A Priori Calculations of Crystal Structures

Some molecules (or substances) have more than one stable crystal form. Such a molecule is said to be polymorphic. These different crystal forms can usually be identified by different physical properties, such as melting points. They may have the molecules present in different conformations. 

Usually, the conformation of a molecule in the crystal is the lowest energy conformation that the molecule can take up in the gas phase. Thus, for example, the n-alkanes tend to crystallize in the all-anti conformation. Occasionally, a crystal is found in which a slightly less stable conformation in the gas phase becomes the only conformation in the crystal phase. For this to happen, the less stable gas-phase molecule must form a crystalline lattice that is more stable than the one formed by the anticipated conformation, and by more than the energy difference between the conformations. Or, it may be an unstable polymorph, which will go over to the stable polymorph (and the stable conformation) under thermodynamic conditions. 

It is also found that whatever forces lead to stability in molecular dimers will typically lead to stability in crystals. Thus, the dipoles tend to align themselves in an anti 

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