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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossman and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 18.1, p. 369
Section 18.1.3. Objectives
a
San Diego Supercomputer Center 0505, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0505, USA, and bStructural, Analytical and Medicinal Chemistry, Pharmacia & Upjohn, Inc., Kalamazoo, MI 49001-0119, USA |
A variety of methods are employed to improve the agreement between observed and calculated macromolecular diffraction patterns. Some of the more popular methods are discussed in the different sections of this chapter. In part, the different methods arise from focusing on different goals during different stages of model refinement. Bias generated by incomplete models, and radius of convergence, are important considerations at early stages of refinement, because the models are usually incomplete, contain significant errors in atom parameters and may carry errors from misinterpretation of poorly phased electron-density maps. During this stage of the process, the primary concern is to determine how the model of the chain tracing and conformation of the residues should be described. In later stages, after the description of the model has been determined, the objective is to determine accurate estimates of the values of the parameters which best explain the observed data. These two stages of the problem have different properties and should be treated differently.
