Flack parameter

From Online Dictionary of Crystallography

Paramètre de Flack (Fr). Flack-Parameter (Ge). Parametro di Flack (It). フラック変数 (Ja). Parámetro de Flack (Sp).


The Flack parameter is the molar fraction [math]x[/math] in the defining equation [math]C=(1-x)X + x\bar X[/math], where [math]C[/math] represents an oriented two-domain-structure crystal, twinned by inversion, consisting of an oriented domain structure [math]X[/math] and an oriented inverted domain structure [math]\bar X[/math]. In reciprocal space, the Flack parameter [math]x[/math] is defined by the structure-amplitude equation

[math]G^2(h,k,l,x)=(1-x)|F(h,k,l)|^2 + x|F({\bar h}, {\bar k}, {\bar l})|^2[/math].

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).

See also

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.


  • Flack, H. D. and Bernardinelli, G. (1999). Acta Cryst. A55, 908–915. Absolute structure and absolute configuration
  • IUPAC (1996). Basic Terminology of Stereochemistry.