1,721,254 research outputs found
First-principles calculation of vibrational Raman spectra in large systems: Signature of small rings in crystalline SiO2
No full textWe present an approach for the efficient calculation of vibrational Raman intensities in periodic systems within density functional theory. The Raman intensities are computed from the second order derivative of the electronic density matrix with respect to a uniform electric field. In contrast to previous approaches, the computational effort required by our method for the evaluation of the intensities is negligible compared to that required for the calculation of vibrational frequencies. As a first application, we study the signature of 3- and 4-membered rings in the Raman spectra of several polymorphs of SiO2, including a zeolite (H-ZSM-18) having 102 atoms per unit cell
Ab initio study of Be (1010) surface dynamical properties
No full textWe present a first-principles calculation of the dynamical properties of the Be (10 (1) over bar 0) surface, as obtained by state-of-the-art density functional perturbation theory. While calculations based on truncated bulk models do not agree with recent electron energy loss spectroscopy experimental measurements, failing to predict even the Rayleigh wave dispersion at zone boundaries, our fully ab initio calculation predicts the correct energies for these and other surface modes. Some of the measured resonances around the Brillouin zone center do not correspond to single localized surface modes but can be identified as due to a high surface density of vibrational states. (C) 2000 Elsevier Science B.V. All rights reserved
First-principles study of the thermal expansion of Be(10(1)over-bar0)
No full textWe present a first-principles study of the thermal expansion of Be(10 (1) over bar0) surface, within the quasiharmonic approximation (QHA). The maximum temperature studied (T=700 K) is well below melting and QHA is adequate in this regime. Many layers are involved in the thermal relaxation of this surface, and in order to apply QHA to this complex system we developed a method for the efficient computation of the third-order derivatives of the total energy in a metallic system using the density-functional perturbation theory . Computer codes for this method are made available on the web. We find that as the temperature increases the short interlayer spacings near the surface contract and the long interlayer spacings expand. The mechanism leading to this behavior is disclosed and compared with that for Mg(10 (1) over bar0) and Al(110) surfaces, where a similar effect was previously observed
Ab initio study of Be (0001) surface thermal expansion
No full textWe present a first-principles study, within the quasiharmonic approximation, of the thermal expansion of Be (0001) surface. The free energy is obtained from full vibrational dispersions computed by density-functional perturbation theory. Our calculation describes very well the thermal expansion in the bull; and has been checked against first-principles molecular dynamics simulations in the surface case. We do not fmd the large thermal expansion recently observed experimentally and we argue that the morphology of the actual surface could be less ideal than assumed
Ab-initio dynamical properties of the Be(0001) surface
No full textWe present a first-principles calculation of the dynamical properties of the Be(0001) surface, as obtained by state of the art density-functional perturbation theory. We calculate the dynamical properties of fully relaxed 12-layer Be slabs, and find significantly different results from those obtained using a bulk-truncated geometry. Our results are in excellent agreement with recent EELS experiments, and show that three layers are involved in the surface dynamics, (C) 1998 Elsevier Science B.V. All rights reserved
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
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