Choosing the correct delta1 and delta2 for fitting

[Tam Nguyen]

Good morning,

I have recently used PDFGui for fitting with my data and I have a question related to choosing the correct delta1 and delta2 for fitting.

Currently, when I fit the chosen structure with my data, I found that at least 3 sets of delta1 & delta2 providing a good fit.

Set 1: delta1 = 1.3, delta2 = 0.2 (Rw = 0.081); with relaxed delta1 and fixed delta2

Set 2: delta1 = 0.8, delta2 = 0.9 (Rw = 0.12); no relaxation

Set 3: delta1 = 0.3, delta2 = 1.5 (Rw = 0.088); no relaxation

How do I know which set to choose?

Thank you all!

Tam Nguyen

[Simon Billinge]

Thanks for the question Tam, This is described in the PDF primer (formerly PDF to the people) book if you can get your hands on a copy:

https://global.oup.com/academic/product/atomic-pair-distribution-function-analysis-9780198885801?cc=us&lang=en&

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Simon Billinge

Professor, 

Department of Applied Physics and Applied Mathematics

Columbia University

[Mikkel Juelsholt]

Hi Tam

It is also described in the pdfgui manual available here

https://www.diffpy.org/products/pdfgui.html

Bottom of the page. 

The short answer is that delta1 and delta2 correlate quite heavily so they should not be refined at the same time

This is why you get multiple set of values giving a good fit. 

At room temperature delta2 is normally the one use

Cheers Mikkel