[RMCProfile-users] X-ray PDF use

[matt.tucker at stfc.ac.uk]

Dear Christopher,


1)      Can RMCProfile fit x-ray PDF?: Yes



What Chris Kerr says is not strictly true. RMCProfile can fit x-ray PDF data.



The x-ray F(Q) can be fitted with the XRAY_RECIPROCAL_DATA keyword. However, the x-ray form factors have to be in the data file and it has to be convolved with the simulation box size. There is a program call xray_to_rmc that will do this for you if supplied the data in the usual rmc format.



For the x-ray PDF, since the q-dependant part has been divided out you can treat this as a neutron PDF with different weights. These weightings are defined using the keyword NEUTRON_COEFFICIENTS (sorry a bit confusing I know).



We are writing a tutorial example to show how all this works and it will be in the minor update 6.5.2. This will be out in the next few weeks.



As Chris Kerr says there are plans to fit the x-ray PDF in a more rigorous way. Once this is implemented it will be useful for all x-ray PDF data, as it will make a fairer comparison regardless of the atomic radii



2)      How do you fit nanoparticles with RMCProfile?



Sadly this is not an easy question to answer, there are three possible approaches.

a.       The first is to treat the data as a bulk sample if the particle is large. If not then a modification function for the particle size could be applied but this isn't implemented at present.

b.      The second (as Chris says) is to put one particle in a large box and only fit out to a distance where it does not see its periodic repeat (RMC uses periodic boundary conditions).

c.       Finally if you can build a starting configuration with particles in a matrix that is not too large (> ~40,000 atoms), then you can use the various constraints/restraints to hold things together and model it as normal. This of course is not trivial but probably the ideal solution.

If you tell us some more about your system we can advise what your best approach is most likely to be.


Best wishes,
Matt


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From: rmcprofile-users-bounces at rmcprofile.org [mailto:rmcprofile-users-bounces at rmcprofile.org] On Behalf Of Christopher J. Wright
Sent: 02 December 2013 00:36
To: rmcprofile-users at rmcprofile.org
Subject: [RMCProfile-users] X-ray PDF use

Hi,
I am working with PDF data collected from an x-ray synchrotron source.  The samples are nanoparticles of various size and compositions.  Is it possible to model these particles using RMCprofile?
Thank you very much,
Christopher J. Wright
Brown University Chemistry Department
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