ISSN:
1600-5724
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
The probability density of phase angles for structures with one or more atoms on known positions [Sim (1959). Acta Cryst. 12, 813-818] is expressed in terms of normalized difference-structure factors, and used to calculate standard deviations of phases and expectation values for amplitudes (|Er|) of the normalized difference-structure factors. Numerical results are tabulated for various values of |E1| and |E2|, i.e. the minimum and maximum |Er| value a given reflection can have. Applications to the DIRDIF procedures [van den Hark, Prick & Beurskens (1976). Acta Cryst. A32, 816-821] are described. New applications are the calculation of the a priori probability density function for |Er| values, and a statistical rriethod for the detection of a centre of symmetry in the remaining part of the structure. For the statistical method, a centricity parameter, Xc, which is unity for centric and zero for acentric distributions, is defined; for the difference structure, Xc is calculated by an iterative procedure, extrapolating towards zero contribution of the known part of the structure. Numerical results for 13 test structures are given.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0567739479001753
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