5.11: R factor
The term R factor in crystallography is commonly taken to refer to the 'conventional' R factor
\(R =
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a measure of agreement between the amplitudes of the structure factors calculated from a crystallographic model and those from the original X-ray diffraction data. The R factor is calculated during each cycle of least-squares structure refinement to assess progress. The final R factor is one measure of model quality.
More generally, a variety of R factors may be determined to measure analogous residuals during least-squares optimization procedures. Where the refinement attempts to minimize the deviates of the squares of the structure factors (refinement against F 2 ), the R factor based on F 2 is used to monitor the progress of refinement:
\(R(F^2) =
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Likewise, refinement against I can be tracked using the Bragg R factor
\(R_B =
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Even for refinement against F 2 or I , the 'conventional' R factor may be calculated and quoted as a measure of model quality, in order to compare the resulting quality of models calculated at different times and with different refinement strategies.
The R factor is sometimes described as the discrepancy index .