EXTI x [0]An extinction parameter x is refined by least-squares, where Fc is multiplied by:
k [ 1 + 0.001 * x * Fc2 * lambda3 / sin(2theta) ]-1/4
where k is the overall scale factor. Note that it has been necessary to change this expression from SHELX-76 (which used an even cruder approximation) and SHELXTL (which used 0.002 instead of 0.001*lambda3). The wavelength dependence is needed for HKLF 2 (Laue) data. The program will print a warning if extinction (or SWAT - see below) may be worth refining, but it is not normally advisable to introduce it until all the non-hydrogen atoms have been found. For twinned and powder data, the Fc2 value used in the above expression is based on the total calculated intensity summed over all components rather than the individual contributions, which would be easier to justify theoretically (but makes little difference in practice). For the analysis of variance and '.fcf' output file, the Fo2 values are brought onto the absolute scale of Fc2 by dividing them by the scale factor(s) and the extinction factor. The above expression for the extinction is empirical and represents a compromise to cover both primary and secondary extinction; it has been shown to work well in practice but does not appear to correspond exactly to any of the expressions discussed in the literature. The article by A.C. Larson in Crystallographic Computing (1970), Ed. F.R. Ahmed, Munksgaard, Copenhagen, pp. 291-294 comes closest and should be consulted for further information.
Back to TWIN
Back to Reflection Data Input and Massaging
Back to Table of Contents