The derivation of non-merohedral twin laws during refinement by analysis of poorly fitting intensity data and the refinement of non-merohedrally twinned crystal structures in the programCRYSTALS

2002 ◽  
Vol 35 (2) ◽  
pp. 168-174 ◽  
Author(s):  
Richard I. Cooper ◽  
Robert O. Gould ◽  
Simon Parsons ◽  
David J. Watkin
Keyword(s):  
Computer Program ◽  
Crystal Structures ◽  
Reciprocal Lattice ◽  
Careful Analysis ◽  
Intensity Data

Although non-merohedrally twinned crystal structures can normally be solved without difficulty, problems usually arise during refinement. Careful analysis of poorly fitting data reveals that they belong predominantly to certain distinct zones in which |Fo|2is systematically larger than |Fc|2. In the computer programROTAX, a set of data with the largest values of (|F_{o}^{\,2}| − |F_{c}^{\,2}|)/u(|F_{o}^{\,2}|) is identified and their indices transformed by rotations or roto-inversions about possible direct- and reciprocal-lattice directions. Matrices that transform the indices of the poorly fitting data to integers are identified as possible twin laws.

LAYER– a computer program for the graphic display of intensity data as simulated precession photographs

1999 ◽  
Vol 32 (2) ◽  
pp. 351-352 ◽  
Author(s):  
Leonard J. Barbour
Keyword(s):  
Computer Program ◽  
Diffraction Data ◽  
Reciprocal Lattice ◽  
Intensity Data ◽  
Graphic Display ◽  
Space Group ◽  
Microsoft Windows

LAYERis a 32-bit Microsoft Windows-based program that reads intensity data inSHELXtype 4 format and displays any level of the reciprocal-lattice net. The program is primarily intended for instructive purposes, but may also prove useful as a visual complement to statistically based numeric analysis of diffraction data for space-group determination.

scholarly journals SUPERFLIP– a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions

2007 ◽  
Vol 40 (4) ◽  
pp. 786-790 ◽  
Author(s):  
Lukáš Palatinus ◽  
Gervais Chapuis
Keyword(s):  
Computer Program ◽  
Crystal Structures ◽  
Diffraction Data ◽  
Periodic Structures ◽  
Powder Diffraction Data ◽  
X Ray ◽  
Modulated Structures ◽  
Structure Solution ◽  
Matching Procedure ◽  
Arbitrary Dimensions

SUPERFLIPis a computer program that can solve crystal structures from diffraction data using the recently developed charge-flipping algorithm. It can solve periodic structures, incommensurately modulated structures and quasicrystals from X-ray and neutron diffraction data. Structure solution from powder diffraction data is supported by combining the charge-flipping algorithm with a histogram-matching procedure.SUPERFLIPis written in Fortran90 and is distributed as a source code and as precompiled binaries. It has been successfully compiled and tested on a variety of operating systems.

Electron crystallography of zeolites. 3. Calcined MCM-22 and MCM-49, a case of subtle differences

2003 ◽  
Vol 218 (9) ◽  
Author(s):  
Douglas L. Dorset
Keyword(s):  
Electron Diffraction ◽  
Single Crystal ◽  
Crystal Structures ◽  
Direct Methods ◽  
Intensity Data ◽  
Electron Crystallography ◽  
Diffraction Intensity ◽  
Crystal Electron

AbstractSingle crystal electron diffraction intensity data, analyzed by direct methods for determining crystallographic phases, have been employed to seek differences between the crystal structures of calcined MCM-22 and MCM-49. A direct comparison of

Correction of integrated Bragg intensities for anisotropic thermal scattering

1978 ◽  
Vol 11 (3) ◽  
pp. 179-183 ◽  
Author(s):  
M. Merisalo ◽  
J. Kurittu
Keyword(s):  
Computer Program ◽  
Single Crystals ◽  
Crystal Structures ◽  
Phonon Scattering ◽  
Phonon Contribution ◽  
Tetragonal Crystal ◽  
Thermal Scattering ◽  
The One ◽  
Integrated Intensities ◽  
The Continuum

A readily applicable procedure is presented for calculating the contribution of one-phonon scattering (TDS) to measured integrated intensities for single crystals of any symmetry under conventional experimental situations. The anisotropy of the scattering is included correctly within the continuum elasticity approximation. A computer program is presented for the calculation of TDS contribution for cubic, tetragonal, orthorhombic, hexagonal and trigonal crystal structures. The procedure is applied to the calculation of the one-phonon contribution for the hexagonal and tetragonal crystal structures.

