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Methods, Problems and Solutions

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

GSAS and Brian Toby's EXPGUI - cumulative chi squared plots [W.I.F. David, "Beyond least-squares analysis in profile refinement"; Accuracy in Powder Diffraction-III, 2001] in EXPGUI's Liveplot

(In this case comparing "Rod Hill and Ian Madsen" Variable Count Time X-ray data vs Fixed Count Time X-ray data)

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]

The following is performed using Brian Toby's EXPGUI which can be a very user friendly way to control GSAS refinements. It is written in Tcl/Tk so works in Windows and UNIX, is freely available, and the source can be freely modified and added to.

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

Download FTP Site and Mirrors

For information and source code on the Rod Hill and Ian Madsen Variable Count Time (VCT) data collection software. Refer:

  • Madsen I.C. and Hill R.J. (1991) "Rietveld Analysis Using Para-Focusing and Debye Scherrer Geometry Data Collected with a Bragg-Bentano Diffractometer", Zeitschrift fur Kristallographie, 196, 73-92
  • Madsen I.C. and Hill R.J. (1992), "Variable Step-Counting Times For Rietveld Analysis, or, Getting The Most Out of Your Experiment Time", Advances in X-ray Analysis, 35, 39-47
  • Madsen I.C. and Hill R.J. (1994), "Collection and Analysis of Powder Diffraction Data with Near Constant Counting Statistics", J.Appl. Cryst, 27, 385-392
  • VCT source Code, documentation and MS-DOS executables: http://www.ccp14.ac.uk/ccp/ccp14/ftp-mirror/csirominerals-anon-ftp/pub/xtallography/variable-count-time/

Note: The following is done with refinements that are not yet fully converged

In EXPGUI, refine a structure as per normal. If you just want to try this out, just quickly refine the scale.

Then click on the Liveplot ICON and select the Options, Cumulative Chi2 menu option which will then overlay the Cumulative Chi2. Examples follow below.

cumulative chi squared menu option in LivePlot

Fixed Count Time X-ray Data

Following is an example of Fixed Count Time XRD data refined in GSAS and displayed with EXPGUIs Liveplot and cumulative chi squared option.

cumulative chi squared in EXPGUI's Liveplot

Variable Count Time X-ray (VCT) Data

For the papers on Variable Count Time data collection, refer:

  • Madsen I.C. and Hill R.J. (1991) "Rietveld Analysis Using Para-Focusing and Debye Scherrer Geometry Data Collected with a Bragg-Bentano Diffractometer", Zeitschrift fur Kristallographie, 196, 73-92
  • Madsen I.C. and Hill R.J. (1992), "Variable Step-Counting Times For Rietveld Analysis, or, Getting The Most Out of Your Experiment Time", Advances in X-ray Analysis, 35, 39-47
  • Madsen I.C. and Hill R.J. (1994), "Collection and Analysis of Powder Diffraction Data with Near Constant Counting Statistics", J.Appl. Cryst, 27, 385-392
  • VCT source Code, documentation and MS-DOS executables: http://www.ccp14.ac.uk/ccp/ccp14/ftp-mirror/csirominerals-anon-ftp/pub/xtallography/variable-count-time/

Following is an example of Variable Count Time (VCT) XRD data refined in GSAS and displayed with EXPGUIs Liveplot and cumulative chi squared option.

cumulative chi squared in EXPGUI's Liveplot

Variable Count Time data displayed in a native format

VCT Data in it's native form

As a bonus: Constant Wavelength Neutron data

Following is an example of Constant Wavelength Neutron data refined in GSAS and displayed with EXPGUIs Liveplot and cumulative chi squared option.

cumulative chi squared in EXPGUI's Liveplot

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