From free molecules, theoretical outline

The following theoretical outline is limited to a presentation of the definitions and equations necessary for the description of the diffraction of electrons from free molecules. More detailed theoretical surveys, including the deduction of the equations, have been given by various authors. > ... 

The expected local symmetry, as assumed from theoretical calculations, is generally not established in the crystal lattice. Instead it is usual that the crystal packing distorts the local symmetry of the molecule. Thus, if the molecule has no local symmetry within the 3o criterion—i.e., geometric parameters obey the assumed local symmetry within the threefold of the given standard deviation—the free molecule should be considered adopting the local symmetry. On the other hand, there exist many examples in which the crystal symmetry requires a local symmetry which should not exist for the free molecule. This very often holds for molecules which reveal disorder in the crystal lattice, another problem which is outlined in detail below in Section TV. 

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