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## CRYSTALS Single Crystal Suite

### Hints on using Crystals to refine a higher symmetry framework (highly ordered host network) containing a lower symmetry guest molecule

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#### From David Watkin:

```Date: Thu, 8 Nov 2001 08:51:34 +0000 (GMT)
From: David Watkin [david.watkin@chemistry.oxford.ac.uk]
To: Lachlan Cranswick [l.m.d.cranswick@dl.ac.uk]
Subject: RE: Crystals Request

Hello
Tis is not a problem. You enter the space group of lowest symmetry
for the whole structure, then apply addtional constraints/restraints to
the higher symmetry part.

eg, for 2 atoms on opposite sides of a pseudo centre
1) Ensure that the starting values for the parameters are as you would
expect (since non-linear LSQ only computes increments which are applied to
the refernced paramters)
2) set up constraints. First ensure that the same shifts are applied to
the x,y,z parameters pairwise
RIDE c(1,x's) c(101,x's)
Then make sure that the inverse shifts are applied to one of the atoms
WEIGHT -1 c(101,x's)
If you want to do whole groups
LINK C(1,X'S) UNTIL C(6) AND C(101,X'S) UNTIL C(106)
WEIGHT -1 C(101,X'S) UNTIL C(106)

Alternatively, it can be done via restraints, eg
RESTRAIN 0.0, 0.001 = C(1,X) + C(101,X)
RESTRAIN 0.0, 0.001 = C)1,Y) + C(101,Y)

CONSTRAINTS AND RESTRAINs can be mixed, though it is up to the user to
ensure they make sense.

When the CONSTRAINT list is processes, CRYSTALS checks to see if any of
the atoms in the structure are subject to spacegroup symmetry
restrictions. If they are, and they are also a subject to a user
CONSTRAINT, the space group symmetry is applied as an additional
RESTRAINT, otherwise an additional CONSTRAINT is applied

Hope this helps

Best wishes
David```

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