Last modified 7 months ago Last modified on 08/06/14 21:28:01


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 (and coming soon, thanks to support from Oak Ridge National Lab, neutron TOF.) At present, code is being developed for all the various steps in diffraction analysis, such as data reduction, peak analysis, indexing, structure solution and structure refinement. The source code can be found at

At present GSAS-II can be used for processing of area detector data, peak fitting, auto-indexing, structure solution, Pawley, Rietveld and small-angle scattering fitting. It can be used with large collections of related datasets for repeated (sequential) refinements and for parametric fitting to thise results. Many of the nice features of GSAS are present and some things work much better than in GSAS, but a few features are yet to come. At this point only x-rays and constant wavelength neutrons are implemented (TOF is currently being implemented). The GUI is self-documenting with help pages for every GUI section. The code is changing on a regular basis, so expect things to break from time to time and you should '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

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 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 the information allowing us to find and fix it. Before reporting a bug, please confirm you are using the newest GSAS-II version (see Help/Check for Updates).

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..."]

2) Report the startup lines that GSAS-II prints at the beginning of the console output [from "GSAS-II binary directory..." to "This is GSAS--II version..."). Restart, if needed to get this.

3) 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 and include a link in the e-mail.

4) Please tell us exactly what you need to do to reproduce the error from the .gpx file (please provide the order that you click on menus and buttons, they may matter.) If we can't reproduce it, we can't fix it.

5) If your error involves processing an image, we will need that too. Again please don’t send that to the full list, 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.