Hi
Stressing the fact that these methods circumvent the phase problem by
more or less randomly shuffling atoms rather than trying to solve it
from scratch, maybe "reverse methods" would be an appropriate term,
comparably to reverse engineering.
Best Wishes,
Joachim.
Peter Y. Zavalij wrote:
How about "reciprocal methods" as opposite to "direct methods" which work in
reciprocal space?
Peter
______________________________________
Peter Zavalij
Director, X-ray Crystallographic Center University of Maryland, College Park,
MD 20742
http:/www.chem.umd.edu/crystallography
-----Original Message-----
From: rietveld_l-requ...@ill.fr [mailto:rietveld_l-requ...@ill.fr] On Behalf Of
ian.mad...@csiro.au
Sent: Wednesday, December 16, 2015 6:29 PM
To: yaroslav.filinc...@uclouvain.be; uachs...@savba.sk
Cc: rietveld_l@ill.fr
Subject: RE: structure solution terminology
Hhmmm. A method which uses a model to calculate a diffraction pattern which is compared
with observed data and the difference minimised by refining some parameters. Then
restarting the process by randomly shuffling some parameters. I propose RRWAS - Rietveld
Refinement With Atom Shuffling. Or maybe this is what you would define as a "bad
joke".
Cheers
o----------------------------oo0oo----------------------------o
Ian Madsen
Honorary Fellow
CSIRO Mineral Resources Flagship
Private Bag 10, Clayton South 3169
Victoria, AUSTRALIA
Phone +61 3 9545 8785 direct
+61 3 9545 8500 switch
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Email ian.mad...@csiro.au
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________________________________________
From: rietveld_l-requ...@ill.fr [rietveld_l-requ...@ill.fr] on behalf of
Yaroslav Filinchuk UCL [yaroslav.filinc...@uclouvain.be]
Sent: Thursday, December 17, 2015 9:17 AM
To: Lubomir Smrcok
Cc: rietveld_l@ill.fr
Subject: Re: structure solution terminology
Hi Lubo,
if we speak of diffraction, we have a Fourier transformation, so we are not
100% in the direct space with any structure solution method.
However, the "structure solution in direct space" does not attempt to work in the
reciprocal space - all the manipulations are done in the direct space. Also, it does not attempt to
address properly the phase problem, which is defined in the reciprocal space, but is simply trying
to find a model "directly" in the direct space. There is even no inverse FT calculated!
So, it is the fair name, IMHO.
Best regards,
Yaroslav
Wednesday, December 16, 2015, 11:05:05 PM, you wrote:
Hi,
I feel I have a problem with terminology. What's the right nickname
for the method of structure solution from powders, which instead of
analyzing electron (nuclear) density shuffles atoms or molecules
around a cell and compares experimental and calculated patterns ? It
could be well used also for single crystal data, I know, but it is not
routinely done (yet ?).
I remember that colloquially used phrase "structure solution in direct
space" woke me up several times during plenary lectures at several
conferences. What are we having in direct space ? IMHO just electron
(nuclear) density. Its FT is our diffractogram so that if we fit
diffractogram, are we still in direct space ? Of course, reciprocal
space is a pure math and diffracted beam is quite realistic (BTW, are
there photons also in reciprocal space :-) ?). But in which space are
we when doing any crystallographic calculation ? OK, OK, you know what
I am asking ... no bad jokes, please.
May I ask for your 2 cents ?
Thanks,
Lubo
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