Ab initio atomistic simulations can be used to predict the mechanical properties of materials subjected to shock waves, using a minimum of calibration data. The equation of state and elasticity can be calculated from quantum mechanical treatments of the electrons and lattice vibrations. Typically, the only experimental input is a pressure correction to match the ambient density. The theoretical treatment is being extended to include dynamic contributions to polymorphic phase changes and a microstructural model of plasticity. Equilibrium polymorphic equations of state were calculated for silicon; these suggest that shocks with pressures between about 15 and 80 GPa should split into two waves with different speeds. Transient X-ray diffraction results from laser-driven shocks appear to agree with the calculated behavior. ©2000 American Institute of Physics.