GSAS-II is used to analyze all types of x-ray and neutron diffraction data, including single-crystal, powder, constant-wavelength, pink-beam and time-of-flight, lab, synchrotron, spallation and reactor sources, including Rietveld analysis. It can handle large numbers of datasets. GSAS-II is free open source software.

• The home page for GSAS-II has recently been moved to GitHub, but some content may still be only at the old location. Note the new installation instructions.

• This repo is the main repository for the GSAS-II source code, but note that there is a separate repo for the GSAS-II installation tools and another for GSAS-II web content (including tutorials). The old subversion site and its associated web site will be maintained for the immediate future.

• Code documentation: https://gsasii.readthedocs.io.

If you use GSAS-II in any part of your project, please cite it in your publications. This is the most valuable way you can demonstrate your support of the project. 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.
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 

GSAS-II is a unique and comprehensive Python project for the calibration, reduction and analysis of all types of x-ray and neutron diffraction data, including single-crystal and powder data, including constant-wavelength, pink-beam and time-of-flight data types and from lab, synchrotron, spallation and reactor sources. Its primary use is for determination of crystal structures and diffraction-based materials characterization for crystalline solids on all scales, from perovskites through protein. Refinements can combine measurements from multiple data types and large groups of data can be analyzed together via "sequential fitting". It also provides powerful and flexible capabilities for integration of 2D powder diffraction image data. In addition to single-crystal and powder diffraction, GSAS-II offers small-angle scattering and reflectometry analysis, structure solution capabilities and interfaces to several other types of analysis tools, such as for pair distribution functions, faulted materials, maximum entropy Fourier maps and symmetry analysis.

GSAS-II offers extensive visualization capabilities and a complete GUI implementation. An applications-interface (API) allows for scripted use of much of the GSAS-II functionality.

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. For incommensurate structures, a generalized form of 3+1 superstructures can be handled. From 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. Sequential fitting is a novel process that allows large numbers of data sets, measured with parametric changes in measurement settings, to be fit single refinement with subsequent parametric fitting.

GSAS-II is freely distributed as open source software; see the license file for more details. GSAS-II runs on Windows, MacOS, Linux and Raspberry Pi computers. It currently receives >600 citations/year.