models for ceramics by the incorporation of charge transfer will be discussed.