Dj. Willock et al., THE NEED FOR REALISTIC ELECTROSTATIC MODELS TO PREDICT THE CRYSTAL-STRUCTURES OF NLO MOLECULES, Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals, 234, 1993, pp. 499-506
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.