Advanced Rietveld Techniques: Tricks and Tips with EXPGUI

This is a web/video version of a presentation made to the research group of John Mitchell at the Argonne Materials Science Division on February 22, 2006. It was originally planned to be released as a combined slide+video+audio (SofTV) recording of the presentation, but this did not come to pass due to incompatibilities between the Mac and Windows versions of PowerPoint. For this reason the slides have many more screen dumps than I would have done, if I had planned to provide recorded demos. I hope the extra detail is not a major hindrance.

Brian Toby, April 2006

Presentation Goal

  • Introduce advanced & less obvious features of EXPGUI
  • Format: Problems & Solutions

Prerequisites

  • Crystallography
  • Some experience with GSAS & EXPGUI
  • Desire to learn new tricks & easier approaches

Outline

  1. Installing the software
    This section shows how GSAS, EXPGUI, CMPR & DRAWxtl (and GRACE) are obtained and the demo shows the installation of the software on a PC running Windows.
    Presentation Notes
    Demo: Installing & updating GSAS & EXPGUI (10 min)
    Demo: Installing CMPR & DRAWxtl (8 min)
    Demo: Customize EXPGUI to start in a selected directory (1 min)
  2. How to get starting profile terms
    Problem: Starting a refinement with bad profile coefficients & only an approximate structure is quite hard
    Solution: In CMPR, fit peaks widths and then fit a profile function to the widths; then in EXPGUI write the profile function into an instrument parameter file
    Presentation Notes
    Demo: Fit widths and fit a profile function (15 min)
    Demo: Use INSTEDIT in EXPGUI to write profile values into an instrument parameter file (9 min)
  3. Getting coordinates into GSAS
    Problem: How do I get coordinates electronically into GSAS without retyping them?
    Solution: EXPGUI can read a variety of coordinate formats, most importantly CIF; it can reset Origin 1 settings into Origin 2; atoms can also be displayed in an order other than the sequence they have been input.
    Presentation Notes
    Demo: Import Coordinates into EXPGUI from a CIF (7 min)
    Demo: Transfer Origin 1 settings to Origin 2 (4 min)
    Demo: Change display order of atoms (1 min)
  4. Graphics options in EXPGUI
    Problem: Are there good ways to graphically review my fit or structure? Can
    Solution: EXPGUI includes LIVEPLOT for review
    Presentation Notes
    Demo: Reviewing fits with LIVEPLOT; survey of LIVEPLOT capabilities (7 min)
    Demo: Exporting graphics and data from LIVEPLOT (3 min)
    Demo: Exporting diffraction data from GSAS into CMPR (3 min)
    Demo: EXPGUI interface to DRAWxtl to display molecular graphics (6 min)
  5. Simulation of diffraction patterns
    Problem: How can I easily simulate a diffraction pattern, for example from a CIF?
    Solution: Use EXPGUI to set up the simulation for GSAS.
    Presentation Notes
    Demo: Using EXPGUI to do quick diffraction simulations with GSAS (6 min)
  6. Bootstrapping the background
    Problem: Fitting background is easy, except when the structure, profile and initial background are all wrong (or when doing a Le Bail fit). How can I just put the background where I want it to be?
    Solution: Use of fixed background points is not a great idea, but using such points to generate an initial background model that is later refined when the model is in good shape. BKGEDIT can be used to do exactly that.
    Presentation Notes
    Demo: Using BKGFIT to fit tough background problems (16 min)
  7. Returning to a prior refinement
    Problem:Yesterday's Fit Was Better Than What I Have Done Since. How Do I Get It Back?
    Solution:GSAS keeps a historical list of old experiment files. EXPGUI has a tool to browse them and load one.
    Presentation Notes
    Demo: Reviewing and loading a previous refinement with the "include archived files" feature (5 min)
  8. Changing flags & settings for multiple histograms en mass
    Problem: I have a 10 histogram refinement with 3 phases, but I don't want to have to edit 10 sets of background parameters or 30 sets of profile refinement flags. Can I set everything at the same time?
    Solution: Yes - use multiple histogram selection.
    Presentation Notes
    Demo: EXPGUI features for simplifying multiple dataset refinements (10 min)
  9. Creating and Viewing Fourier Maps
    Problem: How do I create and analyze Fourier maps.
    Solution: Fourier maps are set up in EXPEDT (not EXPGUI, sorry); GSAS has tools for finding peaks in the map, as well as plotting them; Another option is DRAWxtl, which plots structures and maps quite nicely.
    Presentation Notes
    Demo: Setting up and computing a difference Fourier map (7 min)
    Demo: Exporting a Fourier map and viewing it in DRAWxtl (8 min)
  10. Constraining Atom Parameters
    Problem: When I refine all the atomic parameters in my model, the structure blows up. How do I deal with two atoms that share a site, etc.?
    Solution: GSAS allows the number of refined parameters to be reduced by forcing the shifts on more than one parameter to be the same (or related by a ratio). EXPGUI offers some tools to make this easier to do, particularly when two atoms are placed at the same site and the fraction of the two atoms is to be varied while the total occupancy is to remain constant.
    Presentation Notes
    Demo: Setting up GSAS atom contraints in EXPGUI (9 min)
  11. Other Tips, Tools & Tricks
    Problem: How do I learn about other obscure features in EXPGUI, without reading the tedious and humorless web-page documentation that comes with the software?
    Solution: You can learn some cute tricks here: controlling output in the .LST file; tricks in LSTVIEW, using detachable menus, benefits of the "Change Profile" window; the composition tool; editing excluded regions. Don't look for much humor here either.
    Presentation Notes
    Demo: Miscellaneous EXPGUI tricks (17 min)

Links

Last modified: Thu Apr 6 10:07:23 CDT 2006