Determination of atomic point charges and point dipoles from the Cartesian derivatives of the molecular dipole moment and second moments, and from energy second derivatives of planar dimers. II. Applications to model systems

The use of Cartesian derivatives of molecular observables in the analysis of intermolecular interactions is demonstrated through ab initio calculations for some model planar systems. It is shown that the perpendicular Cartesian second derivatives of the total energy extract from the latter the parameters of the atom-atom pairwise interactions and that in the limit of large intermolecular separation these parameters are the corresponding Cartesian derivatives of specific components of the molecular dipole and second moments. The latter can be used to define unique and unambiguous atomic charges and atomic dipoles. Trends in the atomic moments are observed and correlated with the molecular bonding structure. The ability of these atomic multipoles to reproduce the molecular electrostatic potential at the intermediate and close range is also examined and found to be comparable to that of atomic point charges that are derived by fitting the electrostatic potential.