Examples of PLATON/ADDSYM (= MISSYM(C) Clone)    22/10/97
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Missed symmetry is a serious problem. For a recent discussion see
D.Marsh, Acta Cryst. (1997). B53, 317-322.

Checks for missed symmetry can be done for datasets supplied as
shelx.res, csd.dat, shelxl.cif structured files with PLATON
using the CALC ADDSYM instruction.

Example 1:
==========
In the order of 10 % of the structures published in Space group Cc
can be shown to be better described with higher symmetry. A common
case is the omission of an inversion centre. This example shows
how to detect and correct automatically for that problem.

Run:

platon ex1.spf

with instructions 

CALC ADDSYM
END

A file will be written: ex1.eld containing data for the 
required origin shift and additional inversion centre.

When desired, a new 'ex1.res' file may be written with the instructions

platon ex1.eld

CALC SHELX
END
==================

Example 2:
==========
This example shows how to detect and correct for missed symmetry
for a structure initially solved and refined as a structure in 
space group Cc. The structure is actually taken from the Cambridge
Crystallographic Database and was shown by Marsh & Herbstein 
(Acta Cryst (1988), B44, 77-88) to be better described in space group
Fdd2.

The example 'shelx.res' is given as ex2.res
To detect the missing symmetry run the command:

platon ex2.res

with the interactive instructions:

CALC ADDSYM
END

The console output and trailer listing will indicate the possible
higher symmetry
A file, 'ex2.eld', will be written with the associated transformation
matrix to the new F-centered orthorhombic lattice. 

(The proposed alternative space group, transformations matrix and
shift of origin are listed)

This new file, can be run through PLATON as well with

PLATON ex2.eld

From this output, it can be dirived that a shift of the origin
by (0.375, 0.769,0.0) and the addition of an additional 
symmetry element 'SYMM 3/4+x, 1/4-y, 3/4+z' should result in
a possible better description (i.e. Fdd2).

A new '.res' file may now be produced (using the modified file
given as 'ex2.eld') with the instruction

platon ex2.eld

and

CALC SHELX
END

A new file (to be edited further for the H-atoms in this case)
is written: ex2.res.

Note: Transformation matrices supplied via 'TRNS' are effective
on the following data items such as cell, symmetry & coordinates.
Symmetry items not to be transformed should be protected via [].
