On the origin of topological differences between experimental and theoretical crystal charge densities

Topological analysis of experimental and theoretical (molecular and crystal ) electron densities of p-nitroaniline and p-amino-p'-nitrobiphenyl reveals considerable discrepancies between experiment and theory for the bond crit ical points properties. Particularly large differences occur for the positi ve curvature along the bond path (lambda(3)). The differences become somewh at smaller when more extended basis sets and correlation effects are introd uced in the theoretical calculations. The effect of the crystal matrix on t he properties of bond critical points is evaluated for the p-nitroaniline m olecule using the 6-21G** and 6-31G** basis sets. The differences between t he isolated molecule and the molecule in the crystal are too small to expla in the quantitative disagreement between the theoretical and experimental t opologies reported in the literature and found in the current study. For mo st bonds, the observed changes in the properties of the electron density ag ree well for both basis sets but some discrepancies are found for changes i n lambda(3) for N-H and aromatic C-C bonds. When the theoretical densities are projected into the multipole density functions through refinement of th e theoretical structure factors, the topological properties change and diff erences between theory and experiment are reduced. The main origin of the o bserved discrepancies is attributed to the nature of the radial functions i n the experimental multipole model.