# 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).

## 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

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

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).