Interactions between molecules in crystals

Like the interaction energy between two molecules, the total lattice energy of a molecular crystal contains several contributions from the different types of interactions. We may write 

In this expression, the dipole dipole interactions are included in the electrostatic term rather than in the van der Waals interactions as in Eq. (9.43). Of the four contributions, the electrostatic energy can be derived directly from the charge distribution. As discussed in section 9.2, information on the nonelectrostatic terms can be deduced indirectly from the charge density. The polarizability a, which occurs in the expressions for the Debye and dispersion terms of Eqs. (9.41) and (9.42), can be expressed as a functional of the density (Matsuzawa and Dixon 1994), and also obtained from the quadrupole moments of the experimental charge density distribution (see section 12.3.2). However, most frequently, empirical atom-atom pair potential functions like Eqs. (9.45) and (9.46) are used in the calculation of the nonelectrostatic contributions to the intermolecular interactions. 

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Gavezzotti A (2010) The Unes-of-force landscape of interactions between molecules in crystals. Acta Cryst B66 396 06... 

Molecular modeling is extensively covered in other articles, and since the interactions between molecules in crystals are the same as in other aggregation states of matter, no really new principles are involved. Therefore, the following discussion is limited to subjects that are direcdy related to the symmetry of crystals. 

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