The 0 K isotherm for the organic molecular crystal 1,1‐diamino‐2,2‐dinitroethylene is calculated reasonably accurately using the Hartree‐Fock approximation to the solutions of the many‐body Schro¨dinger equation for a periodic system as implemented in the computer program CRYSTAL. The equilibrium lattice parameters are predicted to within one percent of the values reported by Bemm and O¨stmark. Pressure values on the isotherm agree extremely well with the values measured by Peiris and co‐workers. The key to obtaining such accuracy is the relaxation of all the molecular coordinates as well as the lattice parameters under a fixed volume constraint. It was found that Density Functional Theory (DFT) calculations gave much poorer results, but optimizations were not as extensively investigated in this case. © 2004 American Institute of Physics