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,
129-47, 148-153.
Zachariasen, W. H. (1967). Acta Cryst. 23, 558-564.
Larson, A. C. (1967). Acta Cryst. 23, 664-665.