1,721,050 research outputs found
All-electron magnetic response with pseudopotentials: NMR chemical shifts
No full textA theory for the ab initio calculation of all-electron NMR chemical shifts in insulators using pseudopotentials is presented. It is formulated for both finite and infinitely periodic systems and is based on an extension to the projector augmented-wave approach of Blochl [P. E. Blochl, Phys. Rev. B 50, 17 953 (1994)] and the method of Mauri et al. [F. Mauri, B. G. Pfrommer, and S. G. Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated for molecules by comparison with a selection of quantum chemical results, and in periodic systems by comparison with plane-wave all-electron results for diamond
Accurate first principles prediction of O-17 NMR parameters in SiO2: Assignment of the zeolite ferrierite spectrum
No full textO-17 NMR parameters, both the chemical shifts and the quadrupolar parameters, are calculated for SiO2 polymorphs using density functional theory with the generalized gradient-corrected PBE functional. The gauge including projector augmented wave (GIPAW) method (Pickard, C. J.; Mauri, F. Phys. Rev. B 2001, 63, 245101) ensures the reproduction of all electron results while using computationally efficient pseudopotentials. The use of plane-waves permits fully converged calculations to be performed on structures containing 144 atoms in the unit cell, without the need to resort to the cluster approximation. The calculated NMR parameters of cristobalite, quartz, coesite, and faujasite are in excellent agreement with experimental data. This demonstrates that density functional theory is able to reproduce with high accuracy the O-17 NMR parameters in SiO2 systems. This precision is used to assign the spectrum of the zeolite ferrierite. The data calculated for SiO2 are used to confirm that no simple correlation between the chemical shift and Cq NMR parameters and Si-O-Si angle exists, emphasizing the importance of predictive theories in this field
Atomic structure of icosahedral B4C boron carbide from a first principles analysis of NMR spectra
No full textDensity functional theory is demonstrated to reproduce the C-13 and B-11 NMR chemical shifts of icosahedral boron carbides with sufficient accuracy to extract previously unresolved structural information from experimental NMR spectra. B4C can be viewed as an arrangement of 3-atom linear chains and 12-atom icosahedra. According to our results, all the chains have a CBC structure. Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few percent have a B-12 structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
What bridges mineral platelets of bone?
Full text linkCommentary on: Davies E, Müller KH, Wong WC, Pickard CJ, Reid DG, Skepper JN, Duer MH. Citrate bridges between mineral platelets in bone. Proc Natl Acad Sci USA 2014;111:E1354–E1363
Theoretical investigation of moganite
No full textThe structure and properties of two different modifications of moganite have been studied using density functional theory, and the results have been compared to quartz. It is shown that the enthalpy difference between quartz and moganite, whose structure can be understood as Brazil twinning of quartz on a unit cell length scale, is negligible. This explains the significant amount of moganite in fine-grained quartz samples, as well as the frequent occurrence of Brazilian twinning in quartz. The compression mechanism of moganite has been elucidated, and it is argued that moganite is significantly more compressible than quartz. Observed and calculated NMR spectra are compared, and it is found that the bonding in quartz and moganite is very similar, consistent with the results of a Mulliken population analysis. The elastic stiffness coefficients of moganite have been predicted, and it is shown that formal-charge shell model interatomic potentials appear to be more transferable from quartz to moganite than partial-charge rigid ion equivalents. © 2005 E. Schweizerbart'sche Verlagsbuchhandlung
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
