Last modified 7 weeks ago Last modified on 10/13/15 13:06:57


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

The code is changing on a regular basis, so expect things to break 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, please sign up for the GSAS-II mailing list, see web page and please cite it.

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.

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.

GSAS-II Documentation

Help file:

Tutorials: (this is a great place to start)

Developer Notes: see wiki page. (Includes an extensive guide to the code base.)

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

Updates to the source code can be followed using the Timeline Trac tool at this website. To enter a ticket, send an e-mail to Brian Toby. The Roadmap feature is not currently in use.

For a complete list of GSAS-II wiki pages, see TitleIndex.