Molecular packing analysis

A general method of molecular packing analysis based on the minimization of the crystal lattice energy is described. The lattice energy was approximated by a pairwise sum over nonbonded atoms in different molecules, using (exp–6–1) potential functions. Increased speed of calculation of the lattice sums was achieved by a convergence acceleration technique. The variables considered were six rigid body rotations and translations for each molecule in the asymmetric unit, and the six lattice constants. Molecular flexibility was allowed in the form of internal rotations about bonds (subrotations). In this event additional subrotation potentials of the cos² ψ type could be used to allow for conjugation energy, and the subrotation angles were additional variables. An optional thermal correction, based on the mean square thermal amplitudes and the potential anharmonicities, was applied to calculate the anisotropic thermal expansion. Nonbonded potential parameters, including conjugation energies, were found by fitting them to observed crystal structures.