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Why direct and post-refinement determinations of absolute structure may give different results.
David John Watkin1, Richard Ian Cooper1
1Chemical Crystallography Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford, Oxfordshire OX1 3TA, England.
Summary
Directly refining the Flack parameter (x) differs from post-refinement methods. Discrepancies suggest data or model issues, with post-refinement methods using normal probability plots being most robust for absolute structure determination.
Area of Science:
- Crystallography
- Chemical structure determination
Background:
- The Flack parameter (x) is crucial for determining absolute structure in crystallography.
- Direct refinement and post-refinement methods for estimating the Flack parameter can yield different results.
Purpose of the Study:
- To investigate the causes of discrepancies between direct and post-refinement Flack parameter determinations.
- To evaluate the influence of weighting schemes on Flack parameter estimation.
Main Methods:
- Analysis of 28 crystallographic data sets from the literature.
- Comparison of Flack parameter values obtained through direct refinement versus various post-refinement approaches.
- Assessment of the impact of different weighting schemes on parameter uncertainties.
Main Results:
- Directly refined Flack parameters are generally insensitive to weighting scheme changes, unlike post-refinement estimates.
- Post-refinement estimates are highly sensitive to the choice of weights.
- Disagreements between methods often indicate underlying data or structural model problems.
Conclusions:
- Post-refinement methods utilizing normal probability plots offer robust absolute structure estimation.
- Discrepancies between direct and post-refinement Flack parameter values signal potential data quality issues.
- Reporting both direct and post-refinement Flack parameters is recommended for comprehensive analysis.
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