Acta Crystallographica Section A

Items from 1 to 20 out of 122 results

article

An alternative method for the calculation of joint probability distributions. Application to the expectation of the triplet invariant

J. Brosius

Acta Crystallographica Section A > 71 > 1 > 76 - 81

This paper presents a completely new method for the calculation of expectations (and thus joint probability distributions) of structure factors or phase invariants. As an example, a first approximation of the expectation of the triplet invariant (up to a constant) is given and a complex number is obtained. Instead of considering the atomic vector positions or reciprocal vectors as the fundamental...

article

Scanning of two‐dimensional space groups

Daniel B. Litvin

Acta Crystallographica Section A > 71 > 1 > 111 - 113

Tables of the scanning of two‐dimensional space groups are presented to determine the frieze‐group symmetry of lines that transect two‐dimensional crystals. It is shown how these tables can be used to predict the (001) projection symmetries of migration‐related segments of coincidence site lattice tilt boundaries with [001] tilt axis.

article

SHELXT– Integrated space‐group and crystal‐structure determination

George M. Sheldrick

Acta Crystallographica Section A > 71 > 1 > 3 - 8

The new computer program SHELXT employs a novel dual‐space algorithm to solve the phase problem for single‐crystal reflection data expanded to the space group P1. Missing data are taken into account and the resolution extended if necessary. All space groups in the specified Laue group are tested to find which are consistent with the P1 phases. After applying the resulting origin shifts and space‐group...

article

Diffuse multiple scattering

A. G. A. Nisbet, G. Beutier, F. Fabrizi, B. Moser, more

Acta Crystallographica Section A > 71 > 1 > 20 - 25

A new form of diffraction lines has been identified, similar to Rutherford, Kikuchi and Kossel lines. This paper highlights some of the properties of these lines and shows how they can be used to eliminate the need for sample/source matching in Lonsdale's triple convergent line method in lattice‐parameter determination.

article

Nuclear‐weighted X‐ray maximum entropy method – NXMEM

Sebastian Christensen, Niels Bindzus, Mogens Christensen, Bo Brummerstedt Iversen

Acta Crystallographica Section A > 71 > 1 > 9 - 19

Subtle structural features such as disorder and anharmonic motion may be accurately characterized from nuclear density distributions (NDDs). As a viable alternative to neutron diffraction, this paper introduces a new approach named the nuclear‐weighted X‐ray maximum entropy method (NXMEM) for reconstructing pseudo NDDs. It calculates an electron‐weighted nuclear density distribution (eNDD), exploiting...

article

Symmetry groups associated with tilings on a flat torus

Mark L. Loyola, Ma. Louise Antonette N. De Las Peñas, Grace M. Estrada, Eko Budi Santoso

Acta Crystallographica Section A > 71 > 1 > 99 - 110

This work investigates symmetry and color symmetry properties of Kepler, Heesch and Laves tilings embedded on a flat torus and their geometric realizations as tilings on a round torus in Euclidean 3‐space. The symmetry group of the tiling on the round torus is determined by analyzing relevant symmetries of the planar tiling that are transformed to axial symmetries of the three‐dimensional tiling....

article

From direct‐space discrepancy functions to crystallographic least squares

Carmelo Giacovazzo

Acta Crystallographica Section A > 71 > 1 > 36 - 45

Crystallographic least squares are a fundamental tool for crystal structure analysis. In this paper their properties are derived from functions estimating the degree of similarity between two electron‐density maps. The new approach leads also to modifications of the standard least‐squares procedures, potentially able to improve their efficiency. The role of the scaling factor between observed and...

article

Celebrating the past, looking to the future

Simon J. L. Billinge, Jianwei Miao

Acta Crystallographica Section A > 71 > 1 > 1 - 2

article

Report of the Executive Committee for 2013

M. Dacombe

Acta Crystallographica Section A > 71 > 1 > 114 - 140

The report of the Executive Committee for 2013 is presented.

article

High‐symmetry embeddings of interpenetrating periodic nets. Essential rings and patterns of catenation

Charlotte Bonneau, Michael O'Keeffe

Acta Crystallographica Section A > 71 > 1 > 82 - 91

Symmetrical embeddings are given for multiply intergrown sets of some commonly occurring nets such as dia (diamond), qtz (quartz), pcu (net of primitive cubic lattice) and srs (labyrinth net of the G minimal surface). Data are also given for all known pairs of nets which have edge‐transitive self‐dual tilings. Examples are given for symmetrical polycatenation of the 2‐periodic nets sql (square lattice)...

article

Analysis of rapidly synthesized guest‐filled porous complexes with synchrotron radiation: practical guidelines for the crystalline sponge method

Timothy R. Ramadhar, Shao‐Liang Zheng, Yu‐Sheng Chen, Jon Clardy

Acta Crystallographica Section A > 71 > 1 > 46 - 58

A detailed set of synthetic and crystallographic guidelines for the crystalline sponge method based upon the analysis of expediently synthesized crystal sponges using third‐generation synchrotron radiation are reported. The procedure for the synthesis of the zinc‐based metal–organic framework used in initial crystal sponge reports has been modified to yield competent crystals in 3 days instead of...

