Thermodynamic functions of molecular solids

The electron and nuclear contributions are also sums on the number of atoms contained in the molecule  

An ionic solid is made up of two kinds of ions anions and cations. These ions are located at the nodes of two interlocking periodic lattices the anionic sublattice and the cationic sublattice. 

We model the energy of an ionic crystal limiting us to the case of a diatomic crystal with the formula AaBb linking a cation A with electrovalence za and an anion B with electrovalence zb- We assume that these ions ate comparable to spheres on which the charges are uniformly distributed. The overall cohesion of the crystal is provided by the Coulombian field at l/r created by each ion, plus a repulsive potential 

Let us eonsider an ion / in the lattice, bearing the charge qu At that point, there is an eleetrostatie potential 0 created by all the other ions, and the energy of interaction between that ion / and the rest of the lattice is  

In order to find the total attraction energy, we only need to add the above expression for all the ions, taking care only to count the interaction between each pair of ions once (which we do by using the coefficient Vz), as follows  

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