Numerical error during Fitting

General
1

While trying to perform a Lorentzians model selection to some synthetic data, some unexpected things happened. At the end, I tried to adjust three Lorentzians to a curve created adding exactly three Lorentzians with no error or convolution. The returned fitting was relatively bad, even though it was already initialized with the optimum. Finally I detected that the Fitting Algorithm did not return the curve that was supposed to return, in fact, the third Lorentzian was cut to zero value at a certain distance from the zero.

Our example was made with the following parameters (Amplitude, PeakCentre, FWHM):
1, 0, 2
1, 0, 0.4
1, 0, 0.04
An screenshot of the point where it fails is attached.

2

Open the curve fitting settings: View → Preferences, then in the “Mantid” section under “General” click on the “Curve Fitting” tab. Enter a big number for “Peak Radius (in FWHM)”, something like 10000. The Peak Radius sets the Peak function to zero for distances d > Peak_Radius*FWHM, and the Lorentzian is a Peak function.
Save this setting, redo the fit, and see if the third lorentzian still goes to zero.

.Jose

1 Like
3

Thank you very much. Now I have seen there was already a post debating on this topic, although I was not able to find it before, sorry.
Many thanks again.