article

Prediction of molecular crystal structures by a crystallographic QM/MM model with full space‐group symmetry

Philipp Mörschel, Martin U. Schmidt

Acta Crystallographica Section A > 71 > 1 > 26 - 35

A crystallographic quantum‐mechanical/molecular‐mechanical model (c‐QM/MM model) with full space‐group symmetry has been developed for molecular crystals. The lattice energy was calculated by quantum‐mechanical methods for short‐range interactions and force‐field methods for long‐range interactions. The quantum‐mechanical calculations covered the interactions within the molecule and the interactions...

article

The anatomy of a comprehensive constrained, restrained refinement program for the modern computing environment –Olex2 dissected

Luc J. Bourhis, Oleg V. Dolomanov, Richard J. Gildea, Judith A. K. Howard, more

Acta Crystallographica Section A > 71 > 1 > 59 - 75

This paper describes the mathematical basis for olex2.refine, the new refinement engine which is integrated within the Olex2 program. Precise and clear equations are provided for every computation performed by this engine, including structure factors and their derivatives, constraints, restraints and twinning; a general overview is also given of the different components of the engine and their relation...

article

Direct phasing of protein crystals with high solvent content

Hongxing He, Wu‐Pei Su

Acta Crystallographica Section A > 71 > 1 > 92 - 98

An iterative transform method is proposed for solving the phase problem in protein crystallography. In each iteration, a weighted average electron‐density map is constructed to define an estimated protein mask. Solvent flattening is then imposed through the hybrid input–output algorithm [Fienup (1982). Appl. Opt.21, 2758–2769]. Starting from random initial phases, after thousands of iterations the...

article

Symmetry of Crystals and Molecules. By Mark Ladd. Oxford University Press, 2014. Pp. xxi + 433. Price GBP 55.00 (hardback). ISBN 978‐0‐19‐967088‐8.

Massimo Nespolo

Acta Crystallographica Section A > 71 > 2 > 250 - 252

article

Computational analysis of thermal‐motion effects on the topological properties of the electron density

J. Robert Michael, Tibor Koritsanszky

Acta Crystallographica Section A > 71 > 2 > 225 - 234

The distributions of bond topological properties (BTPs) of the electron density upon thermal vibrations of the nuclei are computationally examined to estimate different statistical figures, especially uncertainties, of these properties. The statistical analysis is based on a large ensemble of BTPs of the electron densities for thermally perturbed nuclear geometries of the formamide molecule. Each...

article

Statistical tests against systematic errors in data sets based on the equality of residual means and variances from control samples: theory and applications

Julian Henn, Kathrin Meindl

Acta Crystallographica Section A > 71 > 2 > 203 - 211

Statistical tests are applied for the detection of systematic errors in data sets from least‐squares refinements or other residual‐based reconstruction processes. Samples of the residuals of the data are tested against the hypothesis that they belong to the same distribution. For this it is necessary that they show the same mean values and variances within the limits given by statistical fluctuations...

article

Group‐theoretical analysis of aperiodic tilings from projections of higher‐dimensional lattices B_n

Mehmet Koca, Nazife Ozdes Koca, Ramazan Koc

Acta Crystallographica Section A > 71 > 2 > 175 - 185

A group‐theoretical discussion on the hypercubic lattice described by the affine Coxeter–Weyl group W_a(B_n) is presented. When the lattice is projected onto the Coxeter plane it is noted that the maximal dihedral subgroup D_h of W(B_n) with h = 2n representing the Coxeter number describes the h‐fold symmetric aperiodic tilings. Higher‐dimensional cubic lattices are explicitly constructed for n = 4, 5,...

article

Hans Wondratschek (1925–2014)

Theo Hahn

Acta Crystallographica Section A > 71 > 2 > 253 - 254

article

Mathematical aspects of molecular replacement. III. Properties of space groups preferred by proteins in the Protein Data Bank

G. Chirikjian, S. Sajjadi, D. Toptygin, Y. Yan

Acta Crystallographica Section A > 71 > 2 > 186 - 194

The main goal of molecular replacement in macromolecular crystallography is to find the appropriate rigid‐body transformations that situate identical copies of model proteins in the crystallographic unit cell. The search for such transformations can be thought of as taking place in the coset space Γ\G where Γ is the Sohncke group of the macromolecular crystal and G is the continuous group of rigid‐body...

