Fitting of low-r region in pdfgui

[Kirstin Wilson]

Hello all,

I have been trying to fit the neutron PDF of several MOF structures on pdfgui. This has resulted in fits like the one attached where the majority of the fit is quite good but in the low r region up to around 1.1 Angstroms the fit oscillates around zero rather than following the downwards trend of the start of the PDF data.

I have also fitted a line from 0 to 1.1 angstroms to calculate density but this value is only half the expected density using the atoms and lattice parameters of a rietveld refinement of the same material.

Could you confirm if it is possible to fit this low r region on pdfgui and if so which parameters should be changed to do this. If you also have any ideas why the calculated density from the experimental data does not match the expected density that would be great too.

Best wishes,

Kirstin

[Simon Billinge]

wow, that is an interesting conundrum.    Does your transition metal node have negative neutron scattering length by any chance?

Congrats on an amazing fit btw....at least to the structural region of the curve.  I can assure you that the misfitting low-r region won't affect any of the refined structural parameters at higher-r so fitting the low-r region will just resolve an interesting conundrum and be aesthetically more pleasing but shouldn't affect any scientific conclusions you may be able to draw....

S

On Fri, Dec 5, 2025 at 4:06 AM Kirstin Wilson <kirstin....@gmail.com> wrote:

Hello all, I have been trying to fit the neutron PDF of several MOF structures on pdfgui. This has resulted in fits like the one attached where the majority of the fit is quite good but in the low r region up to around 1. 1 Angstroms the fit

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Simon Billinge

Professor, 

Department of Applied Physics and Applied Mathematics

Columbia University