Chemical Constraints Window

 

The Chemical Constraints window allows you to restrict the refinement in various ways. For all the windows you pick either a pair of types (Ni and O) and change as appropriate the fields to the right, for instance the bond distance. You then can add, modify or delete values. For each of the different constraints the "Weight" field affects how strongly the constraint is applied, larger numbers being stronger. (You should probably use smaller values at the beginning and only increase them later on.) The "Active" checkbox can be used to turn off or on a particular constraint.

Be aware that these are both very useful, and dangerous. They restrict the range of the refinements which can be useful. However, if you overdo it you are not considering what might be viable structures. It is important to remember that the values are only in two dimensions

Bond Constraint. By setting these you enforce a constraint that for all of the selected species there is an appropriate atom of the other type within a certain distance in projection.

Bumping Constraint. This is the opposite of a Bond Constraint: you enforce a constraint that for all of the selected species there is not an appropriate atom of the other type closer than a certain distance in projection.

Stoichiometry Constraint. You can use this to, for instance, for a surface or the overall structure to have a particular stoichiometry. For each atom you give a particular valence, e.g. 2- for oxygen, 2+ for Ni. If you set Base=0 then the sum of the valences will be contrained to be zero. Alternatively, if you set the Base to some other value then this is added to the total sum. So, for instance, if you had MgO (111) where the arrangement is alternatively Mg2+ and O2- for a NxN reconstruction setting Base=+NxN would be equivalent to forcing a charge compensated reconstruction onto a Mg termination; setting it to -NxN would do the same for a O termination.

Co-ordination. This will force atoms of a given type to have an appropriate number of neighbours with the distance set by the bond contraint for the pair. You set both a minimum and a maximum, and using this you could (for instance) force all the cations to be bonded to at least two oxygen atoms.

Cell Content. Using this you can bound the number of atoms of each type, both a maxmimum and a minimum.