Real-space residual

The real-space residual, RSR, is a measure of the similarity between an electron-density map calculated directly from a structural model and one calculated from experimental data. It is calculated most often in the refinement of biological macromolecular structures.

[math]\textrm{RSR} = {{\sum | \rho_{obs} - \rho_{calc} |} \over {\sum | \rho_{obs} + \rho_{calc} |}}[/math]

([math]\rho[/math]'s are electron density values at grid points that cover the residue in question, obs and calc refer to experimental and model electron density, respectively.)

The measure of similarity is often provided in the form of a graph of RSR values against residue number, showing clearly which residues give best and worst agreement with the experimental electron-density map. For nucleic acid structures, RSR may also be calculated separately for base, sugar and phosphate moieties of the nucleic acid monomer. RSR is generally considered an excellent model-validation tool.