A simple model for calculating ground- and excited-state properties of molecular and rare-gas crystals is presented. The electrons are considered to be tightly bound to their molecular or atomic sites and the effects of the crystal potential, calculated with local-density funcationals, are treated as a perturbation of the molecules or atoms. Results for Ar to 500 kbar show that the ground-state atoms compress as the pressure is increased and that there is a gradual increase in excition energies. Preliminary results on ground-state Xe to 1.5 Mbar show that, to about 1 Mbar, the electronic distributions of the Xe atoms compress, but beyond that there is a slight expansion.