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.

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).

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

  2. 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:

  1. save the original .hkl file

  2. copy file .hkp to .hkl

  3. copy latest .res to .ins

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

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


12-Oct-2001 A.L.Spek