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staphlee

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[资源] 一篇有关Ueq(displacement parameter,温度因子)的文献(作者建議不要稱它溫度因子)

一篇有关Ueq(atomic displacement parameter (ADP), or 温度因子)的文献
这篇主要讲到有关Ueq现在的命名合不合适...有很多计算....我把最主要总结的地方给copy出来给各位虫友....对计算有兴趣的自己去下载全文吧....其中在第6.的地方有讨论到不适合称这个term为温度因子...这也是作者的意见啦...还是满多人再用这个名词的...大家看看. ...参考一下.....

from Acta Cryst. (1996). A52, 770-781.
http://d.namipan.com/d/aa88c4255 ... e4ff079c2bbb6f71300

4. Recommendations
1. The term anisotropic displacement parameters (abbreviated
ADPs) should be used in referring to the
individual atomic coefficients in the exponent of the
factor that describes the effects of atomic motion and
static displacement.

2. The elements of the tensors U and/5 should always
be superscripted when the refinement is referred to a
crystal system rather than to a Cartesian system. This
definition follows from the definition of the elements
of U and /5 as contravariant tensor components (see
§2.1). The frequent use of subscripts for the ADPs, and
specifically for those not referred to Cartesian systems,
is inconsistent with their tensorial properties.

3. With the common Gaussian approximation, use
either the quantities U ij, which have dimension (length) 2,
defined in (25), or the dimensionless/3/j, defined in (22).

4. When the Gaussian approximation to the probability
density function is not deemed valid, the use of
the Gram-Charlier expansion of (62) is recommended,
although other formalisms may sometimes be advantageous
for special problems.

5. Standard uncertainties of ADPs obtained from a
full-matrix refinement are valid within the system in
which the refinement is made. If ADPs are transformed
to any other axial system, Cartesian or not, then the
uncertainties may also be calculated by transforming
the original variance-covariance matrix to this new
axial system and taking the square roots of its
diagonal elements, i.e. the variances. The required
variance-covariance matrix is usually not available
for ADPs taken from the literature. Hence, although
ADPs can still be transformed, their uncertainties cannot
be. Calculations involving published ADPs and their
(published) uncertainties should therefore be referred to
the same system of coordinates as the original refinement
in order to retain the significance of the published
uncertainties.

6. Avoid using the term 'temperature factor', both
because the phenomenon represented may not be due
entirely to thermal motion and because that phrase has
in the past been used in several quite distinct senses
(see §1.5).

7. Avoid using the Gaussian anisotropic parameters
that are now usually symbolized as B ij and are defined
in (26). These quantities are directly proportional to the
recommended U ij, the ratio being 87r 2.

8. Avoid using ADPs that do not represent matrix elements.
In some early references and computer programs,
cross terms were sometimes doubled in magnitude, being
represented, for example, as hk~ 12 instead of 2hk~ 12 for
programming convenience. This was possible because
the matrix representing the ADP is symmetric, with only
six independent terms. This practice is not found in
modern crystallographic software.

9. Published values of Ueq should always be accompanied
by their standard uncertainties. The ratio
of the minimum to the 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 and crystallographers
who maintain their own software are encouraged
to introduce the minor modifications that are required
for the implementation of these Recommendations.

[ Last edited by staphlee on 2009-7-13 at 09:14 ]

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