Day, Graeme Matthew; (2003) Lattice dynamical studies of molecular crystals with application to polymorphism and structure prediction. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis develops the use of lattice dynamical methods for organic molecular crystals. Rigid molecule lattice dynamics have been implemented for anisotropic atom-atom models and applied to polymorphic systems and crystal structure prediction. Test sets of crystals were used to determine the accuracy of the elastic constant and phonon calculations in the context of approximations in the theory and experimental errors. An anisotropic atom-atom description of the electrostatic interactions between molecules can be important for modelling structures and lattice energies. Here, distributed multipole models are applied to second derivative property calculations. Accurate electrostatics offer important improvements over isotropic atom models in the phonon spectra of some polar crystals, while the effect is less pronounced for the elastic stiffness tensor. The usual empirical fitting of the repulsion and dispersion parameters incorporates average thermal effects and limits the possible accuracy of thermodynamical calculations. Hence, a non-empirical model potential has been developed for a series of chlorobenzenes, where anisotropy of the repulsive wall around the chlorine atoms is important. The ability of this non-empirical model to reproduce structural, lattice dynamical, and thermodynamic properties is assessed. Several other molecules that are known to be polymorphic were studied and the relative vibrational free energies between forms are non-negligible. Hence, the vibrational energy is included in searches for additional polymorphs of these molecules, as well as in a blind search for the crystal structure of a small, rigid organic molecule. The differences in mechanical stability and crystal growth rates were also helpful in assessing whether energetically feasible structures are likely to be observed. This work demonstrates the usefulness of lattice dynamical calculations in modelling molecular organic crystals and the possibilities for use in polymorph prediction. While limitations of some of the theoretical and practical approximations have been found, the necessary developments are mostly understood and represent possible future work.

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