Angle restraints also operate on atomic coordinates. The notebook tab for them is labeled “Angle”. They are most commonly used for keeping angles between bonded atoms at a target, but they can be used on any pair of atoms in a structure, whether bonded or not. As was noted for bond distance constraints, you should have an idea of the bonding environment for each type atom that you wish to constrain. Since the searching is done using atomic radii, it is worthwhile to the Phase entry in the data tree and use the Atoms tab and the Compute/“Show Distances & Angles” menu command and confirm that the radii settings locate the angles you wish to constraint. You may need to change the radius values for atoms. Note that the results are listed on the console window. Note that the atoms in an angle are the outer or A/C atom (where the angle is listed as A-B-C) and the central or B atom.
Then, select the “Angle” tab, and make note of the “Search range” factor. This will be used to find angles for atoms where the A-B and C-B distances are shorter than the sum of the atom radii distances times the Search range”. Pairs of atoms with distances longer than that will be ignored. You will then use the “Edit Restr.”/“Add restraint” command and then you will be asked to:
Once this is done, located angles are used to create constraints that are placed into the table of constraints shown in the data window. You can edit the target and esd (\(\sigma \)) value directly in the table. Or you can select restraints in the table and change the target or esd values for the selected restraints using “Edit Restr.” menu commands. There is also a menu command that will delete the selected restraints. If no selection has been made in the table before entering any of these the menu commands, then a window will be displayed where restraints can be selected.
As noted, the Mercury program that tabulates distances and angles within a molecule also prepares the statistically expected values for those values from the structures in the Cambridge Crystallographic Database using the CSD-Core/Mogul Geometry Check. This is primarily of use for organic moieties in structures. See the GSAS-II Help page on Restraints for more information on this command. When used from the Angles tab, constraints will be created for angles from this file.