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Gotthard Seifert - One of the best experts on this subject based on the ideXlab platform.
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stretching and breaking of monolayer mos 2 an atomistic simulation
2D Materials, 2014Co-Authors: Tommy Lorenz, Janole Joswig, Gotthard SeifertAbstract:We report on the simulation of the nanoindentation process of monolayer MoS2 using molecular-dynamics simulations and a density-functional based Tight-Binding Method. A circular sheet of MoS2 with clamped boundaries was indented by a slowly moved tip, which deformed and finally pierced the layer. We found the Young’s modulus of monolayer MoS2 to be 262 GPa, which is in good agreement with experimental observations. Furthermore, the energetic and structural behavior during the indentation process was analyzed. Elasticity theory supplies the necessary equations to explain the experiment. Thereby, the nature of the linear term in the force-deflection relation is discussed.
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defect induced conductivity anisotropy in mos 2 monolayers
Physical Review B, 2013Co-Authors: Mahdi Ghorbaniasl, Andrey N Enyashin, Gotthard Seifert, Agnieszka Kuc, Thomas HeineAbstract:Various types of defects in MoS${}_{2}$ monolayers and their influence on the electronic structure and transport properties have been studied using the density-functional-based Tight-Binding Method in conjunction with the Green's function approach. Intrinsic defects in MoS${}_{2}$ monolayers significantly affect their electronic properties. Even at low concentration they considerably alter the quantum conductance. While the electron transport is practically isotropic in pristine MoS${}_{2}$, strong anisotropy is observed in the presence of defects. Localized midgap states are observed in semiconducting MoS${}_{2}$ that do not contribute to the conductivity but direction-dependent scatter the current, and that the conductivity is strongly reduced across line defects and selected grain boundary models.
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theoretical study of the mechanical behavior of individual tis2 and mos2 nanotubes
Journal of Physical Chemistry C, 2012Co-Authors: Tommy Lorenz, Janole Joswig, David Teich, Gotthard SeifertAbstract:Similar to carbon, several transition-metal chalcogenides are able to form tubular structures. Here, we present results from systematic theoretical investigations of structural and mechanical properties of MoS2 and TiS2 nanotubes in comparison to each other, to carbon nanotubes, and to corresponding experimental results. We have obtained the nanotube’s Young’s moduli (Y), Poisson ratios (ν), and shear moduli (G) as functions of diameter and chirality, using a density-functional-based Tight-Binding Method. Additionally, we have simulated tensile tests by Born–Oppenheimer molecular dynamics simulations. The influence of structural defects on the investigated mechanical properties has been studied as well. As a result of the simulated stretching experiments, we found that TiS2 nanotubes can be stretched only half as much as MoS2 nanotubes.
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structural electronic and mechanical properties of single walled halloysite nanotube models
Journal of Physical Chemistry C, 2010Co-Authors: Luciana Guimaraes, Gotthard Seifert, Andrey N Enyashin, Hélio A. DuarteAbstract:Halloysite is a clay mineral with stoichiometry Al2Si2O5(OH)4·nH2O that can grow into long tubules and is chemically similar to kaolinite. In this work we present a systematic study on the stability, electronic, and mechanical properties of zigzag and armchair single-walled halloysite nanotubes by means of the self-consistent charge density-functional Tight-Binding Method (SCC-DFTB). The detailed analysis is focused on structural properties, strain energy, and band gap as a function of tube radii and Mulliken charge distribution. The strain energy of halloysite nanotubes does not have a monotonic character and the most stable structures should be observed in the region of radii above 24 A, in agreement with experimental data. Analysis of the electronic density of states shows that all tubes are insulators. Our calculations predict that single-walled halloysite nanotubes have Young modulus in the same order of imogolite and inorganic nanotubes, but smaller than that of carbon nanotubes. Even though most of...
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adsorption of phosphonic and ethylphosphonic acid on aluminum oxide surfaces
Surface Science, 2008Co-Authors: R Luschtinetz, Johannes Frenzel, Gotthard Seifert, Janole Joswig, Augusto F Oliveira, Hélio A. DuarteAbstract:Abstract Phosphonic acid and ethylphosphonic acid chemisorbed on aluminum oxide surfaces have been investigated using a density-functional based Tight-Binding Method. We have in particular focused on hydroxylated surface models based on corundum α - Al 2 O 3 (0 0 0 1), bayerite β - Al(OH) 3 (0 0 1) and boehmite γ -AlOOH (0 1 0). On these we have studied monodentate, bidentate and tridentate adsorption of the acids on all possible adsorption sites on the surfaces considering different surface coverages. By comparing the energies of the adsorption complexes we determined the favored adsorption sites for each coordination mode and surface structure. We found that the preference of an adsorption site is strongly influenced by its geometry and the regioselectivity increases when going from mono- to tridentate adsorption complexes. Moreover, the ethyl chain has no influence on the selection of the preferred adsorption sites.
