Dear Christoph (and others),
The constraints within GSAS are limited to pairs of parameters. You describe a
circumstance in which the total unit cell contents from a multiple atom sites is known
from stoichiometry; this is best handled with a "restraint". The restraints in GSAS
are not accessible from EXPGUI but only from EXPEDT. Look for an "S" in the main least
squares menu - one of the entries in the restraints menu will refer to unit cell
contents or chemical restraints ("K"). These can be simply numbers of atoms or could
be sums of charges, etc. Pick an "esd" for the restraint according to how well known
you think the restraint value should be. In this case, I'd pick a pretty small value
(0.01) as water has a rather well known stoichiometry.
Bob Von Dreele
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From: Christoph Salzmann [mailto:[EMAIL PROTECTED]
Sent: Tue 8/3/2004 6:47 AM
To: [EMAIL PROTECTED]
Dear all,
I'm using the EXPGUI software to refine the structure of a monoclinic phase of ice.
(ice V) The diffractograms were recorded on the D2B instrument at the ILL. As a result
of this study, we would like to see changes in the occupancies of the deuteria
positions.
Due to the low symmetry of this phase, I have to use some constraints during the
refinement. There are 14 different sites of deuteria in this phase. If the
stochiometry of this phase is taken into account, the occupancies can be described by
only 4 ordering parameters.
I'm now reaching the limits of the "Constraints" function in EXPGUI: I can refine the
single ordering parameters. But additionally, all four ordering parameters should add
up to 4. This seems to be impossible in EXGUI?
So my question: Can I use the EXGUI software AND define further constraints in GSAS?
How can I do this?
Thank you very much for your help in advance!
Yours, Christoph
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Dr. Christoph G. Salzmann
Institute of General, Inorganic and Theoretical Chemistry
Innrain 52a
University of Innsbruck
6020 Innsbruck
Austria / Europe
e-mail: [EMAIL PROTECTED]
phone: +43 512 507 5122
fax: +43 512 507 2934
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