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.