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Fit Model Choices¶
Mantid contains an increasing catalog of predefined fitting function, which can be used to create a model. Some are demonstrated here.
In addition, it is possible to create new fitting functions using the User Function toolbox.
Multiple peaks + background¶
It is possible to fit overlapping peaks on a common background. Load the file GEM38370_Focussed and plot spectrum number 6. Zoom in on the 3 peaks around 2.15 Angstroms. Add a Linear Background, add a Lorentzian peak to the large central peak, and add a Gaussian peak to both of the smaller peaks, as below. Run the Fit, and inspect the results.
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Non peak model + background¶
Fitting is not limited to peaks. You can select any other function from our list via right-click “Add other function”.
Load EMU00020884 and plot spectrum number 6. Add the other function : ExpDecay Run the Fit and inspect the results. You may need to edit the fitting range (StartX and EndX) to get a good fit!
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Next, add the other function : GausOsc and run the Fit again. You should find this fit is better.
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Custom fitting function¶
User defined function¶
You may have spotted “UserFunction”. It can be chosen by right-clicking on a plot and selecting “Add other function”. It accepts a “Formula” as text string of a mathematical formula. All variables in the formula are treated as parameters, except for “x” which is the argument.
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The formula can simply be entered into Fit Property Browser. Alternatively, click on the … button in the Formula input box to open the User Function Dialog:
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Type your function in the large lower text field. Browse the built-in functions above and add () them
to your formula. The fitting parameters are displayed in the Parameters
field (read-only). If the field is empty then your formula contains errors.
You can save the function you have defined, for future use. Click
the Save button to see the dialog:
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Now your function appears in the list of available functions:
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Any unwanted function can be removed from the list using
button.
When finished click the Use button to insert the formula
into the Fit Function browser.
Tabulated function¶
A TabulatedFunction takes its values from a file or a workspace
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For more information on this, see here.