Marcus Elstner - One of the best experts on this subject based on the ideXlab platform.
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time dependent extension of the long range corrected density functional based tight binding Method
Journal of Chemical Theory and Computation, 2017Co-Authors: Julian J Kranz, Marcus Elstner, Balint Aradi, Thomas Frauenheim, Vitalij Lutsker, Adriel Dominguez Garcia, Thomas A. NiehausAbstract:We present a consistent linear response formulation of the density functional based Tight-Binding Method for long-range corrected exchange-correlation functionals (LC-DFTB). Besides a detailed account of derivation and implementation of the Method, we also test the new scheme on a variety of systems considered to be problematic for conventional local/semilocal time-dependent density functional theory (TD-DFT). To this class belong the optical properties of polyacenes and nucleobases, as well as charge transfer excited states in molecular dimers. We find that the approximate LC-DFTB Method exhibits the same general trends and similar accuracy as range-separated DFT Methods at significantly reduced computational cost. The scheme should be especially useful in the determination of the electronic excited states of very large molecules, for which conventional TD-DFT is supposed to fail due to a multitude of artificial low energy states.
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parametrization and benchmark of dftb3 for organic molecules
Journal of Chemical Theory and Computation, 2013Co-Authors: Michael Gaus, Albrecht Goez, Marcus ElstnerAbstract:DFTB3 is a recent extension of the self-consistent-charge density-functional Tight-Binding Method (SCC-DFTB) and derived from a third order expansion of the density functional theory (DFT) total energy around a given reference density. Being applied in combination with the parametrization of its predecessor (MIO), DFTB3 improves for hydrogen binding energies, proton affinities, and hydrogen transfer barriers. In the present study, parameters especially designed for DFTB3 are presented, and its performance is evaluated for small organic molecules focusing on thermochemistry, geometries, and vibrational frequencies from our own and several databases from literature. The new parameters remove significant overbinding errors, reduce errors for geometries of noncovalent interactions, and improve the overall performance.
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dftb3 extension of the self consistent charge density functional tight binding Method scc dftb
Journal of Chemical Theory and Computation, 2011Co-Authors: Michael Gaus, Marcus ElstnerAbstract:The self-consistent-charge density-functional Tight-Binding Method (SCC-DFTB) is an approximate quantum chemical Method derived from density functional theory (DFT) based on a second-order expansion of the DFT total energy around a reference density. In the present study we combine earlier extensions and improve them consistently with, first, an improved Coulomb interaction between atomic partial charges, and second, the complete third-order expansion of the DFT total energy. These modifications lead us to the next generation of the DFTB Methodology called DFTB3, which substantially improves the description of charged systems containing elements C, H, N, O, and P, especially regarding hydrogen binding energies and proton affinities. As a result, DFTB3 is particularly applicable to biomolecular systems. Remaining challenges and possible solutions are also briefly discussed.
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coarse grained time dependent density functional simulation of charge transfer in complex systems application to hole transfer in dna
Journal of Physical Chemistry B, 2010Co-Authors: Tomas Kubar, Marcus ElstnerAbstract:We present a coarse-grained Tight-Binding Method based on density functional theory (DFT) for the simulation of charge transfer in complex materials. The charge-transfer parameters are computed using a fragment-orbital approach combined with the approximative DFT Method self-consistent charge density functional tight binding (SCC-DFTB), which allows to follow the dynamics of excess charge along nanosecond MD trajectories, still accounting for the important impact of structural fluctuations and solvent effects. Since DFT suffers from the self-interaction error, which would lead to a delocalization of the hole charge over the entire system, we study the effect of an empirical self-interaction correction in detail. The wave function of the excess charge is propagated within the framework of time-dependent DFT, where the electron (hole) and the atomic system are propagated simultaneously according to the derived coupled equations of motion. In the case of DNA, the solvent polarization effects are a dominant f...
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vibrational raman spectra from the self consistent charge density functional tight binding Method via classical time correlation functions
Journal of Chemical Theory and Computation, 2010Co-Authors: Steve Kaminski, Michael Gaus, Marcus Elstner, Prasad Phatak, David Von Stetten, Maria Andrea MroginskiAbstract:The Self-Consistent Charge Density Functional Tight Binding (SCC-DFTB) Method has been extended for the calculation of vibrational Raman spectra employing the Fourier Transform of Time-Correlation Function (FTTCF) formalism. As Witek and co-workers have already shown for a set of various organic molecules, the minimal basis SCC-DFTB approach performs surprisingly good in terms of polarizability calculations. Therefore, we were encouraged to use this electronic structure Method for the purpose of Raman spectra calculations via FTTCF. The molecular polarizability was accessed via second order numeric derivatives of the SCC-DFTB energy with respect to the components of an external electric field “on-the-fly” during a molecular dynamics (MD) simulation. The finite electric field approach delivers Raman spectra that are in overall good agreement for most of 10 small organic model compounds examined in the gas phase compared to a standard Normal Mode Analysis (NMA) approach at the same (SCC-DFTB) and at a highe...
