THE NEED FOR REALISTIC ELECTROSTATIC MODELS TO PREDICT THE CRYSTAL-STRUCTURES OF NLO MOLECULES

Organic molecules with high non-linear optic coefficients have polar g roups often linked by delocalised pi-electron systems. Thus the charge distribution around many atoms in such molecules will be highly non-s pherical. Current methods of predicting the crystal structures of thes e molecules, and thus the macroscopic second harmonic generation capab ility, take no account of these features as they use fixed charges at each atomic site to model the electrostatic interaction. A more realis tic representation of the electrostatic interaction is to use a set of multipole moments on each atom, which may be derived from a distribut ed multipole analysis of an ab-initio wavefunction for the isolated mo lecule. We have adapted the widely used THBREL program to simulate the crystal packing of organic molecules, using the distributed multipole electrostatic model. Preliminary results for the prediction of the cr ystal structures of molecules such as meta-nitroaniline are presented and contrasted with an atomic point charge model. The electrostatic ef fects of the lone pair and pi-electron density makes a qualitative dif ference to the crystal packing.