The ab initio calculation of the 0 K isotherm of the organic molecular crystal 1,1-diamino-2,2-dinitroethylene (C2H4N4O4), also known as FOX-7, is accomplished by means of solutions of the many-body Schrodinger equation in a periodic crystal lattice. It was found that the Hartree-Fock method is adequate to represent the behavior of the material and that, in general, density functional methods give inferior results. Initially, calculations were done assuming rigid molecules under compression. In further calculations the internal molecular bond lengths were optimized for each value of compression. Finally, calculations were performed in which all the molecular coordinates were optimized. The results are compared with experimental x-ray diffraction data obtained by compressing samples in diamond anvil cells. Excellent agreement is obtained when the molecular coordinates are completely optimized. In addition, FOX-7 is highly anisotropic and this anisotropy must be taken into account to obtain good agreement with hydrostatic compression data. (C) 2003 American Institute of Physics.