ADAPTATION OF THE PIMM FORCE FIELD FOR THE PURPOSE OF CRYSTA STRUCTURE PREDICTION
T. Goerdten and H. J. Lindner
Institut für Organische Chemie, Technische Universit"at Darmstadt, Petersenstr. 22, D-64287 Darmstadt, Germany
In the last years some new features have been added to the PIMM force field program, one being the treatment of crystal packings of organic compounds. Although the PIMM91 (gas phase) parameter set[1] results in a good reproduction of experimental sublimation enthalpies, the tendency to a slight compression of observed crystal structures leads to false global minima during crystal structure prediction. To eliminate this deficiency the Giglio potential functions for intermolecular interactions used With the current parameter set for hydrocarbons the deviations for sublimation enthalpies are within the experimental errors and cell constants are calculated with a RMS deviation of 0.12 A.
Input for the implemented crystal structure prediction procedure are the molecular geometry and the space group. The algorithm starts with the generation of a loose packing by a MD simulation or input of trial cell constants. The search for the global minimum is performed by exploring the parameter space with randomly chosen values in the first stage. In further stages the structural parameters are refined using Gaussian redistribution[2] of the worst probes with each structure optimized by a con