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.
Recommendations on the nomenclature of thermal diffuse scattering
[e.g., Willis (1993)] and disorder diffuse scattering [e.g.,
Jagodzinski & Frey (1993)] were considered as outside the scope of this
report.