R-factor (crystallography)
Encyclopedia
In crystallography
, the R-factor (sometimes called residual factor or reliability factor or the R-value or RWork) is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data. In other words, it is a measure of how well the refined structure predicts the observed 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:
For large molecules, 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 Ångström). Small molecules (up to 300 atoms) usually form more ordered crystals than large molecules, it is possible to attain lower R-factors. In the Cambridge Structural Database
more than 95% of the 500,000+ crystals have an R-factor lower than 0.15 and 9.5% have an R-factor lower than 0.03.
Crystallographers also use the Free R-Factor (
) to describe the quality of a model.
The quantities
and 
are similarly used to describe the internal agreement of measurements in a crystallographic data set.
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
, the R-factor (sometimes called residual factor or reliability factor or the R-value or RWork) is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data. In other words, it is a measure of how well the refined structure predicts the observed data. It is defined by the following equation:
where F is the so called structure factor
Structure factor
In condensed matter physics and crystallography, the static structure factor is a mathematical description of how a material scatters incident radiation...
and the sum extends over all the reflections measured and their calculated counterparts respectively. The structure factor
Structure factor
In condensed matter physics and crystallography, the static structure factor is a mathematical description of how a material scatters incident radiation...
F is closely related to the intensity of the reflection it describes:
For large molecules, 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 Ångström). Small molecules (up to 300 atoms) usually form more ordered crystals than large molecules, it is possible to attain lower R-factors. In the Cambridge Structural Database
Cambridge Structural Database
The Cambridge Structural Database , is a repository for small molecule crystal structures. Scientists use single-crystal x-ray crystallography to determine the crystal structure of a compound. Once the structure is solved, information about the structure is saved in a file and deposited in the CSD...
more than 95% of the 500,000+ crystals have an R-factor lower than 0.15 and 9.5% have an R-factor lower than 0.03.
Crystallographers also use the Free R-Factor (
) to describe the quality of a model.The quantities
and are similarly used to describe the internal agreement of measurements in a crystallographic data set.