Anisotropic atom–atom potentials from X-ray charge densities: application to intermolecular interactions and lattice energies in some biological and nonlinear optical materials

Abstract

Anisotropic atom–atom potentials based on X-ray molecular charge densities are applied in the evaluation of intermolecular interactions and lattice energies of crystals of glycylglycine, dl-histidine and dl-proline, p-nitroaniline and p-amino- p′-nitrobiphenyl. In parallel, calculations are performed on the molecular dimers (B3LYP) and on the periodic crystals with both Hartree–Fock (HF) and density functional theory (DFT) methods. The dimer interaction energies agree well with the X-ray based values, except for the strongest interactions, which occur in the glycylglycine crystal and for p-amino- p′-nitrobiphenyl. The lack of agreement in these cases is attributed to the strong induced polarization of the molecular charge densities, which is not reproduced adequately by the dimer calculations. The lattice energies are evaluated as the difference between the molecular interaction energy in the crystal and the molecular relaxation energy upon sublimation. For a given crystal, each of the terms is quite different when calculated with the Periodic HF (PHF) or Periodic DFT (PDFT) methods, but the differences are such that the total lattice energies are similar. The agreement between lattice energies derived from the experimental charge densities and those obtained from the PHF and PDFT calculations is within better than, respectively, 10 and 25 kJ/mol for glycylglycine, dl-histidine and dl-proline. However, while the experimental charge density approach give reasonable estimates of the lattice energies of p-nitroaniline and p-amino- p′-nitrobiphenyl, the PHF and PDFT calculations fail to predict the stability of these crystals.