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111.
F. N. Chukhovskii A. M. Poliakov 《Acta Crystallographica. Section A, Foundations and Advances》2003,59(1):48-53
Direct‐method formalism to determine atomic structures using electron diffraction data is here aimed at a general solution of the phase‐retrieval problem, consequently combining electron diffraction (ED) and high‐resolution transmission electron microscopy (HRTEM) patterns in a `domino' fashion. While there are similarities to conventional (kinematical) direct methods, there remain major differences; in particular, owing to the dynamical effects in the data, the ED structure factors prove to be complex and then the positivity of the reconstructed electron density is no longer a valid constraint for `dynamical' direct methods. Besides, owing to the dynamical effects, heavy atoms no longer dominantly contribute to the HRTEM images. Thus, the `dynamical' direct‐methods concept is based on the phase‐retrieval algorithm utilizing both the dynamical ED and the HRTEM data. The fusion of the traditional direct‐method technique, which is described here, allows realization of a full‐phase restoration of complex structure factors. A numerical example, using the dynamical ED and HRTEM data for (Ga,In)2SnO5 ceramic, shows that the method is capable of yielding a unique phase‐retrieval solution. The clear sense is that the domino transform algorithm proposed works well and represents a valuable method for phasing diffraction patterns in electron structural crystallography using an experiment that is readily performed when the ED and HRTEM data are collected. 相似文献
112.
Ma&#x;gorzata Rabiej 《Journal of Applied Crystallography》2014,47(5):1502-1511
Decomposition of wide‐angle X‐ray diffraction curves into crystalline peaks and amorphous components is one of the most difficult nonlinear optimization problems. For this reason, the elaboration of a reliable method that provides fast unambiguous solutions remains an important and topical task. This work presents a hybrid system dedicated to this aim, combining two methods of artificial intelligence – evolution strategies and an immune algorithm – with the classical method of Rosenbrock. A combination of the mechanisms of these three methods has given a very effective and convergent algorithm that performs very well a multicriterial optimization. Tests have shown that it is faster to converge and less ambiguous than the genetic algorithm. 相似文献
113.
P. Juhs L. Granlund S. R. Gujarathi P. M. Duxbury S. J. L. Billinge 《Journal of Applied Crystallography》2010,43(3):623-629
An extension of the Liga algorithm for structure solution from atomic pair distribution functions (PDFs), to handle periodic crystal structures with multiple elements in the unit cell, is described. The procedure is performed in three separate steps. First, pair distances are extracted from the experimental PDF. In the second step the Liga algorithm is used to find unit‐cell sites consistent with these pair distances. Finally, the atom species are assigned over the cell sites by minimizing the overlap of their empirical atomic radii. The procedure has been demonstrated on synchrotron X‐ray PDF data from 16 test samples. The structure solution was successful for 14 samples, including cases with enlarged supercells. The algorithm success rate and the reasons for the failed cases are discussed, together with enhancements that should improve its convergence and usability. 相似文献
114.
Min Cheng Daoyuan Kang Xiaoli Zhang Huawei Yin Yuhong Wang 《Crystal Research and Technology》2015,50(4):331-337
Comparing with the traditional concentric rotation method (rotation radius is 0 cm), the effects of different rotation radii on the growth rate of KDP crystals were studied by experimental methods. It was found that with the increase of rotation radius from 0 cm, the growth rate of each direction of crystals first increased and then decreased in a size‐unchanged vessel. The smaller the distance between the crystal and vessel wall, the less the growth rate. This phenomenon was named the “wall collision effect”. Also, the value of growth rate reached a maximum when the rotation radius was about half of its allowable largest value in the size‐unchanged vessel. In addition, an increase of the rotation radius could improve the crystal growth rate under the same linear velocity of crystal movement. Finally, the uniformity of crystal growth has also been analyzed compared with the concentric rotation radius. It was found that the uniformity of crystal growth was best when the rotation radius was half of its allowable maximum value, and it was more conducive to the actual application of KDP crystals. 相似文献
115.
