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R-factor (crystallography)

In crystallography, the R-factor (sometimes called residual factor or reliability factor) is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data. It is defined by the following equation:

where F is the so called structure factor and the sum extends over all the reflections measured and their calculated counterparts respectively.The structure factor F is closely related to the intensity of the reflection it describes:

The R factor usually ranges between 0.6 (when comparing a random set of reflections with a given model) and 0.2 (for example for a well refined macro-molecular model at a resolution of 2.5 Angstrom). When working on small molecules (up to 300 atoms), which form extremely well ordered crystals, it is possible to attain R-factors as low as 0.015. Crystallographers also use the Free R-Factor and the symmetric R-Factor to describe the quality of a model.

See also

• Structure factor
• Patterson function