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Principles Calculations (principle + calculation)
Kinds of Principles Calculations Selected AbstractsStructural Stability of Alkali Boron Tetrahydrides ABH4 (A: Li, Na, K, Rb, Cs) from First Principle Calculation.CHEMINFORM, Issue 15 2005P. Vajeeston No abstract is available for this article. [source] Using First Principles Calculations to Identify New Destabilized Metal Hydride Reactions for Reversible Hydrogen Storage.CHEMINFORM, Issue 25 2007Sudhakar V. Alapati Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Transients from initial conditions in cosmological simulationsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006Martín Crocce ABSTRACT We study the impact of setting initial conditions in numerical simulations using the standard procedure based on the Zel'dovich approximation (ZA). As it is well known from the perturbation theory, ZA initial conditions have incorrect second- and higher-order growth and therefore excite long-lived transients in the evolution of the statistical properties of density and velocity fields. We also study the improvement brought by using more accurate initial conditions based on second-order Lagrangian perturbation theory (2LPT). We show that 2LPT initial conditions reduce transients significantly and thus are much more appropriate for numerical simulations devoted to precision cosmology. Using controlled numerical experiments with ZA and 2LPT initial conditions, we show that simulations started at redshift zi= 49 using the ZA underestimate the power spectrum in the non-linear regime by about 2, 4 and 8 per cent at z= 0, 1, and 3, respectively, whereas the mass function of dark matter haloes is underestimated by 5 per cent at m= 1015 M, h,1 (z= 0) and 10 per cent at m= 2 × 1014 M, h,1 (z= 1). The clustering of haloes is also affected to the few per cent level at z= 0. These systematics effects are typically larger than statistical uncertainties in recent mass function and power spectrum fitting formulae extracted from numerical simulations. At large scales, the measured transients in higher-order correlations can be understood from first principle calculations based on perturbation theory. [source] Multiscale simulation of polycrystal mechanics of textured ,-Ti alloys using ab initio and crystal-based finite element methodsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2008D. Ma Abstract Crystal-based finite element methods (FEM) are versatile continuum approaches for predicting mechanical properties and deformation-induced crystallographic textures. They can be applied to both, elastic,plastic and elastic problems. The methodology is based on (i) a detailed understanding of the underlying crystal deformation mechanisms and (ii) a number of constitutive material parameters that are often difficult to measure. First principle calculations, that take into account the discrete nature of matter at the atomic scale, are an alternative way to study mechanical properties of single crystals without using empirical parameters. In this study we demonstrate how to combine these two well-established modeling tools, viz., ab initio modeling and crystal mechanical FEM, for an improved approach to design of polycrystalline materials. The combination is based on (i) the determination of basic thermodynamic and elastic parameter trends in metallurgical alloy design using density-functional (DFT) calculations (P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 (1964), W. Kohn and L. J. Sham, Phys. Rev. 140, A1133 (1965) [1, 2], respectively) and (ii) the up-scale transfer of these results into crystal-based finite element simulations which take into account the anisotropic nature of the elastic,plastic deformation of metals. The method is applied to three body-centered cubic (bcc, ,) Ti,Nb alloys for bio-medical applications. The study addresses two technological processes, namely, the prediction of texture evolution during cold rolling (elastic-plastic problem) and elastic bending of textured polycrystals (elastic problem). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] "Some like it shallower" , p-type doping in SiCPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2003Peter Deák Abstract The usual p-type dopants of SiC, B, and Al, do not produce really shallow levels. In fact, boron can give rise to a secondary very deep acceptor level as well. The picture is complicated by hydrogen which is readily incorporated during in-growth doping into p-type material and can passivate both dopants but to a different degree. First principle calculations are reported regarding the interaction of hydrogen with B and Al in SiC. The results explain why hydrogen is incorporated in much higher amounts into B-doped than into Al-doped samples, and also reveal the influences of hydrogen on boron to produce the shallower acceptor. It will be shown that hydrogen incorporation during growth does not influence Al. Finally an Al,N,Al complex is proposed as a shallower acceptor in SiC. [source] First principles calculation of isolated intermediate bands formation in a transition metal-doped chalcopyrite-type semiconductorPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2006P. Palacios