Martin Karplus - One of the best experts on this subject based on the ideXlab platform.
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lagrangian formulation with dissipation of born oppenheimer molecular dynamics using the density functional tight binding Method
Journal of Chemical Physics, 2011Co-Authors: Guishan Zheng, Anders M N Niklasson, Martin KarplusAbstract:An important element determining the time requirements of Born-Oppenheimer molecular dynamics (BOMD) is the convergence rate of the self-consistent solution of Roothaan equations (SCF). We show here that improved convergence and dynamics stability can be achieved by use of a Lagrangian formalism of BOMD with dissipation (DXL-BOMD). In the DXL-BOMD algorithm, an auxiliary electronic variable (e.g., the electron density or Fock matrix) is propagated and a dissipative force is added in the propagation to maintain the stability of the dynamics. Implementation of the approach in the self-consistent charge density functional Tight-Binding Method makes possible simulations that are several hundred picoseconds in lengths, in contrast to earlier DFT-based BOMD calculations, which have been limited to tens of picoseconds or less. The increase in the simulation time results in a more meaningful evaluation of the DXL-BOMD Method. A comparison is made of the number of iterations (and time) required for convergence of the SCF with DXL-BOMD and a standard Method (starting with a zero charge guess for all atoms at each step), which gives accurate propagation with reasonable SCF convergence criteria. From tests using NVE simulations of C2F4 and 20 neutral amino acid molecules in the gas phase, it is found that DXL-BOMD can improve SCF convergence by up to a factor of two over the standard Method. Corresponding results are obtained in simulations of 32 water molecules in a periodic box. Linear response theory is used to analyze the relationship between the energy drift and the correlation of geometry propagation errors.
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lagrangian formulation with dissipation of born oppenheimer molecular dynamics using the density functional tight binding Method
Journal of Chemical Physics, 2011Co-Authors: Guishan Zheng, Anders M N Niklasson, Martin KarplusAbstract:An important element determining the time requirements of Born-Oppenheimer molecular dynamics (BOMD) is the convergence rate of the self-consistent solution of Roothaan equations (SCF). We show here that improved convergence and dynamics stability can be achieved by use of a Lagrangian formalism of BOMD with dissipation (DXL-BOMD). In the DXL-BOMD algorithm, an auxiliary electronic variable (e.g., the electron density or Fock matrix) is propagated and a dissipative force is added in the propagation to maintain the stability of the dynamics. Implementation of the approach in the self-consistent charge density functional Tight-Binding Method makes possible simulations that are several hundred picoseconds in lengths, in contrast to earlier DFT-based BOMD calculations, which have been limited to tens of picoseconds or less. The increase in the simulation time results in a more meaningful evaluation of the DXL-BOMD Method. A comparison is made of the number of iterations (and time) required for convergence of the SCF with DXL-BOMD and a standard Method (starting with a zero charge guess for all atoms at each step), which gives accurate propagation with reasonable SCF convergence criteria. From tests using NVE simulations of C2F4 and 20 neutral amino acid molecules in the gas phase, it is found that DXL-BOMD can improve SCF convergence by up to a factor of two over the standard Method. Corresponding results are obtained in simulations of 32 water molecules in a periodic box. Linear response theory is used to analyze the relationship between the energy drift and the correlation of geometry propagation errors.
Haydn Chen - One of the best experts on this subject based on the ideXlab platform.
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improvement of the spin polarized self consistent charge extended huckel tight binding Method
Materials Chemistry and Physics, 2000Co-Authors: Michihide Kitamura, Kanryu Inoue, Haydn ChenAbstract:Abstract The spin-polarized self-consistent-charge extended Huckel Tight-Binding (SP-SCC-XHTB) Method has been developed by authors to study the electronic structures of rutile-type transition metal dioxides, MO2, with both the nonmagnetic and magnetic phases. This Method has been used to successfully predict the electronic structures of nonmagnetic (n) MO2’s with M =Ti, V, Nb, Ta, Cr and ferromagnetic (f) CrO2, but is inadequate to predict the value of the energy gap of a semiconductive n-MnO2. The SP-SCC-XHTB band structure calculation, which includes all the relativistic effects in a first principal manner, has only one empirical parameter on the evaluation of the matrix elements of the non-relativistic Hamiltonian H0. In this paper, the SP-SCC-XHTB Method is improved so as to evaluate the matrix elements of the H0 in a more appropriate manner. The band structure calculations are carried out for the n-MO2’s with M =Ti, V, Nb, Ta, Cr and Mn, the f-CrO2 and the af-MnO2. With the improvement it is shown that the SP-SCC-IXHTB band structure calculations give reasonable results for the electronic structures of all the MO2’s considered in the present paper.
