WGHT a [0.1] b [0] c [0] d [0] e [0] f [.33333]The weighting scheme is defined as follows:
w = q / [ sigma2(Fo2) + (a*P)2 + b*P + d + e*sin(theta) ]
where P = [ f * Maximum of(0 or Fo2) + (1-f) * Fc2 ].
It is possible for the experimental Fo2 value to be negative because the background is higher than the peak; such negative values are replaced by 0 to avoid possibly dividing by a very small or even negative number in the expression for w.
For twinned and powder data, the Fc2 value used in the expression for P is the total calculated intensity obtained as a sum over all components.
q is 1 when c is zero, exp[c*(sin(theta)/lambda)2] when c is positive, and 1 - exp[c*(sin(theta)/lambda)2] when c is negative.
The use of P rather than (say) Fo2 reduces statistical bias (A.J.C. Wilson, Acta Cryst., A32 (1976) 994-996). The weighting scheme is NOT refined if a is negative (contrast SHELX-76). The parameters can be set by trial and error so that the variance shows no marked systematic trends with the magnitude of Fc2 or of resolution; the program suggests a suitable WGHT instruction after the analysis of variance. This scheme is chosen to give a flat analysis of variance in terms of Fc2, but does not take the resolution dependence into account. It is usually advisable to retain default weights (WGHT 0.1) until all atoms have been found, when the scheme suggested by the program can be used for the next refinement job by replacing the WGHT instruction (if any) by the one output by the program towards the end of the .res file. This procedure is adequate for most routine refinements.
It may be desirable to use a scheme which does not give a flat analysis of variance to emphasize particular features in the refinement; for example c = +10 or -10 would weight up data at higher 2-theta, e.g. to perform a 'high-angle' refinement (uncontaminated by hydrogen atoms which contribute little at higher diffraction angle) prior to a difference electron density synthesis (FMAP 2) to locate the hydrogens. The exponential weights which are obtained when c is positive were advocated by J.D. Dunitz and P. Seiler, Acta Cryst., B29 (1973) 589-595. Weighting up the high angle reflections will in general give X-ray atomic coordinates which are closer to those from neutron diffraction.
Refinement against F2 requires different weights to refinement against F; in particular, making all the weights equal ('unit weights'), although useful in the initial stages of refinement against F, is NEVER a sensible option for F2. If the program suspects that an unsuitable WGHT instruction has been accidentally retained for a structure which had been refined previously with SHELX-76 or the XLS program in the Siemens SHELXTL system, it will output a warning message.
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