Subject: Center of coordinates

From: Charles Letner <[email-masked]>
Date: Tue, 14 Nov 1995 12:50:17 -0500 (EST)

Hello all,
	This is probably a basic question for anyone who is a
crystallographer.  I'm not and keep wondering about this.  The coordinates
of crystal structure give positive and negative values in the x, y,
and z dimensions (i'm mainly speaking of pdb format files).  This inidates
that the origin of the crystal is located someplace!  What I want to know
is, is there a standard/specification of where the origin is located?
Thanks for your response,
Chuck

Charles Letner
Wright State University
Department of Biochemistry
Dayton, OH 45435
e-mail: [email-masked]

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From: "William T. Winter" <[email-masked]>
Date: Tue, 14 Nov 1995 15:03:36 -0500 (EST)

The choice of origin in a crystal structure is dependent upon the space 
group symmetry.  For example, in space group P2sub1 2sub1 2sub1 it is midway 
between the three sets of non-intersecting two fold screw axes.  Origin 
positions for different space groups may be found in vol I of the International
 Tables for X-ray Crystallography in each space group description.  Of 
course in P1,  which has no symmetry, the choice is arbitrary and in other 
cases individuals have elected to use a non-conventional origin for various 
reasons 

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William T. Winter                      Phone: (315)470-6876
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