Abstract Density Functional Theory (DFT) calculations at the GGA level have been carried out for Ti-substituted chalcopyrite-type CuGaS2, as it might constitute an intermediate band material of the kind that has been proposed to lead to enhanced efficiency photovoltaic cells. According to these calculations an intermediate band appears when Ti substitutes Ga at a 25% level in this structure, resulting in a magnetic half-metallic compound. This intermediate band slightly overlaps the conduction band and, when a higher accuracy calculation approach like the introduction of a Hubbard-type empirical correction is used (GGA + U method), it splits leaving a filled narrow band, well isolated inside the band gap. Considering the nanocrystalline form in which these chalcopyrite-type compounds are used in solar cells, an assessment of the effects of a small crystal size in this system have been carried out with a slab model. In this calculation a decreased bandgap width is observed, which can be as a result of surface termination effects. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First principles calculation of optical and electronic properties with inclusion of exciton effectsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2006Sohrab Ismail-Beigi The cover picture of this issue of physica status solidi (c) has been taken from the article [1]. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Interaction of CO and NO with the spinel CuCr2O4 (100) surface: A DFT studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2008Xiang-Lan Xu Abstract The characteristics of CO and NO molecules at Cu2+ and Cr3+ ion sites on the CuCr2O4 (100) surface have been studied by first principles calculations based on spin-polarized density functional theory (DFT). The calculated results show that adsorption energies for X-down(C, N) adsorption vary in the order: Cu2+ -CO>Cr3+ -NO,Cr3+ -CO>Cu2+ -NO. CO molecules are preferentially adsorbed at Cu sites, whereas NO molecules adsorb favorably at Cu2+ and Cr3+ ion sites. The C-O and N-O stretching frequencies are red-shifted upon adsorption. Combining the analysis of frontier molecular orbitals and Mulliken charge, for CO and NO X-down adsorption systems, the 5, orbitals donate electrons and the 2,* orbitals obtain back-donated electrons. Although for NO with O-down adsorption systems, the NO-2,* orbitals obtain back-donated electrons from substrates without 5,-donation. Coadsorption calculations show the CO/NO mixture adsorb selectively at the Cu2+ion site but simultaneously at the Cr3+ ion site, respectively. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Periodic models in quantum chemical simulations of F centers in crystalline metal oxidesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2007Yuri F. Zhukovskii Abstract We present a survey of recent first principles simulations of the neutral oxygen vacancies (F centers) existing as native or radiation-induced point defects in various crystalline metal oxides in different forms (bulk, bare substrate surface, and on the interface with metal adsorbates). We mainly consider periodic models in calculations of point defects using the metal oxide supercell or cyclic clusters. We compare different formalisms of first principles calculations, mostly the Density Functional Theory (DFT) as implemented in the framework of either localized basis set of atomic orbitals or delocalized basis sets of plane waves. We analyze in detail the structural and electronic properties of F centers in binary oxides of light metals (MgO and Al2O3), and ternary metal oxides (SrTiO3, BaTiO3, PbTiO3, KNbO3, and PbZrO3 perovskites). When available, we compare results of ab initio periodic defect calculations with experimental data, results of the first principles cluster calculations (both embedded and molecular) as well as with semi-empirical calculations. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Ab initio study of spinodal decomposition in (Zn, Cr)TePHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2006T. Fukushima Abstract The spinodal decomposition in (Zn, Cr)Te is simulated by using first principles calculations and Monte Carlo simulation. It is found that the chemical pair interaction between Cr atoms in (Zn, Cr)Te is attractive interaction and leads to spinodal decomposition. Curie temperatures in decomposed situation are estimated by the random phase approximation with taking the magnetic percolation effect into account. This decomposed phase makes the random pattern of high concentration regions which connect each other and have possibility to realize high Curie temperature. