checkCIF procedure

PLAT080

 Convergence of the refinement is proved with a close to zero shift/error   
 value for all refined parameters. Such a convergence is easily achieved
 with a few additional refinement cycles at little cost.
 NOTE: Some SHELXL-97 versions do not allow for negative Flack parameter
 values. Convergence in such a case may be never reached because the Flack
 parameter value is reset to zero.