Version 70 (modified by toby, 21 months ago) (diff)



GSAS-II is a unique and comprehensive open source Python project for determination of crystal structures and diffraction-based materials characterization for crystalline solids on all scales, from perovskites through proteins, using both powder and single-crystal diffraction and with both x-ray and neutron probes. Refinements can combine measurements from laboratory and synchrotron x-rays as well as constant wavelength or time-of-flight* neutron sources. It provides structure solution and refinement as well as extensive visualization capabilities.

Many capabilities of GSAS-II are unique to GSAS-II or are only found in software with very limited scope. For magnetic scattering, all possible color subgroups can be derived and explored. With powder diffraction, GSAS-II supports all stages of data reduction and analysis, including area detector calibration and integration, pattern indexing, LeBail and Pawley intensity extraction and peak fitting. Pair distribution functions (PDF) can be computed from high-energy x-ray diffraction. Instrumental profile parameters can be fit to data from standards or derived from fundamental parameters; sample profile effects (crystallite size and microstrain) are treated independently from the instrument. When large numbers of patterns are measured with parametric changes in measurement settings, GSAS-II provides a novel capability to fit all patterns in a single sequential refinement with subsequent parametric fitting. GSAS-II also provides small-angle scattering and reflectometry fitting and simulation of the effects of stacking faults.

GSAS-II is made available for free use (see license) with open access to the source code. If you use GSAS-II, we encourage you to sign up for the GSAS-II mailing list (see below). Please help us by citing it in your papers.

We are adding new features to the code quite frequently, so we may break things from time to time (see bug reporting, below). Be sure to use the Help/Update capability frequently to stay abreast of new features and fixes as they are added. GSAS-II now provides all of the capabilities in the GSAS/EXPGUI programs, which are no longer supported. GSAS/EXPGUI users are strongly encouraged to switch to use of GSAS-II.

Installation instructions

The license for use is included with the software and is found here. Also, please sign up for the mailing list after installing GSAS-II.

GSAS-II and Python 3.x: We are now supporting GSAS-II with Python 3.7 and later with wxPython 4.0.x. We are tracking down bugs due to changes in wxPython 4.1 and Matplotlib 3.3, but we are no longer supporting Python 2.7 and wxPython 2.x or 3.x. Likely most of GSAS-II will still work, but bugs may be seen, particularly in newer sections of GSAS-II.

Please Cite

The primary citation for GSAS-II is:

Toby, B. H., & Von Dreele, R. B. (2013). "GSAS-II: the genesis of a modern open-source all purpose crystallography software package". Journal of Applied Crystallography, 46(2), 544-549. doi:10.1107/S0021889813003531

If you use GSAS-II in any part of your project, please cite it in your publications. This is the only way you can demonstrate your support of the project. (Citations to GSAS-II have ~doubled every year, with 104 citations in 2017.) Note that some sections of program utilize work by others and will display citations for that. If you use those sections, please cite those papers as well.


To learn how to use different parts of GSAS-II, we strongly encourage running the tutorials here. Note new addition of many video-tutorials so you can see the computer screen as the tutorial is being run.

For background material on powder diffraction crystallography, see the links on our Powder Diffraction Crystallography Educational Materials page.

GSAS-II Documentation

In the future we hope to increase the amount of documentation available but for now this is what is available:

  • Developer Notes: ~200 pages of code documentation are on a separate web site. This is probably not valuable to most users, but code developers should look here. We invite people interested in developing or extending GSAS-II. The code is open source and we are happy to consider collaborations.

Mailing List

Please do subscribe to the mailing list, or use the archives to look for announcements:

  • To subscribe use this link for the Web interface or send an e-mail to "GSAS-II-request @" (remove spaces). Use as the subject subscribe (or subscribe <password> where <password> is your preference for a mailing list login password).


We frequently get bug reports without enough information to find what is wrong or that might have already been fixed. Before reporting a bug, please confirm you are using the newest GSAS-II version (see !Help/"Check for Updates"). If in the latest version, and we can reproduce your bug, we will work on fixing it. Suggestions for improvements are also welcome.

To report a bug, either send an e-mail to the mailing list (gsas-ii "at" or send it to both of us (vondreele "at" and toby "at" Also, please do the following:

1) Send us the error message(s) and traceback from the console window [text beginning with "Traceback (most recent call..."]. Also, tell us if you are running on Windows, Mac or Linux (it sometimes matters).

2) Get us the .gpx file — don’t send that to gsas-ii@…, but you can e-mail it to toby@… and vondreele@…. Better still, upload it to (you will need to set up a guest account) or use some other sharing service, such as; include a link in the e-mail.

3) Please say what you need to do to reproduce the error from the .gpx file (please provide a detailed list of menu commands, button clicks, etc. so we can duplicate that.) If we can't reproduce it, we can't fix it.

4) If your error involves processing an image, we will need that too. Again please don’t send that to gsas-ii@…, either.


A complete list of GSAS-II wiki pages is here.

*Inclusion of Time-of-flight diffraction capabilities in GSAS-II was supported by Oak Ridge National Lab.