Recent theoretical simulations using density functional theory (DFT) andnovel low temperature high energy x-ray diffraction experiments clearly showthe existence of a high pressure morphotropic phase boundary (MPB) in purePbTiO3.The experiments show a richer phase diagram than the simulations, with multiple monoclinic phases(Pm and Cm) in the MPB region. In this paper we examine the MPB region in more detail usinghigh precision DFT calculations within the local-density approximation (LDA)and the Wu–Cohen generalized gradient approximation. Our results support thepolarization rotation theory and open up fresh possibilities for applying chemicalpressure to engineer novel electromechanical materials. We also explain why thezone-boundary mode is more likely to be stable only at higher pressures above∼25 GPa and not atmoderate pressures of ∼10 GPa, using the LDA.