Hello Researchers!
I encountered a system for XRD crystal structure refinement using FullProf Suite, where the one unit cell contains 4 formula units and a total of 30 atoms. For refinement I need to include all the atoms and then refinement gets too difficult and time taking and yet to get useful results. I failed to do this. Could you please help me to find a way to do this easily? Is it possible to do the refinement by taking only one formula unit as input? Any useful crystallographic reference to deal with this type of refinement. All you suggestions will be helpful to me.
Thank you,
With Regards,
Manoj
It sounds as if you have too many parameters to refine. I assume that you already have a reasonable idea about the structure (structure solution has been successful) but the structure is "only" to be refined. In such a case you can typically use further information about the geometry of the structure. One typical thing can be to have equal atomic displacement parameters for all atoms and concentrate only on the fractional coordinates (" x, y, z") of the atoms. Following steps I can suggest:
(i) do you really have the maximum possible/reasonable symmetry? If not, imposing the higher symmetry will avoid unneccessary parameters. By the way, test refinements (and structure solution) in reduced symmetry (e.g. P1) may make sense, but the final refinement should be done in the maximum possible/reaonable symmetry. Everything else I regard as poor practice. Note that depending on the refinement algorhithm used, will nearly automatically generate fancy correlations between refined parameters, if you work with too low symmetry. I sometimes doubt whether robust refinement algorhithms hiding such fancy behaviour are really a benefit, because such fancy behaviour (diverging refinement) is a good warning sign!
(ii) Try to refined x, y, z pertaining to the heavier atoms first and then add the fractional coordinates lighter atoms. Perhaps that will work. Sometimes you may also have to make use of damping options.
(iii) You may also include "reasonable" additional information on your crystal structure/molecular geometry by hard or soft constraints. Depending on the type of material under consideration this may be rigid bodies or it may be restraints on individual bond lengths. This is in particular common practice with organic matials, and I am convinced that Fullprof provides these tools.
Dear Manojkishor Pradhan, would you mind attaching your data and any information about it? Perhaps your *.pcr file.
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