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 the neutron data
whilst no changing the best fit will degrade the least - squares approach to
it. The final structure should fit all data, but are we approaching it
optimally? I know that this is a can of worms, but it is good to think about
what we are doing as combined refinements will continue to become less exotic.
-Andrew
--------------
Andrew Wills
Centre D'Études Nucléaires de Grenoble
"Alan Hewat, ILL Grenoble" <[EMAIL PROTECTED]> wrote:
>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't think this is true, but it is an interesting question. If we were to
extrapolate this argument "ad absurdum" we could say that because some
reflections (for a given radiation) do not give any information about some
parameters (easy to demonstrate) then we would obtain better estimates
for those parameters by removing those reflections from the least squares
process. (Surely untrue :-)
What is true is that if we introduce systematic errors by combining
radiations, we may indeed degrade the result. For example if we have
serious preferred orientation with a very small X-ray sample, it is
probably unwise to introduce this biased information into the refinement
of the neutron data, where there may be less bias because of the average
over a much larger volume.
But if the data is not biased, you must always (?) do better by including
more data, with for example combined X-ray and neutron refinements.
>Surely, it
>would be better to use a new weighting function for the atomic parameters,
>that is dependent on the scattering lengths for each radiation.
Playing around with weighting schemes is to enter dangerous territory.
Alan H.
Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13
ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
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