A two‐phase equilibrium equation of state (EOS) for periclase (MgO) was constructed using ab initio quantum mechanics, including a rigorous calculation of quasiharmonic phonon modes. Much of the shock wave data reported for periclase is on porous material. We compared the theoretical EOS with porous data using a simple ‘snowplough’ treatment and also a model using finite equilibration rates suitable for continuum mechanics simulations. (This model has been applied previously to various heterogeneous explosives as well as other porous materials.) The results were consistent and matched the data well at pressures above the regime affected by strength — and ramp‐wave formation — during compaction. Ab initio predictions of the response of porous material have been cited recently as a novel and advanced capability; we feel that this is a fairly routine extension to established ab initio techniques. © 2004 American Institute of Physics