The Cambridge Structural Database CSD of organic crystals

A complete single-crystal X-ray structure determination (Section 5.5) provides the cell dimensions, the space group, a list of positional coordinates for the atomic nuclei in the asymmetric unit, and the atomic displacement parameters (ADPs). The experiment is nowadays feasible with a modest investment of equipment money and human time, provided that a suitable crystal is available - often, the bottleneck for the productivity of an X-ray diffraction laboratory is not machine time, but the preparation of good crystal samples. The result is a high resolution picture of the molecular structure and a detailed description of the crystal packing. Through the ADPs, X-ray diffraction also provides some hints at the intra- and intermolecular dynamics. 

Of course these pictures are not always perfect and not always unique. The crystal may be disordered or twinned or may have many molecules in the asymmetric unit, in which cases the assignment of space group symmetry may be dubious, with pseudo-symmetries and modulations. Frequently, organic compounds exhibit crystal polymorphism, whereby the same molecule adopts a different conformation and a different packing in different crystals, sometimes at the same temperature and pressure (Section 14.2). Otherwise, crystals can be formed via the interaction of an organic molecule with its solvent, which is carried along into the solvate crystal, or by co-crystallization of different molecules into binary or ternary crystalline compounds. 

In the late 1960s the determination of one crystal structure was still a considerable feat, and crystallographers were satisfied with the discussion of the properties of 

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