[GSAS-II] Local Symmetry Restraint in GSAS-II

[Jing Wang]

Hi, all,

I am a graduate student at The University of Akron working on the Rietveld
refinement of an organic crystal.

The chemical structure of the molecule is illustrated in the attached
figure, the conjugated core (in red color) is recognized as a rigid body.
The space group has been determined as P21 2 21 (No. 18). From the
single-crystal electron diffraction data, we know that the following two
conditions should be satisfied for the final crystal structure:

1) Mirror symmetry in this molecule (denoted by the magenta line in the
attached figure) must be retained during the refinement, i.e.
conformational variation of the two alkyl chains is allowed but should be
always symmetric with respect to the mirror.

2) The centroid of this molecule should be confined in the plane (1/4, y,
z) of the unit cell.

Would you please give me some advice on how to setup these restraint
conditions in GSAS-II? Thank you very much!

Best regards,

Jing Wang
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