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 intermol. 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 org. crystals taken from the literature to cover the most common functionalities of org. and biol. chem. Non-rigid mols. are treated as ensembles of rigid fragments connected by torsional degrees of freedom, for which ad hoc potentials are obtained from MP2-631G** calcns. The performance of the method and force field is assessed by comparison with exptl. structures at 100 and 300 K. Mol. orientation, cell dimensions and sublimation energies are well reproduced, with some exceptions for fluorinated and nitro compds. Simulated d. and energy changes with temp. reproduce exptl. observations. Calcd. radial d. functions and correlation functions reveal details of material behavior at the at. level, including librational amplitudes. Dynamic phenomena like Me group rotations or rotational diffusion in the classic case of the benzene derivs. and adamantane are described in a satisfactory manner. Competition between intra- and intermol. factors in biphenyl and other double-ring compds. 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 at. positions and static packing diagrams, as produced in typical single-crystal X-ray diffraction expts.