Searching the Cambridge Structural Database for the `best' representative of each unique polymorph

2006 ◽  
Vol 62 (4) ◽  
pp. 567-579 ◽  
Author(s):  
Jacco van de Streek
Keyword(s):  
Crystal Structure ◽  
Computer Program ◽  
Crystal Structures ◽  
Cambridge Structural Database ◽  
Structural Database

A computer program has been written that removes suspicious crystal structures from the Cambridge Structural Database and clusters the remaining crystal structures as polymorphs or redeterminations. For every set of redeterminations, one crystal structure is selected to be the best representative of that polymorph. The results, 243 355 well determined crystal structures grouped by unique polymorph, are presented and analysed.

scholarly journals Ambiguity of structure determination from a minimum of diffraction intensities

2014 ◽  
Vol 70 (4) ◽  
pp. 354-357 ◽  
Author(s):  
Ahmed Al-Asadi ◽  
Dimitri Leggas ◽  
Oleg V. Tsodikov
Keyword(s):  
Crystal Structures ◽  
Structure Determination ◽  
Three Dimensional ◽  
Minimum Data Set ◽  
Intensity Data ◽  
Data Set ◽  
One Dimensional ◽  
Minimum Data ◽  
Minimum Intensity ◽  
Dimensional Crystal

Although the ambiguity of the crystal structures determined directly from diffraction intensities has been historically recognized, it is not well understood in quantitative terms. Bernstein's theorem has recently been used to obtain the number of one-dimensional crystal structures of equal point atoms, given a minimum set of diffraction intensities. By a similar approach, the number of two- and three-dimensional crystal structures that can be determined from a minimum intensity data set is estimated herein. The ambiguity of structure determination from the algebraic minimum of data increases at least exponentially fast with the increasing structure size. Substituting lower-resolution intensities by higher-resolution ones in the minimum data set has little or no effect on this ambiguity if the number of such substitutions is relatively small.

The application of phase relationships to complex structures. III. The optimum use of phase relationships

1971 ◽  
Vol 27 (4) ◽  
pp. 368-376 ◽  
Author(s):  
G. Germain ◽  
P. Main ◽  
M. M. Woolfson
Keyword(s):  
Computer Program ◽  
Crystal Structures ◽  
Figure Of Merit ◽  
Complex Structures ◽  
Fourfold Increase ◽  
Phase Relationships ◽  
Strong Relationships

An improvement is described in the automatic procedure for solving crystal structures incorporated in the computer program LSAM. The development of signs from an initial set containing symbols is carried only as far as is necessary to establish strong relationships between the symbols. The information so gained is used in a fresh beginning of the symbolic-addition process. Some failure of relationships between symbols is allowed to give a multisolution method. A phase-permutation computer program for non-centrosymmetric structures, MULTAN, incorporates a weighted tangent formula. This is of the form {\rm tan}\varphi_{\bf h} = {{\sum_{\bf h'}w_{\bf h'},w_{\bf h-h'}|E_{\bf h'}E_{\bf h-h'}| \sin (\varphi_{\bf h'} + \varphi_{\bf h-h'})}\over{\sum_{\bf h'}w_{\bf h'},w_{\bf h-h'}|E_{\bf h'}E_{\bf h-h'}| \cos (\varphi_{\bf h'} + \varphi_{\bf h-h'})}} = {{T_{\bf h}}\over{B_{\bf h}}} and w_{\bf h} = {\rm tanh}\{ \sigma_3\sigma_2^{-3/2}|E_{\bf h}|(T_{\bf h}^2 + B_{\bf h}^2)^{1/2}\}.All phases are accepted as soon as they are found with the associated weight. This gives a fourfold increase in speed in development of the complete phase set. An absolute figure of merit is described to indicate probably correct phase sets for multisolution methods.

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