Anisotropic displacement parameters (ADPs) (see the discussion of terminology in subsection 1.5 and of symbols in subsection 2.1) have typically been determined and reported in the crystallographic literature in a variety of ways, symbolized sometimes as U (as in or ), sometimes as B (as in or ), and sometimes as (as in ). Unfortunately, these terms have not always been used consistently. Dimensionless quantities, e.g., the , have been misidentified as values, which are of dimension (length)2 . When these and related quantities were first determined more than four decades ago, they were considered at best to have qualitative significance, and inconsistencies, when noticed, were often disregarded. Cruickshank's (1956) papers marked a turning point toward quantitative interpretation in terms of simple physical models. Since then, technical improvements, both in the measurement of diffraction data and in computing, have led to increased physical significance in the experimentally determined ADPs. The fact that these quantities are now more meaningful, and the lack of uniformity with which they have been presented, have been discussed informally in recent years by more than one IUCr Commission, but no consensus has previously been reached on an acceptable form of presentation. This situation was brought recently to the attention of the Commission on Crystallographic Nomenclature. That body established this Subcommittee, charging it with examining the merits of adopting a uniform approach to reporting in structural papers quantities that describe atomic displacement, and with making such recommendations as might be deemed appropriate. Discussion within the Subcommittee, conducted principally through electronic correspondence, identified two major areas in which uniform definitions, terminology, and nomenclature would be desirable. These include anisotropic displacement parameters in the Gaussian approximation and in situations in which the Gaussian approximation is inadequate. In considering the uncertainties of experimentally determined ADPs, the reader is referred to the Report of a Working Group on the Expression of Uncertainty in Measurement (Schwarzenbach, et al., 1995), appointed by the IUCr Commission on Crystallographic Nomenclature, which discusses the general concept of uncertainty in the result of a crystallographic measurement, and its application.
The recommendations at the end of this report are proposed for use in all future publications of the International Union of Crystallography. Authors of structure refinement programs, particularly those in widespread use, are encouraged to bring their programs into full conformity with these recommendations.
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