Dialog Box: Space Group Symmetry from Table [ Symmetry dialog ]
Space-group symmetry is obtained using licensed excerpts from the SGInfo program of Ralf Grosse-Kunstleve.
You can specify the space group in any of three ways: 1) the Hermann-Maughin (H-M) or International symbol; 2) the Hall symbol (S.R. Hall: Acta Cryst., A37, 517, 1981); or 3) the number of the group in the International Tables for X-ray Crystallography .
International Tables Volume. You have the option of using the older version of the Tables (called the Second Edition : various revisions and reprints from 1952 to 1979) in which the symmetry information was in Volume I, or the newer version (1983 onwards) in which it is in Volume A. The principal difference between the two versions is that in the older one the unique axis of a monoclinic group is assumed to be the c-axis, whereas in the newer one it is assumed to be the b-axis. Thus entering the symbol P2/m gives two different orientations depending on the volume selected. You can always specify the orientation by entering the long form of the symbol, i.e. P 2/m 1 1, P 1 2/m 1 or P 1 1 2/m.
The H-M or Hall symbol can be typed into the appropriate edit box in either short form or long form, with or without spaces between positions. However, it is usually safer to select the symbol from the list box at the bottom, which gives the standard-form symbols for all the space-groups. Clicking the Select button copies the relevant data to the edit boxes at the top; it does not actually generate the symmetry. Symmetry generation is done after you click OK - this may take a few seconds.
If the space group you select does not appear to be consistent with your choice of axes, a warning box appears, but in most cases consistency is not required. However, if you selected trigonal rhombohedral axes in the Title/Axes dialog, only a rhombohedral space group may be selected. This is done by adding :R to the end of the symbol in the case of H-M symbols, or asterisk (*) in the case of Hall symbols. If you select the rhombohedral setting and then switch to the Custom Symmetry option, the lattice type will be P; it will be R if the axes are hexagonal.
Origin of coordinates. In the International Tables , 24 space groups in the orthorhombic, tetragonal and cubic systems are given with a choice of origin: 1) not on a center of inversion or 2) on a center of inversion. These two origins are selected by adding :1 or :2 respectively to the end of the H-M symbol or the number. Although the origin on the center is second in the Tables and in the list, if the number is omitted this will be the default. This difference in origin is explicit in the Hall symbol, and other choices of origin may be specified for any space group with the Hall symbol. Note also that the origin may be shifted before transformation (see Transformation Parameters).