Properties of molecular crystals by means of theory

A novel quantum-chemical approach to the properties of molecular crystals has recently been developed. This Hartree–Fock-based self-consistent-crystal-field method is outlined and applied to the intramolecular torsion angle in solid hydrogen peroxide, the cold-phase structure of solid dinitrogen and the hot-phase structure of solid acetylene. The potential-energy surfaces are calculated with the assumption of perfectly ordered crystals and coherent molecular potential-energy surfaces are produced. Agreement with experiment is obtained for hydrogen peroxide. It is suggested that discrepancies between calculated and observed structures of N2(s) and C2H2(s) result from static and dynamic disorder, respectively. This interpretation is consistent with the experimentally observed intramolecular-bond shortenings in these two solids. Lattice energies in good agreement with experiment are obtained in all three cases.