I would like to add my two cents to the discussion on ESDs and
weightings.
Paolo wrote:
> Clearly, the ESD on x1 have worsened in this simple case. This does not
> prove the general case, but there might be a proof for that as well.
This conclusion was reached by applying non-linear least squ
Jon Wright wrote :
>PS: Any offers other than GSAS and multipattern fullprof for actually
>doing these fits?
Yes ARITVE, for amorphous compounds :
http://www.cristal.org/aritve.html
Nevertheless, ARITVE can work also for crystallized compounds
with simple profile shape (gaussian only), an
OK, so iff the structure can be properly described by both datasets, the
main problem that we have is what to do with the esd's and chi**2. It seems
that the best thing is for the refinement software to give individual values
for each refinement and then an overall value (perhaps rescaled). At
Well, it looks like my "feeling" might have been wrong this time. The way
you may want to go through the "tedious" proof is the following.
1) You construct the Aij matrix (also sometimes called the Hessian matrix)
Aij=D^2chi^2/DxiDxj (D is the partial derivative sign. E-mail is
On Tue, 25 May 1999, Alan Hewat, ILL Grenoble wrote:
>
> Mainly because the ESD's are only correctly calculated if the model
> is CAPABLE of fitting the data. This is not usually true when systematic
> errors are important compared to statistical errors, since the model is
> usually not capabl
On Tue, 25 May 1999, Alan Hewat, ILL Grenoble wrote:
> >>I guess the degradation which is found would come from parameters which
> >>are determined by both datasets and come out with different values in each
> >>separate refinement.
>
> If they come out differently it is because they are differ
On Tue, 25 May 1999 [EMAIL PROTECTED] wrote:
> Not necessarily. In order to get the ESD, the variance-covariance matrix is
> multiplied by chi^2, and the roots of the diagonal elements are taken.
The justification for multiplying by chi^2 is to assume that the
systematic errors are really just
>>I guess the degradation which is found would come from parameters which
>>are determined by both datasets and come out with different values in each
>>separate refinement.
If they come out differently it is because they are differently biased by
different systematic errors in the data not des
As chi^2 is a function of the number of data points included in the
refinement, combined refinements have considerably improved values for a
total chi^2 when compared with refinements carried out against individual
data sets.
Correspondingly the ESDs in the combined refinement output should be
Jon Wright wrote:
>I guess the degradation which is found would come from parameters which
>are determined by both datasets and come out with different values in each
>separate refinement.
Not necessarily. In order to get the ESD, the variance-covariance matrix is
multiplied by chi^2, and the
Hi all,
Am I right in thinking there are roughly two camps in this dicussion?
Those who think that adding more data degrades the refinement if that data
is not useful and those who think it makes no difference. (I say 'not
useful' in the context of Vanadium in neutron data or deuterium in x-ray
d
Oops, forgive the typos! I haven't found a coffee yet :-)
Andrew
Get your own FREE, personal Netscape WebMail account today at
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Alan,
I am not suggesting removing reflections. But, I think that we should make
sure that we are combining the data in the best possible way. If we know have
strong information on a vanadium position from X-rays and (extrapolate again)
have only noise from neutrons, then stastically introducing
>If we have an atom that is seen by one
>radiation and not by the other there will be a degradation in the quality of
>the parameters by combining the refinement in the current fashion.
Do you mean for example that we might degrade the parameters of a V atom
by introducing neutron data ?
I don
Dear All,
Firstly, it was a pleasure to return to my email and read an interesting
discussion on combined refinements. It is good to have aired some of the
problems and limitations (e.g. are neutrons and X-rays seeing the 'same'
sample?). With a simplistic view, this technique must be the way ah
On Armel's point on using the internet.
The big microscopy centres seem to be going big time for
TelePresence/Collaboratories where collaborators and users
can routinely use the internet rather than expensively hopping
on and off planes to interactively make use of the eqiupmen:
http://tpm.a
Jaap wrote :
>With regard to the "brown envelope" technique. I am not such a fan of
>that. It is very hard for a beam line scientist the know whether the data
>collected are as desired, have a good enough s/r ratio etc, which details
>to look for. In addition, the last few years when we went we h
On Tue, 11 May 1999, Armel Le Bail wrote:
> I already suggested to install such an "automatic" powder diffractometer
> at ILL. As Alan wrote recently, this could be a question of manpower.
