Use of X-ray charge densities in the calculation of intermolecular interactions and lattice energies: Application to glycylglycine, dl-histidine, and dl-proline and comparison with theory

Experimental X-ray charge densities from low-temperature data are used in the evaluation of the intermolecular interactions rind lattice energies of crystals of glycylglycine, DL-histidine, and DL-proline. The X-ray analysis leads to a set of atom-centered distributed multipoles, from which electrostatic interactions are calculated. Nonempirical exp-6 atom-atom potentials are used to calculate the smaller contributions of van der Waals interactions. For comparison, parallel theoretical calculations are performed on the molecular dimers (B3LYP) and the periodic crystals (Periodic Hartree-Fock, PHF). The dimer interactions show good agreement with experimental values, except for the strongest interactions in the glycylglycine crystal. The experimental charge density results correlate well with those based on the PHF calculations, but quantitative agreement for the interaction energies is only obtained after application of a scaling factor of similar to 0.76 to the PHF values. The discrepancy is attributed to the well-known overestimate of molecular polarity in the HF method, resulting from neglect of electron correlation. The agreement between lattice energies derived from the experimental charge density and theoretical values from the PHF calculations is within 10 kJ/mol for the crystals examined in this study. The study provides the basis for use of experimental electrostatic moments in molecular modeling calculations of more complex systems.