[RMCProfile-users] 1. add Bragg data fitting, 2. multiple RMC fitting

[helen.playford at stfc.ac.uk]

Dear Bo,

It has only been a few days since you posted these questions - there are only a few of us trying to answer the mailing list queries and we try to do it as quickly as we can! Also it was a holiday weekend here in the UK so I have not been at work until this morning.

Anyway -

1. When you leave the Bragg data out of a fit, the atoms in your box have more degrees of freedom and so they usually do end up fitting the total scattering data well. However the resulting model is likely to be far more disordered than it should be, because you aren't constraining it to fit the average, long-range structure. The weightings you show would result in the program working hard to fit the Bragg data and almost ignoring the F(Q), so I would try and balance them a bit (smaller numbers mean the dataset will have a bigger influence on the overall goodness-of-fit. If you could send the files you are using to rmcprofile-users-owner at rmcprofile.org I can take a look at what is going on and give you some more suggestions.

2. What you want is basically a refinement "recipe" (i.e. a set of datafiles, a .dat file and a starting configuration) which produces a good fit, and you want to duplicate this many times. If you use an already-refined configuration as your starting point it is already in a kind of equilibrium state where moves have little effect on the goodness-of-fit and you won't properly sample the many possible ways the box of atoms could arrange themselves to fit your data.

I hope this is helpful!

Best wishes,
Helen

-----Original Message-----
From: rmcprofile-users [mailto:rmcprofile-users-bounces at rmcprofile.org] On Behalf Of Bo Jiang
Sent: 29 May 2017 19:49
To: rmcprofile-users at rmcprofile.org
Subject: [RMCProfile-users] 1. add Bragg data fitting, 2. multiple RMC fitting

Hi everyone,

        I am doing synchrotron X-ray PDF for the RMC simulations. The X-ray scattering has a disadvantage about the lighter atoms comparing with neutron, so I also got many problems when I do RMC to synchrotron X-ray PDF. I had two questions still no one answer until now, so I ask this two questions again:



1.  I got a very good fitting with just fitting F(Q) and G(r), but why the RMC fitting go to terrible noise again after I add Bragg data, with an increased weighting of the F(Q)? Below is the weight setting for my job:



BRAGG ::

  > WEIGHT ::   0.01

XRAY_RECIPROCAL_SPACE_DATA ::

  > DATA_TYPE :: F(Q)

  > WEIGHT :: 0.5



2. For multiple RMC boxes refinement, can I do the the multiple RMC refinement based on the structure which has a good RMC refinement already?



Bests



Bo Jiang
Postdoc
NTNU, Department of Materials Science and Engineering Cell phone: (+47) 94 24 98 88

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