Dear Jools, You can fix or hold one atomic parameter in EXPEDT by accessing the atoms list (L A) and then generating a hold in any refinable parameter. After you are in the atomic edition screen type F and then I to insert a hold. There is another tricky way of doing that without using EXPEDT, but I am not sure if it has other consequences. You can always put a constrain on EXPGUI/Constraints thumbnail where any variable (in your case the z coordinate) of one or more atoms have a 0 multiplier. This will generate a message in the GENLES window that states that the linear constraint conrresponding to that variable has not been refined, which is effectively the desired situation. I have used this for fastly constraining magnetic moments in NPD and works, it doesn't prevent convergence or things like that. But when doing the final refinements I always use the hold method into EXPEDT. Hope this helps. Leo
Dr. Leopoldo Suescun Postodoctoral Appointee Argonne National Laboratory Materials Science Division - Bldg 223 9700 S. Cass Avenue, Argonne, IL 60439 Phone: 1 (630) 252 9760 Fax: 1 (630) 252 7777 URL: http://www.msd.anl.gov/groups/nxrs/personnel/suescun/index.html -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Thursday, July 14, 2005 2:40 PM To: [email protected] Dear all, I'm using GSAS and having a problem with refined atom positions "wandering" in the z direction. I'm guessing this is due to the lack of a special position which fixes the origin in z. Accordingly i'd like to fix the z position of one of my atoms to compensate. The problem is that i'd also like to refine the x and y positions for that atom. GSAS doesn't seem to give me the option to do this - i tried inserting a dummy atom (which isn't refined) and constraining the z of my candidate atom with that, but GSAS doesn't seem to lock the two z parameters as i'd hoped. Any ideas? jools --------------------------------------------- This message was sent using UNIS MailSystem.
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