In this approach, one determines the absolute structure of a crystal in a Sohncke Space group.
The processes used to decide the absolute structure use the anomalous dispersion effect.
Light atoms usually show only a small anomalous dispersion effect.
In this case, the Flack parameter can refine to a physically unrealistic value (less than 0 or greater than 1) and has no meaning.
This parameter, introduced by H. D. Flack [1] became one of a standard set of values being checked for structures with noncentrosymmetric space groups.