By Peter Müller, Regine Herbst-Irmer, Anthony Spek, Thomas Schneider, Michael Sawaya
Crystal constitution Refinement is a mix of textbook and instructional. As A Crystallographers consultant to SHELXL it covers complicated features of sensible crystal constitution refinement, that have now not been a lot addressed through textbooks to this point. After an advent to SHELXL within the first bankruptcy, a quick survey of crystal constitution refinement is equipped. Chapters 3 and better tackle a number of the points of constitution refinement, from the remedy of hydrogen atoms to the project of atom varieties, to sickness, to non-crystallographic symmetry and twinning. One bankruptcy is devoted to the refinement of macromolecular constructions and brief chapters take care of constitution validation (one for small molecule buildings and one for macromolecules). In all of the chapters the publication provides refinement examples, in response to this system SHELXL, describing each challenge intimately. It comes with a CD-ROM with all records essential to reproduce the refinements.
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Additional info for Crystal Structure Refinement: A Crystallographer's Guide to SHELXL (International Union of Crystallography Texts on Crystallography)
Pdf ﬁle on the CD-ROM that accompanies this book, exhaustively elaborates on all restraints, the following pages brieﬂy describe the most common restraints as they are used by this program. 1 Geometrical restraints Besides a restraint on chiral volumes (CHIV) and a restraint for atoms that are supposed to lie on a common plane (FLAT),8 SHELXL has two kinds of distance restraints: direct and relative distance restraints. The former restrain distances to a given target value (DFIX, DANG), while the latter restrain equivalent distances to be equal (SADI, SAME).
1991). cif ﬁle. ACTA automatically sets the BOND, FMAP 2, PLAN and LIST 4 instructions and ACTA cannot be combined with other FMAP or LIST commands. lst ﬁle. 9 Reﬁnement problems There are many more or less difﬁcult problems a crystallographer can encounter when reﬁning a crystal structure. The most prominent problems are twinning, disorder, pseudo-symmetry and atom type ambiguities. A whole set of additional difﬁculties is related to the reﬁnement of protein structures. The following chapters are intended to address the most common problems in a way that can easily be understood by scientists who have basic crystallographic knowledge and a minimum of experience reﬁning simple crystal structures.
For macromolecular structures, there is one additional residual factor, the Rfree , introduced by Axel Brünger (1992) which provides a tool to detect overﬁtting (see also Chapters 10 and 11). 4 Parameters For every atom in the model that is located on a general position in the unit cell, there are three atomic coordinates and one or six atomic displacement parameters (one for isotropic, six for anisotropic models) to be reﬁned. 7) and possibly several additional scale factors, like the batch scale factors in the reﬁnement of twinned structures, the Flack-x parameter for non-centrosymmetric structures, one parameter for extinction, etc.