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Re: [sdpd] Re: Calculation of Fhkl or/and Ihkl by different software
I had the same problem with PowderCell some time ago.
best regards Natale Perchiazzi
----- Original Message -----
From: "ipozdn" <ipozdn...@yahoo.com>
To: <sdpd...@yahoogroups.com>
Sent: Monday, November 20, 2006 11:46 AM
Subject: [sdpd] Re: Calculation of Fhkl or/and Ihkl by different software
> 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
>>
>
>
>
>
>
>
> Yahoo! Groups Links
>
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>
>
>
>
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