Structure Refinement at Atomic Resolution.

X-Ray diffraction data at atomic resolution, i.e., beyond 1.2 Å, provide the most detailed and reliable information we have about the structure of macromolecules, which is especially important for validating new discoveries and resolving subtle issues of molecular mechanisms. Refinement at atomic resolution allows reliable interpretation of static disorder and solvent structure, as well as modeling of anisotropic atomic vibrations and even of H atoms. Stereochemical restraints can be relaxed or removed, providing unbiased information about macromolecular stereochemistry, which in turn can be used to define improved conformation-dependent libraries, and the surplus of data allows estimation of least-squares uncertainties in the derived parameters. At ultrahigh resolution it is possible to study charge density distribution by multipolar refinement of electrons in non-spherical orbitals.