We propose variable-temperature measurements of 3D total (Bragg plus diffuse) scattering in single crystals of PbMg1/3Nb2/3O3 - PbTiO3 (PMN-PT) ferroelectrics under an applied electric field. This experiment will reveal the role of various correlations in the electromechanical response, which remains unclear despite decades of studies. We will use an electric field to bias polar displacements while cooling the samples from the disordered state while monitoring changes in macroscopic symmetry (Bragg peaks) and local correlations (diffuse scattering). This information will be used to relate our recently derived atomistic models of PMN-PT to the functional response of these materials.