> How many of you have ever had a protein atom on a Wyckoff position > (AKA a special position).
Sorry one final comment! On a point of terminology: a Wyckoff position is _not_ another name for a special position, it's a descriptor of a set of positions in the unit cell of a space group which share the same site point group, but which are not equivalent by space-group symmetry to any other Wyckoff position in that space group. For example go here: http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list and click 'P2:unique axis b'. So here Wyckoff position type 'e' is one of the set of points (x,y,z) or (-x,y,-z), all of which have site point group '1': this is a 'general Wyckoff position' (aka 'general equivalent position'). This obviously includes almost every position in the unit cell, so almost every atom in a protein crystal must lie on a general Wyckoff position! This might appear trivial, nevertheless it arises because an important property of any group is that the identity operator must by definition be a member (otherwise it's not a group, it's merely a set). Then there are the more interesting 'special Wyckoff positions' (this is what is aka 'special position'); for P2 there are 4: classes 'a' through 'd'. These all have point symmetry '2', but they are listed separately because they are not equivalent by the space-group symmetry (compare entry for C2). Cheers -- Ian
