Symmetry and space groups

Space group operators can be found here: http://www.embl-hamburg.de/~pinotsis/spacegroup.html

The crystallographic toolbox is at [1]. Its predecessor, SgInfo, is here.

Facts about space groups

There are 230 space groups, 65 of which accessible to proteins.

Empirically, P2₁2₁2₁ is the space group which occurs most often (23%) in the PDB; second place is P2₁ with 14.4%, then comes C2 (9.0%) ( [2]).

Reflection conditions

These are explained and listed in Section 2.2.13 of International Tables of Crystallography, Volume A . If this is not on your desk, check out the IUCr wiki:

Reflection_conditions which explains the topic, and links to lists of the general absences in article integral reflection conditions, and the absences along the screw axes in serial reflection conditions.