11.4 The Auto-Background Feature

A very nice feature in GSAS-II that was contributed by a GSAS-II user is the auto-background feature. This uses the Python pybaselines package to estimate where the baseline should be placed. This package is included in the standard GSAS-II installation, but since one can install GSAS-II in other ways, it might not be present if you installed Python yourself. GSAS-II will run fine without this package being installed, but if it is not, the auto-background feature will not function. The Help menu has a command “Add packages for more functionality” that can be used to install pybaselines if not present.

To invoke the auto-background feature, press the “Compute auto background” button. This creates a smooth spline function that will tend to ignore sharp peaks. Selecting this will open a new window with the following options:

• There is a choice of two functions for the spline: “arpls” and “iarpls”. You are suggested to try each and choose the one that works best for you.
• The “log(Lambda)” term is a spline coefficient that determines how much the background can “wiggle” (have sharper variations). There is a slider provided for this value. As the value is changed, the resulting baseline position is shown as a red line.

Once a good background choice has been found, either fixed background points can be created along the red baseline or one can create a new histogram that can be subtracted point-by-point. These are selected by the two buttons on the window:

• “Set Fixed Points & Fit”, which creates fixed background points across the entire pattern and then fits the background terms that are set to be refined. I generally prefer this option.
• “Define Fixed Bkg histogram” creates a histogram of fixed background points and sets this to be subtracted from the current pattern. If the background computed by the auto-background spline does a good job of matching the baseline, but you are not able to fit that with the above options, this may be the optimal choice.