Last modified 3 weeks ago Last modified on 07/22/18 17:17:26


GSAS-II is an open source Python project that addresses all types of crystallographic studies, from simple materials through macromolecules, using both powder and single-crystal diffraction and with both x-ray and neutron probes. Measurements can be constant wavelength or TOF (thanks to support from Oak Ridge National Lab.) GSAS-II supports everything that GSAS/EXPGUI does, except for magnetic scattering, but GSAS-II also handles all the steps in diffraction analysis, such as data reduction, peak analysis, indexing, Pawley fits, small-angle scattering fits, structure solution in addition to structure refinement. It can be used with large collections of related datasets for repeated (sequential) refinements and for parametric fitting to these results. The source code can be found at

We are updating the code quite frequently, so we may break things from time to time (see bug reporting, below). Be sure to 'Update' frequently to stay abreast of new features as they are added.

If you use GSAS-II, we encourage you to sign up for the GSAS-II mailing list here ( and please help us by citing it in your papers.

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 3.6 and wxPython 4.0. It works but expect that some new bugs will show up so we have not made this the default (but please do test this). See Python3 notes for more details.

Please Cite

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.

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.


To learn how to use different parts of GSAS-II, we strongly encourage running the tutorials here.

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.

GSAS-II Project tracking

Lists of the updates made to the source code can be followed using the Timeline Trac tool at this website. Tickets and the Roadmap feature are not currently in use.

APS User's Group

Topical meetings, intended for Argonne-based scientists, are held usually on the second Monday of each month, 1-2 pm in 401/B4100. See the Users Meeting web page page for information on past and future meetings.

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