Calculated lattice energies of molecular crystals

It is evident that the electrostatic interactions constitute a major component of the lattice energy of ionic crystals. According the treatment for NaF described above, the ratio of absolute values of the electrostatic and repulsive forces to the lattice energy is l l/n, where n is the Born coefficient. With n ff = 7.445 (Table 

the electrostatic contribution is % 115% of the total interaction energy. On the other hand, for small nonpolar molecules in the gas phase and dipole-dipole interactions averaged over all mutual orientations, the dispersion forces contribute 90-100% of the interaction (Israelachvili 1992, p. 95). 

In molecular crystals, the relative importance of the electrostatic, repulsive, and van de Waals interactions is strongly dependent on the nature of the molecule. Nevertheless, in many studies the lattice energy of molecular crystals is simply evaluated with the exp-6 model of Eq. (9.45), which in principle accounts for the van der Waals and repulsive interaction only. As underlined by Desiraju (1989), this formalism may give an approximate description, but it ignores many structure-defining interactions which are electrostatic in nature. The electrostatic interactions have a much more complex angular dependence than the pairwise atom-atom potential functions, and are thus important in defining the structure that actually occurs. 

Hirshfeld and Mirsky (1979) evaluated the relative contributions to the lattice energy for the crystal structures of acetylene, carbon dioxide, and cyanogen, using theoretical charge distributions. Local charge, dipole and quadrupole moments are used in the evaluation of the electrostatic interactions. When the unit cell dimensions are allowed to vary, inclusion of the electrostatic forces causes an appreciable contraction of the cell. In this study, the contributions of the electrostatic and van der Waals interactions to the lattice energy are found to be of comparable magnitude. 

Spackman et al. (1988) have used experimental charge densities to sum 

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