Re #22044 Add GSAS section to Engg GUI docs

docs/source/interfaces/Engineering Diffraction.rst

@@ -451,6 +451,69 @@ User may plot single peak fitting workspace in separate window by using
 Plot To Separate Window button, if the *engggui_fitting_single_peaks*
 is available.
 
+.. _gsas-Engineering_Diffraction-ref:
+
+GSAS Fitting
+------------
+
+.. warning:: This is a new capability that is currently in a very
+             early stage of definition and implementation. Not all
+	     options may be supported and/or consistent at the moment.
+
+The GSAS tab provides a graphical interface to the Mantid algorithm
+:ref:`GSASIIRefineFitPeaks <algm-GSASIIRefineFitPeaks>`. This allows
+users to perform GSAS-style fitting on their data from Mantid.
+
+The user must input the following files:
+
+- **Focused run file(s)** - these must have been generated either by
+  the **Fitting** tab or :ref:`EnggFocus <algm-EnggFocus>`.
+- **Instrument Parameter File** - contains DIFA and DIFC GSAS
+  constants, will probably be of ``.prm`` format
+- **Phase file(s)** - contain crystallographic information about the
+  sample in question. Currently only ``.cif`` files are supported
+
+The following parameters are also required:
+
+- **New GSAS-II Project** - GSASIIRefineFitPeaks creates a new
+  ``.gpx`` project here, which can be opened and inspected from the
+  GSAS-II GUI
+- **GSAS-II Installation Directory** - you must have a version of
+  GSAS-II from at least **February 2018** to use the GUI. See
+  :ref:`Installing GSASII` for how to install a compatible version
+- **Refinement method** - can either be **Pawley** or
+  **Rietveld**. Pawley refinement is currently under development, so
+  Rietveld is recommended.
+
+Optionally, you may also supply:
+
+- **XMin** and **XMax** - the limits (in TOF) to perform fitting
+  within
+- **DMin** - the minimum dSpacing to use for refinement when
+  performing Pawley refinement
+- **Negative weight** - The weight for a penalty function applied
+  during a Pawley refinement on resulting negative intensities.
+
+To do a refinement, take the following steps:
+
+1. Load a run by selecting the focused NeXuS file using the
+   corresponding **Browse** button, then clicking **Load**. The run
+   number and bank ID (for example ``123456_1``) should appear in the
+   **Run Number** list in the **Preview** section. Click the label,
+   and the run willl be plotted
+2. Select your input files, and input any additional parameters in the
+   **GSASIIRefineFitPeaks Controls** section
+ 3. Click **Run Refinement**. Once complete, fitted peaks for the run
+    should be overplotted in the fitting area. In addition, Rwp
+    (goodness of fit index), Sigma and Gamma (peak broadening
+    coefficients) and lattice parameters should be displayed in the
+    **Fit Results** section.
+
+You can toggle the fitted peaks on and off with the **Plot Fitted
+Peaks** checkbox, remove runs from the list with the **Remove Run**
+button, and plot the run and fitted peaks to a larger, separate plot
+using **Plot to separate window**.
+
 .. _setting-Engineering_Diffraction-ref:
 
 Settings