> 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
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