THE RELAXATION OF MOLECULAR-CRYSTAL STRUCTURES USING A DISTRIBUTED MULTIPOLE ELECTROSTATIC MODEL

We describe a method for minimizing the lattice energy of molecular cr ystal structures, using a realistic anisotropic atom-atom model for th e intermolecular forces. Molecules are assumed to be rigid, and the st ructure is described by the center of mass positions and orientational parameters for each molecule in the unit cell, as well as external st rain parameters used to optimize the cell geometry. The resulting prog ram uses a distributed multipole description of the electrostatic forc es, which consists of sets of atomic multipoles (charge, dipole, quadr upole, etc.) to represent the lone pair, pi electron density, and othe r nonspherical features in the atomic charge distribution. Such ab ini tio based, electrostatic models are essential for describing the orien tation dependence of the intermolecular forces, including hydrogen bon ding, between polar molecules. Studies on a range of organic crystals containing hydrogen bonds are used to illustrate the use of this new c rystal structure relaxation program, DMAREL, and show that it provides a promising new approach to studying the crystal packing of polar mol ecules. (C) 1995 by John Wiley and Sons, Inc.