From Online Dictionary of Crystallography
The Flack parameter is the molar fraction x in the defining equation , where C represents an oriented two-domain-structure crystal, twinned by inversion, consisting of an oriented domain structure X and an oriented inverted domain structure . In reciprocal space, the Flack parameter x is defined by the structure-amplitude equation
For a multidomain-structure twin of a chiral crystal structure, an equivalent Flack parameter may be calculated according to the method of Flack and Bernardinelli (1999).
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