Asymmetric unit

Unité asymétrique (Fr). Unità asimmetrica (It). 非対称単位 (Ja).

An asymmetric unit of a space group is a simply connected smallest closed part of space from which, by application of all symmetry operations of the space group, the whole space is filled. This implies that:

• mirror planes must form boundary planes of the asymetric unit;
• rotation axes must form boundary edges of the asymetric unit;
• inversion centres must either form vertices of the asymetric unit or be located at the midpoints of boundary planes or boundary edges.

These restrictions do not hold for screw axes and glide planes.

In mathematics, an asymmetric unit is called fundamental region or fundamental domain.

See also

• Unit cell
• Section 2.2.8 in the International Tables for Crystallography, Volume A