While there are some entries that are found in every GSAS-II project, the contents of the GSAS-II data tree will vary with what is being done with GSAS-II. For example, if the program is being used to process a series of diffraction images, the data tree will contain only entries for images and the histogram(s) generated from those images. On the other hand, a Rietveld fit will contain entries for at least one histogram and at least one phase. In this section, I will describe the contents of the data tree in some detail. This information is provided here as reference material for later chapters of the book. If you are reading the book from the beginning, you are recommended to skim over this information and return later as you need to know more information relevant to work you are doing. Note that the data tree is called that because it is a hierarchical structure, where each entry is related to each other as branches in a tree come from the main tree trunk and the there are smaller branches that are attached to the main branches. In GSAS-II there are only three levels allowed. There is a top-level entry, which will usually have several child branches. I will refer to these as the main tree items. Some of those main tree items will have their own child branch, which I will call child tree items. To keep the structure of the program straightforward, he child tree items are not allowed to have their own children.
When a GSAS-II project is initially created, there will only be the initial top-level entry in the data tree, which will be labeled “Loaded Data:”. This will be changed to “Project:...” with the name of the .gpx file that contains the project when the project is saved. Clicking on this entry does not perform any action and the data window will be left blank. Clicking on any main data tree entry or child entry will cause information to be displayed relevant to that selection in the data window and most data tree items (main or child) will present controls where the contents can be changed. Many, but not all data tree items will also cause visualization information to be displayed in the GSAS-II graphics window.
As soon as any information is placed in the project (.gpx) file or the file is saved, a series of main tree items is placed into the project, as presented in Table 15.1. When any form of data is placed in a GSAS-II project, a histogram entry is created, where the prefix on the name assigned for the histogram entry is dictated by the type of data that are included. The prefixes used are presented in Table 15.2. Most histogram main entries will also have child entries, some of which are discussed in later sections of this chapter. Finally, there are a few other types of data tree main items that are listed in Table 15.3
| Tree item name | Description and discussion |
| Notebook | Information tracking the work done in a project chronologically is placed here as well as any note you may choose to include. |
| Controls | Entries that control how refinements are performed are accessed here. |
| Covariance | After a refinement has been performed, this causes plot of the covariance between parameters to be shown as well as a summary of refinement statistics. |
| Constraints | Any constraints on how refinement parameters are limited or linked are placed here. These are discussed in Chapter 25. |
| Restraints | Any restraints that penalize the fit based on values for refinement parameters are found here. These are discussed in Chapter 26. |
| Rigid bodies | Rigid bodies are defined to control the geometry of atoms in a structure model, as discussed in Chapter 27. |
| Tree item prefix | Description and discussion |
| IMG | 2D diffraction images. |
| PWDR | Powder diffraction datasets. See §15.3 for the child entries in a PWDR histogram. |
| HKLF | Single crystal diffraction datasets. |
| Pair Distribution Function histograms computed from powder diffraction data. |
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| SASD | Small Angle diffraction datasets. |
| REFD | Reflectometry datasets. |
| PKS | Powder diffraction peak lists. This is not commonly used. More common is a peak list associated with a PWDR histogram. |
| Tree item name | Description and discussion |
| Phases | When any phases are included in a project, this entry is created. Each phase that is imported or created will be a separate child entry under the Phases main entry. |
| Hist/Phase | This tree item is present when the option to placing the parameters belonging to both phases and histograms (HAP parameters) are placed directly into the data tree, see §14.8.1. |
| Sequential Results | GSAS-II can perform a number of different fits on series of datasets, for example, calibrations and refinements. Results from these are placed in sequential results data tree entries, which will have data tables associated with them. |