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 mask evolves into the correct shape and the phases converge to the correct values with an average error of 30–40° for high‐resolution data for several protein crystals with high solvent content. With the use of non‐crystallographic symmetry, the method could potentially be extended to phase protein crystals with less than 50% solvent fraction. The new phasing algorithm can supplement and enhance the traditional refinement tools. 相似文献
116.
Thomas Weber Arndt Simon Hansjürgen Mattausch Lorenz Kienle Oliver Oeckler 《Acta Crystallographica. Section A, Foundations and Advances》2008,64(6):641-653
Complex disorder and corresponding diffuse scattering from La0.70(1)(Al0.14(1)I0.86(1)) was taken as basis for investigating the reliability, reproducibility and influence of the refinement parameters of evolutionary algorithm refinements of Monte Carlo simulations. Using the same diffuse‐scattering data set, a model relying on reasonable a priori knowledge about the real structure was used, as well as one that includes no presumptions except the average structure and the chemical composition. To strengthen the complementary character of the approaches, different evolutionary algorithms (`differential' and `cooperative' evolution) were employed. It was found that the resulting structures are qualitatively and quantitatively in good agreement independent of the strategy used. It is shown that the method of population averaging (applicable only in differential evolution refinements) allows reasonable estimates about uncertainties of structure parameters if proper refinement parameters are chosen, which makes differential evolution the method of choice for quantitative refinements. Recommendations for the best choices of the parameters are given. The disordered structure of La0.70(1)(Al0.14(1)I0.86(1)) contains clusters including differently interconnected La6Al units. The modes of interconnection and local distortions are discussed in detail. 相似文献
117.
Yao Yao & Xiao-Xia Guo 《数学研究》2015,48(1):53-65
When the matrices $A$ and $Q$ have special structure, the structure-preservingalgorithm was used to compute the stabilizing solution of the complex matrix equation $X+A^TX^{-1}A=Q.$ In this paper, we study the numerical methods to solve the complexsymmetric stabilizing solution of the general matrix equation $X+A^TX^{-1}A=Q.$ Wenot only establish the global convergence for the methods under an assumption, butalso show the feasibility and effectiveness of them by numerical experiments. 相似文献
118.
Jean-Frédéric Gerbeau;Marina Vidrascu 《Mathematical Modelling and Numerical Analysis》2003,37(4):631-647
We propose a quasi-Newton algorithm for solving fluid-structure interaction problems. The basic idea of the method is to build an approximate tangent operator which is cost effective and which takes into account the so-called added mass effect . Various test cases show that the method allows a significant reduction of the computational effort compared to relaxed fixed point algorithms. We present 2D and 3D fluid-structure simulations performed either with a simple 1D structure model or with shells in large displacements.https://doi.org/10.1051/m2an:2003049 相似文献
119.
We analyze a least-squares asymmetric radial basis function
collocation method for solving the modified Helmholtz equations. In
the theoretical part, we proved the convergence of the proposed
method providing that the collocation points are sufficiently dense.
For numerical verification, direct solver and a subspace selection
process for the trial space (the so-called adaptive greedy
algorithm) is employed, respectively, for small and large scale
problems. 相似文献
120.
Ghahraman Solookinejad Amir Sayid Hassan RozatianMohammad Hossein Habibi 《Applied Surface Science》2011,258(1):260-264
Zinc oxide (ZnO) thin film was fabricated by sol-gel spin coating method on glass substrate. X-ray reflectivity (XRR) and its optimization have been used for characterization and extracting physical parameters of the film. Genetic algorithm (GA) has been applied for this optimization process. The model independent information was needed to establish data analyzing process for X-ray reflectivity before optimization process. Independent information was exploited from Fourier transform of Fresnel reflectivity normalized X-ray reflectivity. This Fourier transformation (Auto Correlation Function) yields thickness of each coated layer on substrate. This information is a keynote for constructing optimization process. Specular X-ray reflectivity optimization yields structural parameters such as thickness, roughness of surface and interface and electron density profile of the film. Acceptable agreement exists between results obtained from Fourier transformation and X-ray reflectivity fitting. 相似文献