What does TLS refinement mean?
The theory behind the TLS parameterisation has been presented in detail by Schomaker and Trueblood (Schomaker and Trueblood, 1968) [3], with useful summaries in Howlin et al. (1989) [2] and Schomaker and Trueblood (1998) [4]. TLS parameters describe the possible mean square displacements of rigid bodies. In this context, the rigid bodies are groups of atoms in your protein model. How many groups and the make-up of each group must be chosen by the user. TLS parameters describe anisotropic motion – an anisotropic U factor can be derived for each atom in a TLS group. But these U factors are correlated by virtue of belonging to the same rigid body, and only 20 refinement parameters are required for each TLS group. Thus, refinement of TLS parameters is a method of including anisotropic displacements without requiring the large number of parameters of full anisotropic refinement. TLS refinement can therefore be used at moderate resolution, e.g. 2.0Å. The number of extra parameters depends on
