1,721,092 research outputs found
INTRINSIC DEFECT ENERGIES OF LITHIUM HYDRIDE AND LITHIUM DEUTERIDE CRYSTALS
No full textA theoretical study has been made of the defect structure of lithium hydride and lithium deuteride. A potential model is obtained describing the statics and dynamics of these crystals. Intrinsic defect energies are calculated using the Harwell HADES program which is based on a generalised Mott-Littleton method. The results are in good agreement with the experimental data, and suggest that the vacancy and interstitial migration mechanisms of anions and cations are all comparable in their contribution to ionic conduction
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
CRYSTAL and EMBED, two computational tools for the ab initio study of electronic properties of crystals
No full textSubstitutional boron and nitrogen pairs in diamond. A quantum mechanical vibrational analysis
No full textThe lattice structures, electronic characteristics and spectroscopic features of the three vicinal substitutional, spin zero defects, N s -N s (A-centre), B s -B s , and B s -N s in diamond are reported. They are derived from all-electron B3LYP calculations based on Gaussian basis sets, within a periodic supercell scheme, including both 64- and 216-atom cells. The local geometry reflects the differences between the strong C-C bond of the host lattice and the weaker impurity-impurity bonding in the defective systems. The band structures show two occupied bands 1.06 eV above the VBE for N s -N s , and empty bands 3.36 eV and 0.25 eV below the CBE for B s -B s and for B s -N s respectively. The IR spectra of B s -B s and N s -N s contain sharp peaks at 631 cm −1 , 692 cm −1 and 1373 cm −1 , and 451 cm −1 , 571 cm −1 and 1276 cm −1 , respectively, which might reasonably be considered as ‘finger prints’ for these systems. For B s -N s , a set of medium intensity absorptions are predicted at 890 cm −1 , 945 cm −1 , 1018 cm −1 , 1078 cm −1 and 1281 cm −1 . However, numerous peaks in this region have been predicted for other defective systems, so that the firm identification of B s -N s based on these frequencies is much less certain than B s -B s and N s -N s
The NV-⋯N+charged pair in diamond: A quantum-mechanical investigation
No full textThe NV-⋯N+ charged pair in diamond has been investigated by using a Gaussian-type basis set, the B3LYP functional, the supercell scheme and the CRYSTAL code. It turns out that: (i) when the distance between the two defects is larger than 6-7 Å, the properties of the double defect are the superposition of the properties of the individual defects. (ii) The energy required for the reaction NV0 + Ns → NV- + N+ is roughly -1.3 eV at about 12 Å, irrespective of the basis set and functional adopted, and remains negative at any larger distance. (iii) These results support the observation of a charge transfer mechanism through a Ns → NV0 donation occurring in the ground state, through a tunnelling process, without irradiation. (iv) The IR spectrum of the two subunits is characterized by specific peaks, that might be used as fingerprints. (v) Calculation of electrostatic interaction permitted an estimate of the effective charge of the defects
Large-scale B3LYP simulations of ibuprofen adsorbed in MCM-41 mesoporous silica as drug delivery system
No full textThe atomistic details of the interaction between ibuprofen (one of the most common nonsteroidal anti-inflammatory drugs) and a realistic model of MCM-41 (one of the most studied mesoporous silica materials for drug delivery) were elucidated by quantum mechanical modeling inclusive of London forces. Calculations are based on periodic density functional theory adopting all-electron Gaussian-type basis functions of polarized double-&ζ quality and the B3LYP hybrid functional. By docking the drug on different sites of the MCM-41 pore walls, we have sampled different local features of the potential energy surface of the drug-silica system, both for low and high loadings (one and seven drug molecules per unit cell, respectively). For all cases, ibuprofen adsorption in MCM-41 is exothermic (average ΔH = -99 kJ·mol-1) and exergonic (average ΔG = -33 kJ·mol-1), exclusively when London interactions are taken into account due to their dominant role in dictating all features of this system. The comparison between simulated IR and NMR spectra suggests that static disorder of the adsorbed ibuprofen due to surface sites heterogeneity can also be invoked together with the current interpretation based on a dynamic behavior of the adsorbed ibuprofen to interpret the spectral features. Analysis of H-bond patterns exhibited by the drug interacting with the MCM-41 surface silanol (SiOH) groups revealed the importance of cooperativity in the H-bond strength. The present work shows that large-scale all-electron full quantum mechanical simulations employing accurate hybrid functionals can soon become competitive over modeling studies based on molecular mechanics methods, both in terms of superior accuracy and absence of the problematic parametrization, due to organic/inorganic interface. (Figure Presented)
N2 positively charged defects in diamond. A quantum mechanical investigation of the structural, electronic, EPR and vibrational properties
No full textEPR data of the positively charged N2 defect (N+2 ) are reported from all-electron calculations based on the B3LYP hybrid functional using a local (Gaussian type) basis set, within a supercell scheme, as implemented in the CRYSTAL code. The Fermi contact Aiso term, the components of the hyperfine coupling tensor,
B, and the electric field gradient P (only P8 is available) are all in excellent agreement with two independent sets of experimental data, with errors of less than 4%. The equilibrium geometry, electronic structure, and IR and Raman spectra are also computed, where the two spectra are compared with those ++
of the neutral N2 and double positive N2 , both of which have closed shell ground states. In particular the
IR and Raman spectra of the three species exhibit important differences. Information on the charge, spin and geometry of defective species derived from experimental EPR data are based mostly on relatively simple models; here they are obtained directly from rigorous quantum mechanical simulations. It is shown that the extrapolation from the experimental EPR data can lead to relatively serious errors
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
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