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Methods, Problems and Solutions

GSAS (General Structure Analysis System) Rietveld powder diffraction and Single Crystal software

Using EXPGUI, BKGEDIT and GSAS to manually define a background using Chebyshev Polynomial coefficients

The CCP14 Homepage is at http://www.ccp14.ac.uk
[Back to Problems and Solutions] | [Back to GSAS Hints/Resources]
[The reference to use for GSAS in any resulting publications is: A.C. Larson and R.B. Von Dreele, "General Structure Analysis System (GSAS)", Los Alamos National Laboratory Report LAUR 86-748 (1994).]
[The reference to cite in any resulting publications for using EXPGUI is: B. H. Toby, EXPGUI, a graphical user interface for GSAS, J. Appl. Cryst. (2001). 34, 210-213]

EXPGUI has an ingenious method of manually describing a fixed background. The user clicks on the points that define the background; after which EXPGUI will fit Chebyshev coefficients to these background points and save them into the GSAS EXP file. To then change from a fixed background to a refined background at a later stage can thus done seamlessly with very reasonable starting values generated by the EXPGUI manual background fitting.

A November 2002 note from Brian Toby: 

For what it is worth, BKGFIT has been improved since the tutorial above
was written and will now fit most of the GSAS background functions to a
set of user-supplied points. So, it can also be used to see how
different functions will work with a particular background shape. [IMHO,
Chebyschev is usually fits as well as anything else and typically
refines with more stability, since the terms are orthogonal.]

EXPGUI Homepage with download and installation information on EXPGUI for Windows and UNIX

Download FTP Site and Mirrors

Click here to download the example files used in this tutorial run-through

In EXPGUI, load the EXP file that you intend to manually fit the background. (the example file provided above is a combined X-ray / Neutron refinement)

EXPGUI Interface

Before manually fitting the background, run Powpref.

To manually fit the background, via the EXPGUI menu, select Powder, bkgedit to bring up the BKGEDIT screen.

BKGEDIT Starting Screen

Select the Histogram pattern of interest then use "Click and Drag" via the mouse to zoom up as required

Zoomed BKGEDIT Starting Screen

Click on the Add Icon and click to define the background points. (you can always delete points using the Delete Icon)

Adding background points in BKGEDIT

Now define the number of Chebyshev terms to fit the background to. In the following, we will start with 6 terms. (If you get a divide by zero error, check that you have the latest version of EXPGUI - post 15th November 2001).

Defining number of Chebyshev terms to fit the background to

Now fit using the Start Fit Icon.

fitting with 6 terms

Slowly increase the number of terms until you get the best fit possible without over-fitting.

fitting with 12 terms

When you have the best fit possible, click on the Save in EXP file & Exit, which will then save the Chebyshev terms into the GSAS EXP file.

The Save in EXP file ICON

Back in EXPGUI

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