RECOMMENDATIONS ON ADP NOMENCLATURE AND USAGE. Kenneth N. Trueblood, Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA

Modern x-ray and neutron diffraction techniques can give precise parameters that describe dynamic or static displacements of atoms in crystals. However, confusing and inconsistent terms and symbols for these quantities occur in the crystallographic literature. The recommendations of a subcommittee appointed by the IUCr Commission on Crystallographic Nomenclature concerning these quantities will be described. The report of this subcommittee is now under consideration by the IUCr Executive Committee. We anticipate that the entire report will be published in 1996, in Acta Cryst. A52.

The chief focus is on anisotropic atomic displacement parameters (ADPs) in the Gaussian approximation. The principal recommendations will be presented, with examples to illustrate their implications. These include:

-- With the common Gaussian approximation, use either the quantities U^ij, which have dimension (length)², or the dimensionless [[beta]]^ij (each of which will be defined). Avoid using those now usually symbolized as B^ij which are directly proportional to the U^ij, the ratio being 8[[pi]]².

-- Avoid using the term "temperature factor", because the phenomenon represented may not be due entirely to thermal motion and because that phrase has been used in several distinct senses.

-- Published values of U_eq should always be accompanied by their standard uncertainties. The ratio of the minimum to maximum eigenvalues of the corresponding anisotropic displacement tensors should also be published, either in the primary publication itself, or in the secondary (deposition) publication.

Authors of crystallographic software are encouraged to introduce the minor modifications needed to implement the subcommittee's recommendations.