# Flack parameter

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## Definition

The Flack parameter is the molar fraction x in the defining equation $C=(1-x)X + x\bar X$, 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 $\bar X$. In reciprocal space, the Flack parameter x is defined by the structure-amplitude equation

G2(h,k,l,x) = (1 − x) | F(h,k,l) | 2 + x | F( − h, − k, − l) | 2.

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).

## Related chemical terms

The IUPAC Basic Terminology of Stereochemistry contains definitions of related chemical terms of use to the crystallographer, viz: absolute configuration, chiral, chirality, chirality sense, enantiomer, enantiomerically pure, enantiomorph, racemate, racemic, racemic conglomerate and relative configuration.