> I think that this is rather a local political question : it is not a very
> interesting job for a human be
Dear All,
here is my (biased) opinion on the whole matter.
1) Intensity data: neutron powder diffraction *always* yield better
intensity data than x-ray powder diffraction, including synchrotron.
Contrary to popular belief, this is true not only for mixtures of heavy and
light atoms, but also f
> With regard to the "brown envelope" technique. I am not such a fan of
> that.
I have mixed feelings about the "brown envelope" technique. I do think
that traveling to collect data that are run under routine,
semi-automatic, conditions can be a waste of time and money, so Armel
has a very good
What Armel is describing is precisely the way we started. We used it when
required, discussed over a few pints whether it was a good idea or not,
and came to the conclusion that it was. Later we tried it also on
refinements where the need was not so clear, and now it is routine. I have
not read
Alan Hewat wrote:
>Before some-one else says they can see hydrogen with x-rays, Mike
>Glazer, Bill David and I saw vanadium with neutrons. So let's just
>say "insensitive" but not "completely". Otherwise I agree :-)
>Combined refinements are sometimes necessary and a good thing.
>
>1. David, W.
Toby wrote,
>Armel you may indeed be more equal than some but, I would like to
>welcome you to visit us in the suburbs of Washington, along with some of
>your favorite samples. DC is no match for Paris or even Grenoble, and
>our cafeteria does not come near the quality of the ILL, but there are
>
>this discussion has gone too far from the starting point. The question
>really isn't "neutrons Yes/No", but if all of us have equal access to all
>sources. People from both NIST, Grenoble or RAL would, no doubt, answer
>yes, why not ? Well, if I were there I wouldn't hesitate for a moment,
>but
On Mon, 10 May 1999, Armel Le Bail wrote:
> published combined X-ray and neutron refinements. Am I so far
> from the truth if I estimate the number of published works combining
> X-ray and neutron in a single refinement to, say less than 20 cases ? I
> would like to see, in the 10 next years, th
At 07:08 PM 5/10/99 +0200, Armel wrote:
>PS- take the Rietveld Round Robin PbSO4 X-ray pattern and omit
>the O atoms, you will have RB~15%. Make a Fourier difference
>and you will see if the "light" atoms are so light, when using good data.
>When I remember my crystallography courses 25 years ago
Jaap wrote :
>Finally Armel, in my view an attempt the split the Rietveld community in
>two, i.e. in house X-ray and central facility neutrons, is artificial. By
>teh way did you note that someone called Mark Weller was on your neutron
>list as well. As far as I know, he has been working in South
>The neutron fit will be
>completely insensitive to the V positions and the x-rays insensitive to
>the D positions. (This is easily verified.)
Before some-one else says they can see hydrogen with x-rays, Mike
Glazer, Bill David and I saw vanadium with neutrons. So let's just
say "insensitive" b
Armel Le Bail wrote:
> I note that this is mainly the neutron community
> that is not preoccupated by improving the positions of the
> heavy atoms by using X-ray ;-).
With a few exceptions, neutrons are sensitive to most heavy atoms. For
many materials the improvement obtained using both x-rays
Let start to cut the crap and go to Armel's real reason not to use a
combined refinement:
> Of course this is only kidding and provocative opinion, as
> usual. I love both radiations, indeed. However, I tend to think
> that a simultaneous refinement could eventually degrade
> the heavy atom po
>The confirmation of what I suspected : some peoples are
>more equal than others !-).
Perhaps :-) But Armel, if you or some-one else is interested
in setting up a neutron service in Grenoble as fast as at NIST,
we would be very interested. We can provide the equipment
(D1A is quite competiti
Brian wrote:
>Finally, I should mention in response to Armel that at least here at
>NIST, most requests for time are scheduled within 2-8 weeks of when we
>get them (see http://www.ncnr.nist.gov/~toby/bt1.html).
The confirmation of what I suspected : some peoples are
more equal than others !-)
I also use combined CW neutron and synchrotron refinements. A simple
minded justification goes as follows. Most of the problems I work on are
badly underdetermined -- at least by the crystallographic rule-of-ten
(10 crystallographic observations for each structural variable). By
changing scatterin
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