# Flack parameter

### From Online Dictionary of Crystallography

Paramètre de Flack (*Fr*); Parametro di Flack (*It*); フラック媒介変数 (*Ja*).

## Definition

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).

## 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*.