Knowledge of the equation of state is crucially important in materials science and engineering, metallurgy, geophysics, and planetary sciences. However, equilibrium coexistence of phases during a pressure-induced martensitic transformation is extremely difficult to realize experimentally, and most shock and anvil-cell experiments contain various amounts of a nonhydrostatic, anisotropic stress. Hence, an improved understanding of transformations arises when we can better compare idealized theoretical results to realistic experimental data. A long-studied case is iron (Fe), our focus below, but the results remain quite general.