Parameterization of site-site potentials in the spherical expansion formalism: A point charge model for the electrostatic interaction of the aza-benzene molecules

It is shown that application of the two centre spherical expansion furnishes an elegant method to derive model parameters for the intermolecular interaction. The method is applied to the electrostatic interaction and to the electrostatic potential of the aza-benzene molecules. Point charge models are derived from ab initio calculated permanent multipole moments (up to and including the 26 moments) [22]. Point charge models are proposed with single point charges located at the atomic nuclei and with nitrogen lone pair charges in the molecular plane, 1 bohr away from the nitrogen atoms. Without the introduction of these lone pair charges no satisfactory fit of the electrostatic properties of the molecules was obtained. Constraining the C-H bonds and the nitrogen lone pair fragments to electrical neutrality the best charge for H was found to be 0-19 and for N 0-68 a.u. obtained from a fit including all molecules simultaneously