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Flack parameter reporting enquiry

  • To: Coredmg <coredmg@iucr.org>
  • Subject: Flack parameter reporting enquiry
  • From: Mike Hoyland via coreDMG <coredmg@iucr.org>
  • Date: Tue, 21 Jul 2020 12:00:43 +0100
  • Cc: Mike Hoyland <mh@iucr.org>
  • Organization: IUCr
Dear All,
We have had the following correspondence regarding the reporting of Flack parameters which we thought would be good to be discussed in this forum. It was sent to us by David Watkin.
===========================================================================We are updating the cif-generator in CRYSTALS and are faced with a problem when reporting the Flack parameter.
The current dictionary terms seem to be:

Following the publication of the Parsons, Flack & Wagner paper(Simon Parsons et al.  Intensity quotients and absolute structure refinement Acta Cryst. (2013). B69, 249–259) there has been a drift, encouraged by recent releases of SHELXL, to determine the Flack parameter by a post refinement method.  Though this method does involve least squares, it is not part of the structure refinement.
We think that, if the cif is to reflect the actual analysis, some additional (optional) terms need defining - something along the lines of:
_flack_parameter_post-refinement_value      value, su
The value determined by any of the post-refinement method (e.g. Parsons Quotient, Sheldrick Hole-in-One, Bijvoet Difference, Hooft 'y')
_flack_parameter_post-refinement_details;reference etc to actual method used;
_flack_parameter_in_model       value (no su)
The actual value used in computing structure factors, defaulting to zero.  This is needed because by default SHELXL does a Parsons Quotient calculation at the end but sticks with a value of zero unless the user opts to do a BASF/TWIN refinement (in which case the Flack parameter appears as the twin-fraction).
We believe that_refine_ls_abs_structure_Flack  and/or_flack_parameter_post-refinement_value
should be in the cif because they provide the evidence for the reported absolute structure, even though the value used in the model may be different from the determined value. I would give both values because significant differences are indications that there is an issue with the observed data.
Example, where the user or program decides the sample is enantiopure:
_refine_ls_abs_structure_Flack         0.2(4)_refine_ls_abs_structure_details;Refinement together with other structural parameters, Flack(1983);_flack_parameter_post-refinement_value 0.0(7)_flack_parameter_post-refinement_details;0Parsons Quotient, Parsons, Flack & Wagner, (2013);_flack_parameter_in_model  0.0
Take care reading SHELX output - GMS now calls his old Hole-in-One the 'Classical Method', while I would think Flack's 1983 paper really gives the original 'Classical' method.
If you have to Self Isolate in the coming weeks, this paper may help pass the time:Cooper & Watkin, Acta Cryst. (2016). B72, 661-683All is explained therein.======================================================================================
With thanks and best wishes,Mike HoylandSystems DeveloperIUCr, Chester==================================

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