Reliability factor, R-factor

In structure analysis using an X-ray beam or an electron beam, Reliability factor (R-factor) is an index that expresses the degree of reliability for the structure obtained from an experimental structure analysis result. When the crystal structure factor obtained from an experimental diffraction pattern is expressed as F_obs (hkl), and the crystal structure factor calculated from the assumed structure model is expressed as F_cal (hkl), the R-factor is defined by the following equation.

Here, (hkl) is an index of Bragg reflection (Mirror index) and the sum of the right side of the equation is taken for all of the experimentally measured reflections.
In the structure analysis, so as to minimize the R-factor, the parameters (symmetry, lattice constant, atomic coordinate, etc.) of the crystal structure (structure model) are determined. When the experimentally-obtained crystal structure factor coincides fully with the crystal structure factor calculated from the determined structure, the R-factor becomes 0 (zero). When the crystal structure which provides the highest reproducibility of the experimental value is obtained, the value of the R-factor is about 0.05 for X-ray crystallography and is about 0.2 for electron crystallography.

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