Equilibrium structure and dynamics of organic crystals by Monte Carlo simulation: critical assessment of force fields and comparison with static packing analysis

The atom-atom intermolecular force field AA-CLP with subdivision of interaction energies into Coulomb-polarization, dispersion (London) and repulsion (Pauli) terms is applied to the Monte Carlo simulation of 63 organic crystals taken from the literature to cover the most common functionalities of organic and biological chemistry. Non-rigid molecules are treated as ensembles of rigid fragments connected by torsional degrees of freedom, for which ad hoc potentials are obtained from MP2-631G** calculations. The performance of the method and force field is assessed by comparison with experimental structures at 100 and 300 K. Molecular orientation, cell dimensions and sublimation energies are well reproduced, with some exceptions for fluorinated and nitro compounds. Simulated density and energy changes with temperature reproduce experimental observations. Calculated radial density functions and correlation functions reveal details of material behaviour at the atomic level, including librational amplitudes. Dynamic phenomena like methyl group rotations or rotational diffusion in the classic case of the benzene derivatives and adamantane are described in a satisfactory manner. Competition between intra-and intermolecular factors in biphenyl and other double-ring compounds is accurately described. Use of dynamic or Monte Carlo simulation gives a realistic picture of crystal structure and bonding, which often contrasts with simplistic views postulated on the basis of averaged atomic positions and static packing diagrams, as produced in typical single-crystal X-ray diffraction experiments.