[RMCProfile-users] 1. about the loops. 2. Maximums and minimums of the distance window constraints

[helen.playford at stfc.ac.uk]

Hi Bo,

Thanks for your email.

1. Because there are a finite number of atoms in an RMC box, some of the partial pair correlation functions may have only a very small weighting with regard to the total PDF. If those small correlations are of interest then it is necessary to have multiple RMC boxes that have been refined against the same data which can basically be added together to get partial functions with better statistical quality. Furthermore, bond angle distributions calculated from a single box are likely to be very noisy, so the same technique of adding together multiple boxes is used to improve this. As a rule of thumb if you want to get good partial correlation functions then something like 20 - 50 boxes will probably work, but for bond angles you will need closer to 100. This is obviously system dependent as well. 

2. Usually you'll base the distance window values on where the peaks in the PDF go to the baseline, and you'll choose the edges of the peak. If you're using the distance window constraint on overlapping peaks then you will have to use your judgement to estimate a "width" around the average positions. There's no one rule that will work for everything - it might need a bit of trial and error!

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: 22 May 2017 08:48
To: rmcprofile-users at rmcprofile.org
Subject: [RMCProfile-users] 1. about the loops. 2. Maximums and minimums of the distance window constraints

Hi everyone,
        I am working with PDF data collected from X-ray synchrotron source from ESRF, and I have got the final RMC model. I have two questions

1.      Why the author said that 'the modelling was performed 120 times to improve statistics' in Adv. Funct. Mater. 2013, 23, 185-190. So I should do more than hundreds of times to get the average structure?

2.      How to decide the Maximums and minimums of the distance window constraints? Can I  just plus or minus 0.5A or 0.6A base on the average structure atoms distance?

Thank you very much!


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


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