GEOMETRY-DEPENDENT ATOMIC CHARGES - METHODOLOGY AND APPLICATION TO ALKANES, ALDEHYDES, KETONES, AND AMIDES

A general methodology for deriving geometry-dependent atomic charges i s presented. The main ingredient of the method is a model that describ es the molecular dipole moment in terms of geometry-dependent point ch arges. The parameters of the model are determined from ab initio calcu lations of molecular dipole moments and their Cartesian derivatives at various molecular geometries. Transferability of the parameters is bu ilt into the model by fitting ab initio calculations for various molec ules simultaneously. The results show that charge flux along the bonds is a major contributing factor to the geometry dependence of the atom ic charges, with additional contributions from fluxes along valence an gles and adjacent bonds. Torsion flux is found to be smaller in magnit ude than the bond and valence angle fluxes but is not always unimporta nt. A set of electrostatic parameters is presented for alkanes, aldehy des, ketones, and amides. Transferability of these parameters for a ho st of molecules is established to within 3-5% error in the predicted d ipole moments. A possible extension of the method to include atomic di poles is outlined. With the inclusion of such atomic dipoles and with the set of transferable point charges and charge flux parameters, it i s demonstrated that molecular electrostatic potentials as well as elec trostatic forces on nuclei can be reproduced much better than is possi ble with other models (such as potential derived charges). (C) 1995 by John Wiley and Sons, Inc.