Why direct and post-refinement determinations of absolute structure may give different results

What is it about?

In recent years there has been unease at the observation that the well defined theory behind the Flack parameter could give different results when applied to a single set of observed data, depending upon how the theory was applied. This paper explains how these differences arise, and shows that when the differences are significant, they act as a powerful diagnostic for other issues with the structural analysis.

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Why is it important?

The determination of the absolute structure of crystalline materials, and hence the absolute configuration of crystalline molecular materials has become common place, especially in connection with pharmaceutical materials. It has long been observed that direct refinement of the Flack parameter often yields an unexpectedly large standard uncertainty. More recently post-refinement methods for determining the same parameter yield uncertainties inline with expectations. When the two methods give significantly different results, it is quite likely that there are other problems with the analysis which must be addressed and resolved or explained.