              The extinction coefficient used to calculate the correction\
               factor applied to the structure-factor data. The nature of the\
               extinction coefficient is given in the definitions of\
               _refine_ls_extinction_expression and\
               _refine_ls_extinction_method.\
\
               For the 'Zachariasen' method it is the r* value; for the\
               'Becker-Coppens type 1 isotropic' method it is the 'g' value,\
               and for 'Becker-Coppens type 2 isotropic' corrections it is\
               the 'rho' value. Note that the magnitude of these values is\
               usually of the order of 10000.\
\
               Ref:  Becker, P. J. \& Coppens, P. (1974). Acta Cryst. A30,\
                     129-153.\
                     Zachariasen, W. H. (1967). Acta Cryst. 23, 558-564.\
                     Larson, A. C. (1967). Acta Cryst. 23, 664-665.