

Cover illustration: Polar mode GM6 (left) and antiferrodistortive mode M31q (right) present in the monoclinic structure of SrAl_{2}O_{4}, together with a graph of maximal subgroups showing their isotropy subgroups. The amplitude of mode GM6 is much larger, and ab initio calculations confirm its soft character, while distortion M31q is a hard mode frozen as an induced secondary distortion [PerezMato et al. (2010). Acta Cryst. A66, 558590]. The mode figures were drawn using FullProf Studio [RodriguezCarvajal (1993). Physica B, 192, 5569]. 
Acta Cryst. (2011). A67, 18 [ doi:10.1107/S0108767310039140 ] Geometric properties of nucleic acids with potential for autobuildingT. Gruene and G. M. SheldrickSynopsis: Algorithms and geometrical properties are described for the automated building of nucleic acids in experimental electron density. Online 11 November 2010 
Acta Cryst. (2011). A67, 920 [ doi:10.1107/S0108767310041437 ] Ab initio reconstruction of difference densities by charge flippingL. Palatinus, F. Fleischer, P. Pattison, T. Weber and W. SteurerSynopsis: The chargeflipping algorithm in its `bandflipping' variant is used to reconstruct density difference between a superstructure and average structure from superstructure reflections only. This technique can be used for the structure solution of superstructures, some types of twins and intergrowths of polytypes as well as for the solution of modulated structures and superstructures of quasicrystals. Online 11 November 2010 
Acta Cryst. (2011). A67, 2134 [ doi:10.1107/S010876731004287X ] Practical applications of averages and differences of Friedel oppositesH. D. Flack, M. Sadki, A. L. Thompson and D. J. WatkinSynopsis: The practical use of Friedeldifference intensities for spacegroup determination, leastsquares refinement and structure validation is described. Online 7 December 2010 
Acta Cryst. (2011). A67, 3544 [ doi:10.1107/S0108767310042431 ] Coincidence lattices in the hyperbolic planeM. A. RodríguezAndrade, G. AragónGonzález, J. L. Aragón and A. GómezRodríguezSynopsis: Expressions for coincidence indices and bases for coincidence lattices in the hyperbolic plane are obtained using Clifford algebra. Online 11 November 2010 
Acta Cryst. (2011). A67, 4555 [ doi:10.1107/S0108767310042297 ] Generation of (3 + d)dimensional superspace groups for describing the symmetry of modulated crystalline structuresH. T. Stokes, B. J. Campbell and S. van SmaalenSynopsis: A complete table of (3 + 1)D, (3 + 2)D and (3 + 3)D superspace groups has been enumerated that corrects omissions and duplicate entries in previous tables of superspace groups and Bravais classes. A new online data repository provides detailed information about each superspace group. Online 18 November 2010 
Acta Cryst. (2011). A67, 5662 [ doi:10.1107/S0108767310044892 ] PVMR: assembling small helix fragments as structural solutions for molecular replacementF. Jiang and W. DingSynopsis: A new realspace implementation of the molecularreplacement method is described, in which the search model vectors are directly matched with the Patterson self and cross vectors. This method enables the searching for small helix fragments in the target crystal and is able to produce fragmentassembled solutions that are structurally homologous to the target structure. Online 7 December 2010 
Acta Cryst. (2011). A67, 6367 [ doi:10.1107/S0108767310047823 ] Pattersonfunction direct methods for structure determination of organic compounds from powder diffraction data. XVIJ. RiusSynopsis: A new type of direct methods are presented that directly explore the Patterson instead of the modulus function. Since they work with the experimental intensities, they are particularly well suited for handling powder diffraction data of organic compounds. Online 7 December 2010 
Acta Cryst. (2011). A67, 6886 [ doi:10.1107/S0108767310042832 ] Euclidian embeddings of periodic nets: definition of a topologically induced complete set of geometric descriptors for crystal structuresJ.G. EonSynopsis: Given the topology, the lattice together with the set of distance vectors from each atom to the centre of gravity of its neighbours determine the geometry of a crystal structure. A special construction provides the lattice type. Online 15 December 2010 
Acta Cryst. (2011). A67, 8792 [ doi:10.1107/S0108767310047513 ] Isomorphism of lattices of subgroups of the layer and rod groups with sublattices of subgroups of the space groupsJ. Fuksa and V. KopskýSynopsis: It is shown that lattices of subgroups of reducible space groups contain copies of lattices of subgroups of layer and rod groups as sublattices. Online 15 December 2010 
Acta Cryst. (2011). A67, 9395 [ doi:10.1107/S0108767310044296 ] On the allowed values for the triclinic unitcell anglesJ. Foadi and G. EvansSynopsis: The possible range of angles for a triclinic unit cell are explored in detail. Online 25 November 2010 
Acta Cryst. (2011). A67, 96124 [ doi:10.1107/S0108767310031089 ] Report of the Executive Committee for 2009Online 30 November 2010 
Acta Cryst. (2011). A67, 125129 [ doi:10.1107/S0108767310045940 ] Notes for authors 2011Online 15 December 2010 
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