Ab initio solid-state heats of formation of molecular salts from ion packing and crystal modelling:: application to ammonium crystals

A method to estimate heats of formation of ionic molecular crystals prior to their actual synthesis is put forward. It relies on systematic searches for stable polymorphs through ion packing, explicit evaluations of the cohesive energies using these computer-generated crystal structures, and separate density-functional estimations of the gas-phase contributions. Preliminary results indicate that suitable trial structures are readily found even with a moderate sampling of the search space. Moreover, the density functional procedure employed turns out to be satisfactory for charged species. On the other hand, using observed crystal geometries, a point-charge model yields acceptable values for the cohesive energies. Finally, the need for a transferable force field to relax the trial structures appears to be the most critical point.