This is used to place restraints on occupancies and is found with the notebook tab labeled “Chem. comp.” This probably the next most commonly used restraint after bonds and is quite versatile. This constraint uses the number of atoms in the unit cell, which is determined from the site multiplicity times the occupancy.
The first step in creating a composition constraint is to select the “Chem. comp.” tab and then use the “Edit Restr.”/“Add restraint” menu command. The first window to be opened allows selection of which atoms to include into the constraint. What you will select here will depend on what you want the constraint to do. In the next window you specify the target value that will be used in the constraint. (This is easily changed later). The constraint is then created as a list of the selected atoms. For each atom, there are four entries: the atom name, the number of atoms in the cell (the multiplicity * occupancy), a factor that you supply (which defaults as 1) and the product of the factor times the number of atoms. The sum of these products is the calculated value for the restraint, and this is shown as the last line for the restraint, along with the target and \(\sigma \) (esd) value. Note that the “Edit Restr.”/“Delete restraint” menu command can be used to either delete an entire restraint or remove one or more atom from a restraint, depending on which row(s) are selected. Rows can be selected before the menu command is used by clicking on row labels or if no entries are selected, a window will be opened to select row(s).
How this restraint will work will depend on what atoms are included and how the factor and target values are set. Here are some ideas:
You can create as many compositional restraints as you wish.