[GSAS-II] Inquire about some program variables.

[Luiz Felipe]

Dear GSAS-II Users,

I have been working on Rietveld Refinement for some powder X-ray diffraction patterns acquired from lab-synthesized samples. As always, I find myself questioning the expected values in the instrumental parameters tab and how these values (U, V, W, X, and Y) correlate to the Gaussian, Lorentzian, and G+L curves displayed in the plot.

More specifically, what should the shape of the G, L and G+L curves look like? Sometimes I observe curves that resemble a decreasing exponential, while other times they look like a saturation exponential. I am unsure of the expected profile I should be aiming for. Could you please clarify this, or point me to a reference that could help me better understand the subject, maybe even a more recent version of the program manual, since I only have the old GSAS-I manual?

Additionally, I have questions regarding the thermal parameters U_iso in the Phase->Atoms tab. In the fluoroapatite tutorial, it is recommended to refine these parameters together with the atomic positions (X). While I understand the physical relationship of atomic positions to the refinement via wave mechanics, I am wondering about how the thermal parameters fit into the underlying mathematics.

Lastly, when I retrieve the wR and χ2RED values in the covariance tab, I see a colorful matrix with a gradient ranging from green to red. Although I usually ignore this matrix, I would very much like to understand what these colors represent, apart from the numerical range between -1 and 1.

Thank you in advance for your assistance.


Best regards,
Luiz Felipe Pompermaier
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