checkCIF procedure

PLAT987

 A BASF/TWIN refinement of the Flack x parameter is indicated when its value
 deviates significantly from zero as measured against its associated s.u.
 value. This applies in particular for structures containing significant
 anomalous scatterers. With a default SHELXL refinement, the Flack x parameter
 value is not taken into account in the refinement model as part of the
 calculation of F(calc) and the creation of the FCF file. Inclusion of a Flack
 x in the refinement model that deviates significantly from zero will lead to
 lower R-values and a better model.