A SEMIEMPIRICAL QUANTUM CHEMICAL METHOD TO CALCULATE THE LATTICE ENERGIES OF ORGANIC MOLECULAR CRYSTALS. By Gerhard Raabe, Institut fur Organische Chemie, RWTH Aachen, Prof.-Pirlet-Str. 1, D-52056 Aachen, Germany

A semiempirical quantum chemical method based on a perturbative approach [1] is presented which retains the original meaning of the contributions (dispersion, electrostatic, induction, and repulsion energy) to the lattice energy. These components are obtained using the results of the semiempirical MINDO/3 method [2]. Thus the dispersion energy is calculated by means of the London formula employing MINDO/3-FP [3] atom-in-molecule polarizabilities and vertical (Koopmans) ionization potentials. Only the Coulomb part of the electrostatic energy is considered employing MINDO/3 atomic charges which are also used together with the polarizabilities to obtain the induction energy. An approximate method is used to calculate the exchange repulsion energy which includes the repulsive part of the lattice energy via MINDO/3 molecular orbitals and overlap integrals.

Results obtained for several organic compounds including some polymorphs are presented. The choice of the molecular model is discussed, emphasizing the importance of a correct positioning of the hydrogen atoms.

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[2] R. C. Bingham, M. J. S. Dewar, D. H. Lo (1975). J. Am. Chem. Soc.; 97,1285 - 1293.

[3] R. M. Metzger (1981). J. Chem. Phys.; 74, 3444 - 3457.