This short tutorial explains how to create and edit a simple crystal structure (NaCl). To define a crystal structure in ReX, a parent phase has to be created first inside a sample model; the structure can then be used as a starting model in a Rietveld analysis, or exported to be used in another program.
First of all, start ReX by double clicking on the program icon (to install the ReX program, follow these instructions). The main application window will appear with the default layout. For this tutorial, it is useful to make the 3d-structure view visible; to do so, open the View menu in the main application menu and click on the Show View -> Structure menu item.
Next, right click on the Sample item located under the Analysis node in the project view; then, select the New phase item from the popup menu that shows up.
A new, empty phase will be created inside the sample; you may need to click on the sample node to expand it and show the new Phase item:
By clicking on the Phase item, the corresponding properties will be displayed in the properties view:
Change the Label text to NaCl; leave all the other properties unchanged. Next, expand the NaCl phase item in the project view to show the corresponding structure node 
:
After selecting it, its properties are displayed in the corresponding view:
We are going to modify the space group settings and the cell parameters. Click on the Select button in the space group panel; the space group edit dialog appears. Choose the Cubic option in the Symmetry combo box, and the -F 4 2 3 symbol in the Hall symbol combo:
After closing the space group dialog, write the the 5.6406 value in the “a” cell parameter text box:
This completes the basic crystal cell editing. Now, we are going to add the two atoms defining the asymmetric unit. To do so, right click on the structure item and select the Add atom command from the popup menu:
A new atom will be added to the crystal structure. Select the newly created atom (atom 1 node in the project tree) to display its properties:
By default, newly created atoms are placed at (0, 0, 0) position in the unit cell and are assigned the H element. Click on the Element combo button and select Cl in the list of the periodic elements that shows up; leave the fractional coordinates unchanged. The structure view should be updated accordingly, with the Chlorine atom replicated in the unit cell according to the space group settings:
Now, add the second atom by following the same procedure and selecting the Na element in the Element combo box. Edit the fractional coordinates text boxes with the (0.5, 0.5, 0.5) values. The complete NaCl structure should now appear in the structure view; you can rotate the cell around the center by dragging the left mouse buttons, while the mouse wheel can be used to zoom in or out.
