Lubo wrote:
>why do you like playing with numbers so much ? Well, I can tell you some
>more :
>suppose you can subdivide your (our) interval (0-180 degs 2theta) into
>small parts, say 0.01 degs. In other words, you may have 180000 channels
>at maximum. If there is an "useful" intensity in a channel (not only a
>background, i.e.) you can attribute that channel "1", "O" otherwise. Than
>you have less than 180kB of information at maximum. What can you do with
>such a information rubbish ?
Do you want to go back to a pre-civilization time ? Numbers matter a lot.
Well, I guess that you mean 0.001 degs for having such information rubbish.
The sample makes the rubbish part, not the diffractometer.
I play with only one number that characterizes the resolution.
I am not interested in rubbish. So that I did not look at many points
of the MgO HRPD pattern (now I know that I have a HRPD at home, indeed ;-),
only at peaks. And, yes, for solving the MgO problem, no need to have
FWHM=0.03° at 1.54A, even FWHM=2° would work. But the fact is that
more and more complex structures are undertaken by powder diffraction
methodology, for which you will not find any rubbish on a HRPD pattern.
Ask to Bob von Dreele if he prefers apples or oranges for looking
at proteins...
And the question is really : if I do not succeed at home, can I really
succeed on a big instrument ? I think yes if those numbers mean
really something. Hence the need to define what they mean.
I have recently posted to the SDPD mailing list a wonderful powder
pattern (synchrotron and conventional X-ray) that I am unable to index.
Some people donwloaded the data. And I am still waiting for a solution...
Feasibility limits really exist. No need to waste time if the problem lies
beyond them. You can refine a protein structure by using a completely
constrained model, from powder data, OK. But you can't solve it from
such data (I will not say never here because of molecular replacement
methods).
Best,
Armel
