A local point of view to intermolecular bonding of saturated organic molecules

The general expression for the common one-electron density matrix (DM) of saturated organic molecules obtained previously in the framework of the Huckel type model [V. Gineityte, J. Mel. Struct. (Theochem) 343 (1995) 183] has been applied to investigate intermolecular bonding. To this end, the total DM of two interacting molecules has been expressed as a sum of two matrices, the first one coinciding with the direct sum of DMs of isolated molecules and the second one being a correction describing the intermolecular bonding. As a result, additive intra- and intermolecular contributions have been revealed both within occupation numbers of basis orbitals and within the total energy of interacting molecules, and an internal additivity of intermolecular terms with respect to increments of pairs of bonds has been established. These results have been used to substantiate the local models of intermolecular bonding commonly used when studying the early stages of chemical reactions between two saturated molecules. Conditions for which the local model is adequate have been formulated explicitly. (C) 1999 Elsevier Science B.V. All rights reserved.