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

Abstract

The widely used atom-atom approximation for evaluating intermolecular energies is deficient in its treatment of the electrostatic interactions. Latticeenergy 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.

Nonbonded potentials for azahydrocarbons: the importance of the Coulombic interaction

“…The work of Hirshfeld & Mirsky (1979) on ethanedinitrile used potential (II) for nitrogen. We repeated these calculations under identical conditions and obtained different results for the Coulombic lattice energy.…”

The influence of Coulombic interactions on thermal parameters for naphthalene and anthracene: a lattice-dynamical approach

“…For these reasons a more convenient model is called for. A very practical one used previously for calculating lattice energies consists of discrete charges placed on the atomic sites (Hirshfeld & Mirsky, 1979). Although a more sophisticated model, the so-called 'distributed dipole model' (Gamba & Bonadeo, 1981), has been proposed, this is only necessary in some special cases (lone-pair atoms, for instance) and even in this case it can always be reproduced by means of two opposite charges (Williams & Cox, 1984).…”

The role of bonding electrons in intermolecular forces; solid acetylene

“…Their approach, which is not formulated in a form suitable for the crystalline state, would in any case, if applied to such a problem involve a prohibitively large amount of computing. Hirshfeld et al [15] performed lattice energy calculations for the C2H 2 crystal in space group Pa3 explicitly including contributions to Eetr162 They obtained a cell at equilibrium of 5-65/~, much smaller than the experimental value; they attributed the discrepancy to too large a (negative) contribution from Eelec 9…”