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.