A description of special aspects of the weighting scheme used
in least-squares refinement. Used to describe the weighting
when the value of _refine.ls_weighting_scheme is specified
as 'calc'.
Sigdel model of Konnert-Hendrickson:
Sigdel =
Afsig + Bfsig*(sin(theta)/lambda-1/6)
Afsig = 22.0, Bfsig = 150.0
at the beginning of refinement.
Afsig = 16.0, Bfsig = 60.0
at the end of refinement.