The electrostatic term in lattice-energy calculations: C2H2, CO2and C2N2

The widely used atom-atom approximation for evaluating intermolecular energies is deficient in its treatment of the electrostatic interactions. Lattice-energy calculations have been performed for three crystal structures with explicit incorporation of the electrostatic energy, at three levels of approximation, based on Hartree-Fock molecular charge distributions. Although the molecules chosen are all non-polar, the electrostatic term in each case provides most of the calculated lattice energy and leads to an appreciable contraction of the predicted equilibrium cell dimensions. In cyanogen the electrostatic contribution appears necessary to account for the observed orthorhombic structure rather than an alternative cubic form. Treating each molecule as a point quadrupole severely overestimates the interaction energies of nearest-neighbor molecules but for more distant neighbors agrees fairly well with more detailed models of the molecular charge distribution. Assigning point charges to the several atoms is an adequate approximation for the three systems examined but greater flexibility is likely to be required for molecules of lower symmetry.