[sdpd] Giant but not so big

[Armel Le Bail <alb...@cristal.org>]

Hi,

I would like to congratulate the authors of the
SDPD of a giant cubic structure (SG; Fd-3m;
a = 72.26 A, V = 387000 A**3) named MIL-100 :
http://www3.interscience.wiley.com/cgi-bin/abstract/109803129/ABSTRACT
http://www3.interscience.wiley.com/cgi-bin/abstract/109605226/ABSTRACT
See also the ESRF Highlights 2004, p. 25-26.

However, if I was asked to explain how the
structure was solved exactly, I would be unable...
- pure prediction ? then matching with the powder pattern ?
- or "prediction" knowing the cell parameters (which
    would be in fact "structure determination") and moving
    moieties inside of the cell by global optimization ?

Would be interesting to know exactly.

Armel

PS - It is said that 387.000 A**3 is "far beyond the
current possibilities for solving the structure from
ab initio procedures."

Well, it was written in the SDPD tutorial in 1992 that
"Triclinic centrosymmetrical cells with up to 3000 Å3
volume or cubic cells with F lattice as large as 300.000 Å3
are the theoretical upper limits which one could expect to
attain by using high resolution synchrotron data !
http://www.cristal.org/iniref/tutorial/part3a.html#3.1

And these limits were estimated for FWHM close to 0.02°
(2-theta). So, for 0.01° FWHM, you would have to multiply by 2...

Not far beyond - just inside the range.

At ACA'97, such 0.01° FWHM was already available
and the estimation given was 576.000A**3 for a F-centered
cubic cell. See the slide :
http://www.cristal.org/iniref/aca97/t18.gif
http://www.cristal.org/iniref/aca97/aca97.html

So the race to the largest structure solved by SDPD is
continuing... But at least, please explain how exactly
you solved your structure...



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