1.45: Flack parameter
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).