QUANTUM MECHANICAL MOLECULAR MECHANICAL SELF-CONSISTENT MADELUNG POTENTIAL METHOD FOR TREATMENT OF POLAR MOLECULAR-CRYSTALS/

The self-consistent Madelung potential (SCMP) approach for calculating molecular wave functions for a subunit embedded in a symmetrical envi ronment constituted by the copies of the subunit is implemented with s emiempirical NDDO model Hamiltonians and supplemented with empirically parameterized dispersion-repulsion interaction potentials. This model yields sublimation enthalpies in good agreement with available experi mental data for a series of molecular crystals, including imidazol, be nzimidazole, urea, urethane, dicyaneamide, formamide, uracil, cytosine , maleic anhydride, succinic anhydride, and 1, 3, 5-triamino-2, 4, 6-t rinitro-benzene. The SCMP-NDDO method, which avoids difficulties conce rning the parametrization of charges in the molecular mechanics force fields, is proposed mainly for the treatment of molecular crystals wit h large unit cells. It might be particularly useful where important ch arge reorganization is expected under the effect of the crystal field. Charge distributions, obtained by the SCMP and the simple dielectric cavity self-consistent reaction field models, are compared and analyze d. (C) 1998 John Wiley & Sons, Inc.