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A.6 Constraints pane

GSAS offers two types of restraints that can be included in a model. So called "soft constraints" or "restraints" penalize the fit if aspects of a model (for example, bond lengths) differ from selected values. So called "hard constraints" reduce the actual number of parameters in a model, by grouping parameters so that a single shift (possibly scaled) is applied to each parameter in the group. The constraints pane is used to set these "hard constraints." At this time the only constraints that can be set in EXPGUI are on atomic parameters (others can be set in EXPEDT).

Note that these constraints are only applied to shifts but not to the actual values. This means that, for example, if two atoms are constrained so that the shift on their occupancy is the same, but the initial occupancies differ, the in occupancy difference will not change as the values refine.

Atom Constraints

The atom constraints editing pane appears below. Note that constraints can be listed sorted by constraint number, phase number, lowest atom number or variable by clicking on the buttons at the top of the pane.

To delete one or more constraints, click on the appropriate checkbuttons in the Delete column and the press the Delete button, below.

Due to the way GSAS implements these constraints, a parameter can be included in only one constraint. Should one or more parameters be constrained twice, an error message is shown as shown below. Note that the constraint number is displayed as red, for constraints that have conflicts in the constraints editing pane (see above.)

When the edit button for a constraint is pressed, the constraint is opened for editing. Controls are available to select the phase, variable, multiplier and atom(s). Note that for atom selection, use of the control key and the left mouse button allow individual atoms to be selected or deselected. Shift and the left mouse button allow ranges of atoms to be selected and the right mouse button selects all atoms (see Mouse Actions).

Note that it is inappropriate to constrain different types of parameters together, for example X and Uiso. The left-most variable button is the "master" which determines what variable are allowed in the other variable buttons.

When the "New Constraint" button is pressed, a blank constraint editing window is opened. Note that any variable to the right may then be selected, including two special codes, Uxx and XYZU. XYZU creates four constraints, requiring the shifts on x, y, z and Uiso to be the same for all the selected atoms. Uxx does the same for the six anisotropic displacement parameters. These special codes are valuable for atoms that share a site.

Note that only 100 parameters may be included in a constraint, with one exception. When shifts for Uiso for all atoms in a phase are constrained, this counts as a single parameter. If more than 100 parameters are entered for a constraint, an error message is shown. Up to 2000 atomic constraints may be entered. EXPGUI does not warn if you exceed this limit.


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Brian Toby (Brian.Toby@NIST.GOV)
$Revision: 223 $ $Date: 2009-12-04 23:02:27 +0000 (Fri, 04 Dec 2009) $