[sdpd] Re: Calculation of Fhkl or/and Ihkl by different software

["ipozdn" <ipozdn...@yahoo.com>]

Dear Jacco,

Thank you very much for detailed and informative answer!
I used the pattern from the link as a reference, and I got that 
neither Diamond, nor PowderCell didn't reproduce it correctly.
Besides of the small variations in intensity, FindIt is OK. And the 
best is FullProf, when I put the occupation numbers correctly (thanks 
to Dr. Roisnel), it works perfect.

Best regards,
Irina


--- In sdpd...@yahoogroups.com, Jacco van de Streek 
<jaccovandestreek...@...> wrote:
>
> > I just tried to compare four different software:
> FindIt, Diamond,
> > PowderCell and FullProf to calculate structure for
> alpha-alumina using
> > ICSD #51687 file (Materials Science Forum(2001),
> 378, 288-293,Unit
> > Cell   4.7597(1) 4.7597(1) 12.9935(3)). I used the
> same geometry,
> > wavelength, profile parameters, no background, same
> space group
> > settings. But all four patterns have different
> intensity, and structure
> > factors also (for software that gives a possibility
> to get one). I can
> > not understand why the structural factors differ.
> The visualized
> > structures are exactly the same. Maybe somebody
> knows?
> 
> Dear Irina,
> 
> I do not know how big the differences are, so maybe
> some of the differences are not big enough to be
> relevant. However, trigonal space groups like R-3c are
> more difficult to get right than the more
> straightforward lower symmetry space groups. This is
> because the matrices representing the symmetry
> operators are no longer diagonal, which means that it
> becomes important to multiply them in the correct
> order (from the left or from the right). Not all
> programs get this right, which could explain the
> differences you are observing.
> 
> Secondly, in trigonal space groups special positions
> can contain values like 1/3 and 2/3, which can no
> longer be represented accurately in text files but are
> truncated to values like 0.3333 and 0.6667, which can
> give problems with rounding errors; however, the
> atomic coordinates of corundum do not contain these
> values, so this cannot be the problem.
> 
> And Vincent already mentioned another problem, namely
> the confusion over the hexagonal versus the
> rhombohedral setting.
> 
> If the differences that you observe are substantial
> (i.e. big enough that at least one of them must
> clearly be wrong), then it is important to first
> establish what the correct answer is. I found an
> experimental powder pattern of corundum (CuKa) on the
> web:
> 
> http://www.ccp14.ac.uk/tutorial/powderx/runthr.htm
> 
> Using the coordinates from ICSD #73724, which can be
> found here:
> 
> 
http://www.crystal.unito.it/mssc2006_cd/tutorials/geometry/geom_tut.ht
ml
> 
> it can easily be tested which program gives the
> correct answer.
> 
> I verified that the visualiser Mercury, freely
> available for a variety of platforms from
> 
> http://www.ccdc.cam.ac.uk/products/csd_system/mercury/
> 
> gives the correct simulated powder pattern. The next
> version of Mercury, Mercury 1.5, due to be released in
> a couple of weeks, will also able to write out
> structure factors, and it will also have the option to
> include CuKa2.
> 
> Kind regards,
> -- 
> Dr Jacco van de Streek
> 
> 
> Send instant messages to your online friends 
http://uk.messenger.yahoo.com
>





 
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