Flack absolute structure parameter x:

(F_c²)* = (1-x) F_c²_hkl + x F_c²_-h-k-l

x = 0 <=> correct absolute structure

x = 1 <=> wrong absolute structure

Inversion of the structure:

MOVE  1  1  1  -1

exceptions for some space groups like Fdd2, I4₁ etc.

sometimes it is necessary to change also the space group, e.g. P3₁ => P3₂

0 < x < 1 <=> racemic twin or wrong absolute structure

TWIN -1  0  0    0  -1  0    0  0  -1    2

BASF  k2

H. D. Flack, Acta Crystallogr.A39, 876 (1983)