SSP Project Summary:
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Student

Daniel Wilson, University of Edinburgh

Supervisors

Lorna Smith, EPCC

Robert Hammond, Department of Mechanical and Chemical Engineering, Heriot-Watt University

In the past crystal structure determination from powder diffraction has mostly involved highly symmetric inorganic systems and has had limited application for organic molecules. This is primarily due to the fact that whereas single crystal reflection intensities are resolved in three dimensions in reciprocal space, powder diffraction patterns contain the same intensity information projected in two dimensions. The problems which arise include peak overlap (either symmetric or accidental), large background intensities which cannot be defined easily and preferred orientation.

To address these issues, a systematic search algorithm (SNIFFER97) (Hammond, Roberts et al., 1997) was developed as part of a collaborative research program with Zeneca Specialities, Blackely. This systematically searches all the possible packing configurations of the asymmetric unit within the specified unit cell and assesses the lattice energy, powder diffraction pattern and close contacts of each structure.

The method is however limited in accuracy due to computational limitations. Simple runs can involve searching millions of configurations, a number that increases dramatically if torsional effects are considered within the molecule and / or two molecules are present in the asymmetric unit. This problem is thus ideal for parallelisation and this project would be a continuation of last year's SSP project, aiming to extend the parallel version of the code to include the Monte Carlo search algorithm and to solve the structure of a catechol/urea complex, a system previous unsolved.

References

Hammond, R.B., Roberts, K.J., Docherty, R., Edmondson, M., J. Phys. Chem. B., 101, 33, 6532, 1997.

The final report for this project is available here.
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