A three-fold twin in B2₁

4/7: Detwin in SHELXL

Our plan is to use the 'good enough' model in space group P2₁ to detwin the dataset using 'LIST 8' in SHELXL. The header of the 'good enough' res file should look like this:

In the above, notice that the TWIN command now specifies three components. A BASF line, with two fractions for the 2nd and 3rd components is also included. The 'LIST 8' instruction tells SHELXL to write a detwinned fcf file that includes the following information for each reflection:

h, k, l, F_o², σ(F_o²), F_c², φ_c, d, σ_shelx

On running this refinement, the usual SHELXL output statistics show an R-value of about 9% (below). That's not so great, but for this exercise it should be fine.

The important stuff is in the file 9mj.fcf, the first few lines of which look like this:

The CIF-format junk in the header of this fcf file is useless to us, so remove it using a text editor. For the diffraction data, we only need the first five fields for each reflection, formatted as '3I4, 2F8.2' to read into XPREP. The conversion is trivial using a simple awk one liner ...

... which generates a detwinned hkl file:

This detwinned dataset is available here:


In part 5, we'll use this detwinned dataset to figure out a strategy to complete the structure.