Evaluation of net atomic charges and atomic and molecular electrostatic moments through topological analysis of the experimental charge density

The atoms in molecules (AIM) theory may be used to derive atomic charges, a tomic volumes and molecular dipole moments from the charge density. The the ory is applied to theoretical periodic Hartree-Fock (PHF), density-function al (DFT) and experimental X-ray densities of p-nitroaniline using the progr am TOPOND and a newly developed program, TOPXD, for topological analysis of densities described by the Coppens-Hansen multipole formalism. Results sho w that, like dipole moments derived directly from the multipole refinement, AIM-derived atomic and molecular moments are dependent on the multipole mo del used. As expected, large differences are found between charges derived from the monopole parameters and those from AIM analysis of the experimenta l model density. Differences between the kappa'-restricted multipole model (KRMM) and the unrestricted multipole model (UMM) results are preserved in the AIM analysis. The enhancement of the molecular dipole moment of p-nitro aniline in the solid state is confirmed by both experiment and theory but t he experimental dipole moment is in much better agreement with theoretical periodic Hartree- Fock and, especially, periodic DFT (PDFT) data when KRMM is used in the refinement. The AIM analysis allows a rigorous definition of the charges of the atoms in molecules and provides a realistic basis for c omparison between molecules and between experiment and theory.