Peak fitting using PDFgui

[Tanu Swami]

Hello everyone

I am trying to fit a data using PDFgui software with the monoclinic symmetry. At r around 22A it is not fitting the peak but bond lengths are present at that value of r. Can someone please explain what could be the possible reason for this?

I am attaching the picture for reference. 

Regards

Tanu

[Simon Billinge]

Dear Tanu,

The model is fitting quite well at 22 A.  The measured signal in blue has noise in it which is not in the model.  After the Fourier transform from Q-space to r-space, the noise becomes correlated and resembles a sine-wave with somewhat random phase shifts in it.  You may be able to see this if you plot the difference curve (often plotted in green below).  The reason the red curve looks flat at that point and the blue curve has a small peak is likely due to these noise ripples.

tl;dr, everything looks fine with your fit, though there may be some things you can modify to make it better.  One thing to check is to make sure that you set the Qma parameter in the data pane to be the same as the actual Qmax you used when you Fourer transformed your data.  This also affects the ripples I was talking about.

S

On Sat, May 17, 2025 at 7:42 AM Tanu Swami <tanusw...@gmail.com> wrote:

Hello everyone I am trying to fit a data using PDFgui software with the monoclinic symmetry. At r around 22A it is not fitting the peak but bond lengths are present at that value of r. Can someone please explain what could be the possible reason

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Simon Billinge

Professor, 

Department of Applied Physics and Applied Mathematics

Columbia University

[Tanu Swami]

Thank you for answering it. One more thing I would like to ask is can these ripples be present at lower r values too because the similar issue I was facing earlier at r around 9 A.

To view this discussion visit https://groups.google.com/d/msgid/diffpy-users/CANugsUEZubm6%3DqHS5sArM58g08CCooLz_N-thQAoDKCLyn1ZDQ%40mail.gmail.com.

[Simon Billinge]

Yes, they will be present at 9A and throughout the PDF 

Simon Billinge

Professor, 

Department of Applied Physics and Applied Mathematics

Columbia University

On Sat, May 17, 2025, 9:53 AM Tanu Swami <tanusw...@gmail.com> wrote:

Thank you for answering it. One more thing I would like to ask is can these ripples be present at lower r values too because the similar issue I was facing earlier at r around 9 A. On Sat, 17 May, 2025, 7: 16 pm Simon Billinge, <sb2896@ columbia. edu>

To view this discussion visit https://groups.google.com/d/msgid/diffpy-users/CA%2BPaf%3DwiiKwnAkZq-U8Pt6C3xCoCjRQ_WWsNiPumeoobcw7i_w%40mail.gmail.com.

[Tanu Swami]

Okay.
Thank you so much.

To view this discussion visit https://groups.google.com/d/msgid/diffpy-users/CANugsUHN0Qv%3D-orfnPj9jzhO99-1GKGSd1MEPjA4otw8QuzBxw%40mail.gmail.com.

[Tanu Swami]

Can you please give one more clarification that are these ripples temperature-dependent because this peak at 22 A is disappearing at higher temperatures?

[Federico Bianconi]

Dear Tanu,

Termination ripples are numerical artifacts resulting from the truncation error in the Fourier transform when calculating the PDF up to a finite Qmax, and therefore they are not directly temperature-dependent.

However, temperature may have an indirect influence. As the temperature increases, the Debye-Waller factors also increase, leading to the broadening and damping of high-R peaks. As a result, termination ripples in those regions may appear differently.

Of course, this comparison holds only if the PDFs were obtained using the same data reduction procedure.

I hope this answers your question. Please don’t hesitate to reach out if anything remains unclear.

Cheers,

Federico Bianconi

[Tanu Swami]

Okay.
Thank you

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