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GSAS-II data tree overview

GSAS-II creates projects that are saved in .gpx files. The GSAS-II Data Tree, which is shown on the left side of the main GSAS-II window shows a hierarchical index to the contents of a project. This index will usually have all of the entries listed below on the Universal Data Tree Items page, which are found in all types of GSAS-II projects and will also have at least one entry representing some sort of experimental data (or simulation) -- called a histogram -- and will likely have one or more phase entries.

When you click on an entry in the data tree, the right side of the GUI, the GSAS-II Data Window will show the associated information with that section of the .gpx file, you may have access to additional menu commands and visualization information related to the selected data tree item may be shown in the GSAS-II Graphics Window.

Most of the subsequent sections of the GSAS-II help documentation describe: what the values shown in the GUI mean and what actions can be performed using the GUI, the menu commands and actions available for each type of data tree item, as well as any graphics that will be displayed and actions that can be performed graphically.

Data tree Histogram items

These constitute the data sets ("Histograms") to be used by GSAS-II for analysis. These are shown in the data tree as top-level entries where the prefix indicates the data type. The name in the data tree is usually followed by a descriptive term or file name. Most histogram types have information divided into sections, where each section is placed into a child (subdata) tree entry under the main histogram data entry. Selection of the main data tree item for a histogram may not produce much information in the data window but usually does provide visualization of the data in the graphics window. The histogram data prefixes are listed below and are described in subsequent sections of this help information.

  • PWDR: Powder Diffraction data
  • HKLF: Single Crystal data
  • PDF: Pair Distribution Functions (derived from data)
  • IMG: 2D Diffraction Images
  • SASD: Small Angle data
  • REFD: Reflectometry data
  • PKS Powder Diffraction peak lists

Data tree Phase information

A GSAS-II phase is the description of a crystal structure, including unit cell parameters, symmetry and atom coordinates. Each phase is placed in the data tree as a child (subdata) tree entry in the phase. Note that while phases do not have any special prefix and do not have child entries in the data tree, the phase information is separated into sections through the use of tabs on the upper part of the data window. The tabs are listed here and each tab is documented separately.

Phase/Histogram Linking

Note that there is no limit to the number of phase and histogram entries that may be placed a project file, beyond that the computer being used must have enough memory to hold this information. Also, all the histogram and phase entries in project need not be utilized. Every phase can be linked to zero, one or more histograms. A single-crystal dataset can only be linked to one phase (or none), but a powder diffraction dataset can be linked to any number of phases, including zero. A phase that is not linked to a histogram is not used in refinements. Likewise, a histogram that is not linked to a phase is also not used. Deleting unlinked entries from a .gpx file is possible, but is not recommended as it may introduce problems; these are bugs that are hard to track down.