Sublimation entropies and vapor pressures of crystals

What are lattice energies useful for In principle, a larger lattice energy means a more stable material. In practice, mechanical stability often depends more on the directional properties of the cohesive energy, such as planes of easy cleavage. Besides, crystals made of organic molecules are mechanically weak anyway, so that minor differences in total cohesive energy may be quite irrelevant. 

The saturated vapor pressure of a crystal can be determined from the standard free energy of sublimation and is related to volatility. The sublimation entropy is  

In correlation studies over large numbers of crystal structures, the calculation of the lattice energy must be relatively fast, and it is very difficult, if not altogether impossible, to calculate accurate point charges or distributed dipoles for thousands of molecules. It is also impossible to analyze all the molecular structures to find the locations of specific charge sites such as lone pairs, bond dipoles, etc. the formulation must be a strictly atom-atom one, with all interaction centers located at easily recognizable atomic nuclear positions, and should either require no separate coulombic terms, or use a ready recipe for the approximate evaluation of atomic point charge parameters without ab initio molecular orbital calculations. 

Another model that can be applied to large-scale computations is the Williams force field (ref. [44], Chapter 4 see Table 8.4) with atomic point charge parameters calculated by the rescaled EHT approach (equation 4.12). This formulation (which we call here WqEHT) does not apply to hydrogen bonded crystals. The more refined forms of the Williams force field cannot be used, because of the difficulty of automatically allocating site charges far from atomic nuclei. 

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