Asymmetric Unit

A crystal structure consists of a basic motif that is repeated in 3D space by the symmetry operators of the crystallographic space group. A crystallographer determines the coordinates of the atoms in this basic motif, called the asymmetric unit. It is the smallest part of a crystal structure from which the complete structure can be built using space group symmetry.

The asymmetric unit may consist of only one molecule or ion, part of a molecule, or several molecules that are not related by crystallographic symmetry. (This is why you sometimes see more than one molecule in Mercury when you first display a structure from the Cambridge Structural Database.) For example, consider structures of formula C12 H18 N4 O2:

If the asymmetric unit contains one molecule, the crystallographer must determine the coordinates of 36 atoms.
If the asymmetric unit contains two molecules, the crystallographer must determine the coordinates of 72 atoms.
If the asymmetric unit is half a molecule, this implies that the molecule possesses symmetry coincident with a crystallographic symmetry element. For example, the molecule might possess a mirror plane, so that half of the atoms are related to the other half by symmetry. In this case, the crystallographer must determine the coordinates of only 18 atoms.

Related Topics

Atomic Labels
Cambridge Structural Database
Crystal Chemical Unit
Molecule
Z' (Number of Molecules in the Asymmetric Unit)

Cambridge Crystallographic Data Centre Web: http://www.ccdc.cam.ac.uk Support Email: support@ccdc.cam.ac.uk Support Phone: +44 1223 336022