Coombes, David Stuart; (1997) Towards the a priori prediction of molecular crystal structures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Previous work on modelling crystal structures of polar organic and hydrogen bonded molecules used an isotropic repulsion-dispersion potential together with a point charge electrostatic model. However, this model does not describe accurately the electrostatic forces arising from the non spherical features of the molecular charge distribution such as lone pair and [pi] electron density. The electrostatic forces can be described far more accurately using a Distributed Multipole Analysis (DMA) of an ab initio wavefunction, which describes the molecular charge distribution as sets of multipoles (charge, dipole, quadrupole etc.) on every atomic site. This thesis investigates the ability of the DMA model, together with an empirical repulsion- dispersion potential, to reproduce the crystal structures of a range of organic molecules, from hydrocarbons to those which contain mixed functional groups and hydrogen bonds. The optimisation of the empirical repulsion-dispersion potential parameters is also attempted, in order to optimise them for use with the DMA electrostatic model. However, little improvement in the already reasonable predicted structures is obtained using these potentials, suggesting the use of a static minimisation technique is the main limitation to improving the crystal structure predictions. This potential is then used together with a powerful crystal structure prediction program to predict possible polymorphs of alloxan. This generates the observed structure and a number of alternative low energy crystal packings. Isolated dimer structures are also predicted. This is used to account for the unusual crystal packing and lack of hydrogen bonds in the observed crystal structure of alloxan. This work shows that the combination of a powerful crystal structure prediction program together with a realistic model for the intermolecular forces represents a major advance in our ability to predict crystal structures, although other factors need to be taken into account before genuine ab initio crystal structure prediction becomes possible.

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