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electronic structures of transition metal dioxides studied by the spin polarized self consistent charge extended huckel tight binding Method
Materials Chemistry and Physics, 1998Co-Authors: Michihide Kitamura, Kanryu Inoue, Haydn ChenAbstract:Abstract Band theory based on the spin-polarized self-consistent-charge extended Huckel Tight-Binding (SP-SCC-XHTB) Method, which includes all the relativistic effects and the spin-polarized self-consistent-field (SP-SCF) atomic-structure calculation based on the Hartree-Fock-Slater Method, has been applied to the electronic-structure calculations of tetragonal rutile-type MO2s, such as nonmagnetic (n) TiO2, VO2, NbO2, TaO2, CrO2, MnO2, ferromagnetic (f) CrO2 and antiferromagnetic (af) MnO2. It is shown that the calculations are consistent with the experimental observations for all systems except the case of MnO2. It is demonstrated that the semiconductive nature of MnO2 can be explained by adding an assumption that three electrons in the Mn t2g orbital are localized in the crystal into the SP-SCC-XHTB band-structure calculation.
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electronic structure of barium titanate studies by the extended huckel tight binding Method
Journal of Applied Physics, 1994Co-Authors: M Holma, M Kitamura, Haydn ChenAbstract:The electronic structure of barium titanate is determined according to the extended Huckel tight‐binding Method using atomic data based on a self‐consistent‐field calculation. The effects of the phase transition on the partial density of states are studied. The electronic contribution to the phase stability of the compound is calculated from the total density of states. The tetragonal phase is found to be stable as compared to the cubic phase. The density of states is compared with available x‐ray photoemission spectra and is found to be in agreement with some features in experimental results.
Tsunenobu Kimoto - One of the best experts on this subject based on the ideXlab platform.
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orientation and size effects on electronic structure of rectangular cross sectional sn nanowires
Journal of Applied Physics, 2021Co-Authors: Masatoshi Sato, Hajime Tanaka, Tsunenobu KimotoAbstract:The band structures of Sn nanowires with various cross-sectional shapes and orientations were calculated by a Tight-Binding Method. The wave functions of bulk Sn and Sn nanowires were also analyzed. The calculation revealed that the conduction band minimum of some Sn nanowires originates from the electronic states in the valence band of bulk Sn. This behavior was discussed in terms of the dependence on the orientation and cross-sectional shape of Sn nanowires. In addition, we clarified that the [110]-oriented nanowires with a narrow (001) sidewall and [111]-oriented nanowires have the lightest effective mass among the considered nanowires.
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quantum confinement effects on conduction band structure of rectangular cross sectional gaas nanowires
Journal of Applied Physics, 2014Co-Authors: Hajime Tanaka, Naoya Morioka, Seigo Mori, Jun Suda, Tsunenobu KimotoAbstract:The conduction band structure and electron effective mass of GaAs nanowires with various cross-sectional shapes and orientations were calculated by two Methods, a Tight-Binding Method and an effective mass equation taking the bulk full-band structure into account. The effective mass of nanowires increases as the cross-sectional size decreases, and this increase in effective mass depends on the orientations and substrate faces of nanowires. Among [001], [110], and [111]-oriented rectangular cross-sectional GaAs nanowires, [110]-oriented nanowires with wider width along the [001] direction showed the lightest effective mass. This dependence originates from the anisotropy of the Γ valley of bulk GaAs. The relationship between effective mass and bulk band structure is discussed.
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quantum confinement effect on holes in silicon nanowires relationship between wave function and band structure
Journal of Applied Physics, 2011Co-Authors: Naoya Morioka, Jun Suda, Hironori Yoshioka, Tsunenobu KimotoAbstract:The authors theoretically studied the valence band structure and hole effective mass of rectangular cross-sectional Si nanowires (NWs) with the crystal orientation of [110], [111], and [001]. The E–k dispersion and the wave function were calculated using an sp3d5s∗ Tight-Binding Method and analyzed with the focus on the nature of p orbitals constituting the subbands. In [110] and [111] nanowires, longitudinal/transverse p orbitals are well separated and longitudinal component makes light (top) subbands and transverse component makes heavy subbands. The heavy subbands are located far below the top light band when NW has square cross-section, but they gain their energy with the increase in the NW width and come near the band edge. This energy shift of heavy bands in [110] NWs shows strong anisotropy to the direction of quantum confinement whereas that in [111] NWs does not have such anisotropy. This anisotropic behavior and the difference among orientations are understandable by the character of the wave fu...
