1,720,957 research outputs found
DFT-LDA Pseudopotentials in Quantum Monte Carlo
No full textWe investigate the portability of standard norm-conserving pseudopotentials outside the density functional theory-local density approximation (DFT-LDA) framework, i.e., their use and interpretation as electron-ion effective potentials in valence-only diffusion Monte Carlo simulations. While first-principles many-body pseudopotentials are not available in the literature yet, the use of approximate pseudopotentials in quantum Monte Carlo simulations is becoming widespread. Here we attempt a systematic analysis of the portability of norm-conserving pseudopotentials generated within DFT-LDA, focusing on a model many-body system, the two-electron valence-only ion. Our results indicate that the portability is good in most cases, hence the use of pseudopotentials in quantum Monte Carlo simulations is in general a reasonable approximation but suggest that in some cases this approximation may be relevant
Variational Monte Carlo on a parallel architecture: an application to graphite
No full textWe present the parallelization strategy adopted to perform Variational Quantum Monte Carlo calculations on solids on distributed-memory architectures and the issues involved in the development of the parallel programs. Results obtained by the calculation of total electronic energy of graphite using nonlocal pseudopotentials in conjunction with the Variational Monte Carlo approach will be shown
Pseudopotential Portability in the QMC Framework
No full textWe have calculated the binding energies of several two-electron pseudoions using the Diffusion Quantum Monte Carlo method. The comparison between our results and the experiment suggests that HSC pseudopotentials are portable from the original single particle theory (DFT-LDA) to the “exact” many body one. Moreover we are able to evaluate the degree of portability of each effective field
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
Relaxations at GaN (1010) and (110) Surfaces
No full textWe present an ab-initio calculation of GaN wurtzite (1010) and zinc-blende (110) surface structures and formation energies. Our method employs ultrasoft pseudopotentials and plane-wave basis. These features enable us to obtain accurate results using small energy cut-off and large supercells. The (110) surface shows a Ga-N surface dimer rotation of ∼ 14°, i.e. about one half that of the ordinary III–V non-nitride compounds, and a 5% contraction of the surface bond-length (more than the double that occurring in GaAs). For the (1010) surface, a layer rotation angle of about 11° and a bond-length contraction of 6% has been found. Zinc-blende GaAs (110) and wurtzite ZnO (1010) surfaces have been studied as well, for the sake of comparison. GaAs results are in good agreement with the experimental findings. For ZnO a large bond contraction and a rotation angle of around 11% result. Thus, our findings place GaN closer in behaviour to the highly ionic II–VI compounds than to the non-nitride III–V semiconductors
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
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