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Vibrational properties of four consecutive carbon picotubesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2008Nils Rosenkranz Abstract Four different picotubes have been synthesized up to now. These highly symmetric, ringlike hydrocarbons are closely related to the smallest armchair nanotubes available. We investigate vibrational properties of the different picotubes using first principles calculations and polarization-dependent Raman spectroscopy. Our results emphasize the strong relationship between picotubes and nanotubes. In this context, two features in the picotube spectra are of particular interest: The high-energy bands and breathing-like modes. Vibrational patterns calculated for the high-energy modes of picotubes are similar to the axial and transversal high-energy vibrations of armchair nanotubes. Concerning the radial breathing modes of picotubes, our calculations suggest a d,1 -dependence of the radial breathing mode (RBM) frequency, which is again in analogy to nanotubes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Calculated high pressure crystal structure transformations for phosphorusPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003Rajeev Ahuja Abstract In this paper we have studied the structural stability of the sp bonded element, P, by means of the first principles calculations. The theoretical calculations made use of a full potential linear muffin-tin orbital (FPLMTO) method adopting the local density approximation to the density functional theory. We reproduce the observed crystallographic phase stability of P as a function of compression. Our results confirm the recent experimental finding of Akahama et al. We have also proposed a new structure for an experimentally reported unidentified intermediate phase in between simple cubic and simple hexagonal phase. This new structure is similar to what has been observed for Si. We have explained the stability of different phases under pressure using our calculated density of states (DOS). [source] Thermal conductance of the AlN/Si and AlN/SiC interfaces calculated with taking into account the detailed phonon spectra of the materials and the interface conditionsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2010M. Kazan Abstract We present a calculation of the thermal conductance (TC) of the interface between aluminium nitride (AlN) and silicon (Si) and that between AlN and silicon carbide (SiC) with taking into account the detailed phonon spectra of the materials, as obtained from first principles calculations, and the interface conditions. On the basis of the results obtained, we discuss the relation between the interface TC, the interface conditions, and the mismatches between the acoustic waves velocities and the phonon densities of states of the materials in contact. Our calculation method is expected to provide a reliable tool for thermal management strategy, independently from the substrate choice (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First principles study of the decomposition processes of AlN in a hydrogen atmospherePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008U. Panyukova Abstract The growth of AlN films by the hydrogen vapor phase epitaxy method is generally carried out at a high temperature over a hydrogen atmosphere. The difficulties concerned with the decomposition processes on the surface during the film growth result in necessity of computer modelling of that processes. First principles calculations of the decomposition processes of AlN in a hydrogen atmosphere are reported. The mechanism of desorption of atoms from the surface was determined. Al atoms desorb as AlH from (0001) surface and as Al from (000-1) surface of AlN. And N atoms desorb as NH3 from (0001) surface and as NH from (000-1) surface of AlN. The desorption of Al atoms is a rate limiting reaction. The calculation results correspond well with the experimental date published earlier. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Theoretical reassessment of Whelk-O1 as an enantioselective receptor for 1-(4-halogeno-phenyl)-1-ethylamine derivativesCHIRALITY, Issue S1 2004Alberto Del Rio Abstract A combination of molecular mechanics and first principles calculations was used to explore the enantioselectivity of receptors, taking into account experimental data from the CHIRBASE database. Interactions between the Whelk-O1 HPLC chiral stationary phase with the complete series of 1-(4-halogeno-phenyl)-1-ethylamine derivative racemates were studied. The objective was to extract information from the interactions between the chiral Whelk-O1 stationary phase and the enantiomers, hence probing the origin of the enantioselective behavior. Calculations correctly reproduce the elution orders and reasonably describe the experimental enantioselectivities and retention factors. Different binding modes were observed for the first eluted enantiomer complexes, whereas the second eluted show only one prevalent diastereomeric binding fashion. Natural bond orbital (NBO) analysis was used on the global minima bound-complexes to quantify donor-acceptor interactions among chiral stationary phase and ligand moieties. Intermolecular hydrogen bonding was found to be the essential energetic interaction for all systems studied. CH-,, aromatic stacking and various charge transfer interactions were found to be smaller in magnitude but still important for the global enantioselective behavior. The three-point interaction model is discussed, pointing out the difficulty of its application for the qualitative prediction of elution orders (absolute configurations). Chirality 16:S1,S11, 2004. © 2004 Wiley-Liss, Inc. [source] | |||||||||||||