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

Mon May 22 10:02:00 BST 2017

Hi Bo,

1. You could also do this by just not including the distance window constraint block at all! But anyway, there's no reason that it shouldn't be a reliable result, but it does depend on whether the partial functions (i.e. things like bond lengths and distributions) make sense for the system you're looking at. This is where you have to use your judgement to determine if a result makes sense.

2. An extreme lower limit on the number of moves generated is that it must be greater than the number of atoms in the box! But again there isn't a rule for this. If the refinement has converged (i.e. the chi^2 is no longer going down) then you've probably done "enough" moves. Usually this isn't a problem because modern computers generate so many moves so quickly. In terms of running time, depending on my system I've needed everything from 6 to 72 hours. 

Best wishes,
Helen

-----Original Message-----
From: rmcprofile-users [mailto:rmcprofile-users-bounces at rmcprofile.org] On Behalf Of youqin5912
Sent: 22 May 2017 09:53
To: rmcprofile-users at rmcprofile.org
Subject: Re: [RMCProfile-users] 1. about the loops. 2. Maximums and minimums of the distance window constraints

Thanks very much for your kind reply! I still have two questions about the RMC.
1. For the second question, I try to set 0 to all the maximums and minimums of the distance window constraints, which means a full relax fitting RMC. I still get a good RMC fitting result, is that a reliable result?
2. How many moves should be generated for each RMC box at least?

Thank you very much!

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

>-----Original Message-----
>From: rmcprofile-users [mailto:rmcprofile-users-bounces at rmcprofile.org] 
>On Behalf Of helen.playford at stfc.ac.uk
>Sent: Monday, May 22, 2017 10:22 AM
>To: rmcprofile-users at rmcprofile.org
>Subject: Re: [RMCProfile-users] 1. about the loops. 2. Maximums and
minimums
>of the distance window constraints
>
>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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