From:   SMTP%"L.M.D.Cranswick@dl.ac.uk" 14-OCT-2000 14:48:20.97
To:     sianc.davies@bbsrc.ac.uk (sianc.davies)
CC:     cockcroft@GORDON.CRYST.BBK.AC.UK (Jeremy Cockcroft)
Subj:   Re: Powder Diffraction Project


> Sorry to bother you, but I'm doing Jeremy Cockcroft's Powder Diffraction
> Certificate course and he said you may be able to help me with using Reitica
> 1.6.7a for Windows NT and 95 P.C.s.  I have refined a structure of PbSO4 using
> isotropic thermal parameters.  Now I would like to refine anisotropic
> parameters.  I have set individual B's to 0.6 and the isotropic parameter to
> 0.0, but I keep getting an error message "Refinement unstable - change
> values/numbers of parameters" and I don't know what this means or what I'm
> doing wrong.
> 
> Could you help?
Hi Sian (and Jeremy),

Sorry for delay.  Just back from a trip to Germany and a firewall
there was stopping me looking at my Email.

Could you send me your input and data file.  
I think the main problem is you may not be using "decent"
starting values for the anisotropic thermals and may not
be handling the codewords for the anisotropic thermal parameters
correctly.  Though it could be a bug given anisotropic refinement
is normally only done on the minority of Neutron data.

Rietica - being of DBWS origin - may not 
handle all the anisotropic symmetry operators
for you - this has to be done manually as per old style single
crystal programs.  I am not sure if this is handled in the PD
course? A quick way of generating starting anisotopic 
thermals is to do this in Shelx using a dummy HKL file.

Lachlan.


-- 
Lachlan M. D. Cranswick

Collaborative Computational Project No 14 (CCP14)
    for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44-1925-603703  Fax: +44-1925-603124
E-mail: l.cranswick@dl.ac.uk  Ext: 3703  Room C14
                           http://www.ccp14.ac.uk

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