International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. F, ch. 25.2, p. 736

In refining macromolecular structures, it is almost always necessary to supplement the diffraction data with chemical information in the form of restraints. A typical restraint is the condition that a bond length should approximate to a target value with a given estimated standard deviation; restraints are treated as extra experimental data items. Even if the crystal diffracts to 1.0 Å, there may well be poorly defined disordered regions for which restraints are essential to obtain a chemically sensible model (the same can be true of small molecules too!). SHELXL is generally not suitable for refinements at resolutions lower than about 2.5 Å because it cannot handle general potentialenergy functions, e.g. for torsion angles or hydrogen bonds; if noncrystallographic symmetry restraints can be employed, this limit can be relaxed a little.
For some purposes (e.g. riding hydrogen atoms, rigidgroup refinement, or occupancies of atoms in disordered side chains), constraints, exact conditions that lead to a reduction in the number of variable parameters, may be more appropriate than restraints; SHELXL allows such constraints and restraints to be mixed freely. Riding hydrogen atoms are defined such that the C—H vector remains constant in magnitude and direction, but the carbon atom is free to move; the same shifts are applied to both atoms, and both atoms contribute to the leastsquares derivative sums. This model may be combined with antibumping restraints that involve hydrogen atoms, which helps to avoid unfavourable sidechain conformations. SHELXL also provides, e.g., methyl groups that can rotate about their local threefold axes; the initial torsion angle may be found using a differenceelectrondensity synthesis calculated around the circle of possible hydrogenatom positions.