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Keywords

BOOK REVIEW (10)
X‐RAY DIFFRACTION (9)
SYNCHROTRON RADIATION (6)
SYMMETRY (5)
CRYSTALLOGRAPHY (4)
DYNAMICAL THEORY (4)
OBITUARY (4)
PAIR DISTRIBUTION FUNCTION (4)
BIOMACROMOLECULES (3)
CHARGE DENSITY (3)
DIRECT METHODS (3)
PHASE PROBLEM (3)
XFELS (3)
X‐RAY CRYSTALLOGRAPHY (3)
X‐RAY FREE‐ELECTRON LASERS (3)
AB INITIO PHASING (2)
APERIODIC STRUCTURES (2)
BRAVAIS LATTICES (2)
CATENATION (2)
COLOR SYMMETRY (2)
COMPUTER SIMULATIONS (2)
CRYSTAL TOPOLOGY (2)
DENSITY FUNCTIONAL THEORY (2)
DIFFRACTION (2)
DISPERSION SURFACE (2)
DYNAMICAL DIFFRACTION (2)
EDITORIAL (2)
ELECTRON DIFFRACTION (2)
FERROELECTRICS (2)
HIGHER‐DIMENSIONAL SPACE (2)
INTERNATIONAL TABLES FOR CRYSTALLOGRAPHY (2)
ITERATIVE PROJECTION ALGORITHMS (2)
LABELLED QUOTIENT GRAPHS (2)
LEAST SQUARES (2)
MATHEMATICAL CRYSTALLOGRAPHY (2)
MICROED (2)
MOLECULAR SYMMETRY (2)
NANOCRYSTALS (2)
NANOTUBES (2)
ORBITS (2)
PERFECT COLORING (2)
PERIODIC NETS (2)
PHASE DETERMINATION (2)
PHASE TRANSITIONS (2)
POWDER DIFFRACTION (2)
PROTON DISORDER (2)
QUASICRYSTALS (2)
REFINEMENT (2)
RIEMANN SURFACE (2)
SECTION TOPOGRAPHY (2)
SINGLE CRYSTALS (2)
STRUCTURE DETERMINATION (2)
STRUCTURE SOLUTION (2)
TWINNING (2)
AB INITIO CALCULATIONS (1)
AB INITIO SOLUTION (1)
ABSOLUTE STRUCTURE (1)
ABSORPTION ANISOTROPY (1)
AFFINE NORMALIZERS (1)
AKAIKE INFORMATION CRITERION (1)
ALGEBRAIC GEOMETRY, AMBIGUITY (1)
ALGORITHM (1)
ALICE GRACE JENNY BRAGG (1)
AMORPHOUS MATERIALS (1)
ANION ORDER (1)
ANISOTROPIC DISPLACEMENT TENSOR (1)
ANTISYMMETRY (1)
APATITES (1)
APERIODIC TILINGS (1)
ARAGONITE (1)
ARISTOTYPES (1)
ATOM AND ORBITAL ORDERS (1)
AVERAGE UNIT CELL (1)
AVERAGE UNIT CELL CONCEPT (1)
AXIAL POINT GROUPS (1)
BA8GA16SN30 (1)
BAYESIAN MODEL (1)
BEAM DEFLECTION (1)
BENT PERFECT CRYSTALS (1)
BIEBERBACH GROUPS (1)
BIOCRYSTALLOGRAPHY (1)
BIOLOGICAL SPECIMENS (1)
BIVARIANT LATTICE COMPLEXES (1)
BLIND TEST (1)
BOND TOPOLOGICAL PROPERTIES (1)
BORROMEAN NETS (1)
BRAGG CASE (1)
BUILDING BLOCKS (1)
CARBAMAZEPINE (1)
CDI (1)
CELL REDUCTION (1)
CHARGE‐DENSITY MAPS (1)
CHIRAL INDEXES (1)
CHIRAL NANOTUBES (1)
CHIRALITY (1)
CLATHRATE HYDRATES (1)
CLATHRATES (1)
COHERENCE (1)
COHERENT DIFFRACTION IMAGING (1)
COHERENT X‐RAY DIFFRACTION IMAGING (1)
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INFONA - science communication portal

Acta Crystallographica Section A

An alternative method for the calculation of joint probability distributions. Application to the expectation of the triplet invariant

Scanning of two‐dimensional space groups

SHELXT– Integrated space‐group and crystal‐structure determination

Diffuse multiple scattering

Nuclear‐weighted X‐ray maximum entropy method – NXMEM

Symmetry groups associated with tilings on a flat torus

From direct‐space discrepancy functions to crystallographic least squares

Celebrating the past, looking to the future

Report of the Executive Committee for 2013

High‐symmetry embeddings of interpenetrating periodic nets. Essential rings and patterns of catenation

Analysis of rapidly synthesized guest‐filled porous complexes with synchrotron radiation: practical guidelines for the crystalline sponge method

Prediction of molecular crystal structures by a crystallographic QM/MM model with full space‐group symmetry

The anatomy of a comprehensive constrained, restrained refinement program for the modern computing environment –Olex2 dissected

Direct phasing of protein crystals with high solvent content

Symmetry of Crystals and Molecules. By Mark Ladd. Oxford University Press, 2014. Pp. xxi + 433. Price GBP 55.00 (hardback). ISBN 978‐0‐19‐967088‐8.

Computational analysis of thermal‐motion effects on the topological properties of the electron density

Statistical tests against systematic errors in data sets based on the equality of residual means and variances from control samples: theory and applications

Group‐theoretical analysis of aperiodic tilings from projections of higher‐dimensional lattices B_n

Hans Wondratschek (1925–2014)

Mathematical aspects of molecular replacement. III. Properties of space groups preferred by proteins in the Protein Data Bank

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