# TwinRotMat

## Determine Twin Matrix from Fo/Fc Data

This function carries out an after-the-fact analysis for missed
or unaccounted for twinning from the analysis of an Fo/Fc CIF.
TwinRotMat addresses the same issue as the Windows program ROTAX by Simon
Parsons and Bob Gould but is not identical, both in terms of underlying
algorithms and approach. It might be useful to compare the results of both.

Ideally, twinning is already discovered in the early
stages of data collection and taken care of. This applies in particular
for data collected on CCD detector systems. Exceptions are cases of
(pseudo)merohedral twinning.

This routine
addresses cases of (reticular)(pseudo) merohedral twinning.
Cases where data are based on a the artificially higher volume
twin lattice are not covered. (i.e. monoclinic structure in
orthorhombic supercell twin lattice).

A tentative rotation twin axis and associated matrix is produced.

The analysis is based on the well known fact that unaccounted for
twinning shows up in a significant number of reflections with
I(obs) much greater than I(calc).

The program needs an '.fcf' file (e.g. shelxl.fcf) [or an .ins + .hkl]
for the analysis

An HKLF 5 type of file is written for SHELXL refinement.

Parameters:

Iobs-Icalc/sigma Crit: PAR(413) = 10.0

Theta Criterium: PAR(414) = 0.05

The analysis can be invoked in various ways. (The -T option bypasses the
alternative invocation from the PLATON menu).

- Given an '.fcf' run
**platon -T compound.fcf**

- Given an .ins & .hkl run
**platon -T compound.ins**

In this case, the missing calculated structure factors will be
calculated prior to the analysis for twinning.

Example:s103b.fcf.
Monoclinic, twinned about (1 0 0). Overlap in 0, 6 & 12 th l-layer.
PLATON Analysis Result:

2-Rotation about ( 1 0 0 )

Rotation Matrix:

1 0 0.8347

0 -1 0

0 0 -1

An HKLF 5 type SHELXL file is written. In order to proceed with
the SHELXL refinement:

- save the original
**.hkl** file

- copy file
**.hkp** to **.hkl**

- copy latest
**.res** to **.ins**

- add 'BASF 0.0' to the '.ins' instruction set.

- change 'HKLF' line into 'HKLF 5' (no transformation matrix !

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*12-Oct-2001 A.L.Spek*