Multilevel domain decomposition for electronic structure calculations
Barrault, M. . E-mail: maxime.barrault@edf.fr; Cances, E. . E-mail: cances@cermics.enpc.fr; Hager, W.W. . E-mail: hager@math.ufl.edu; Le Bris, C. . E-mail: lebris@cermics.enpc.fr
2007-03-01
We introduce a new multilevel domain decomposition method (MDD) for electronic structure calculations within semi-empirical and density functional theory (DFT) frameworks. This method iterates between local fine solvers and global coarse solvers, in the spirit of domain decomposition methods. Using this approach, calculations have been successfully performed on several linear polymer chains containing up to 40,000 atoms and 200,000 atomic orbitals. Both the computational cost and the memory requirement scale linearly with the number of atoms. Additional speed-up can easily be obtained by parallelization. We show that this domain decomposition method outperforms the density matrix minimization (DMM) method for poor initial guesses. Our method provides an efficient preconditioner for DMM and other linear scaling methods, variational in nature, such as the orbital minimization (OM) procedure.
Multigrid Methods in Electronic Structure Calculations
NASA Astrophysics Data System (ADS)
Briggs, Emil
1996-03-01
Multigrid techniques have become the method of choice for a broad range of computational problems. Their use in electronic structure calculations introduces a new set of issues when compared to traditional plane wave approaches. We have developed a set of techniques that address these issues and permit multigrid algorithms to be applied to the electronic structure problem in an efficient manner. In our approach the Kohn-Sham equations are discretized on a real-space mesh using a compact representation of the Hamiltonian. The resulting equations are solved directly on the mesh using multigrid iterations. This produces rapid convergence rates even for ill-conditioned systems with large length and/or energy scales. The method has been applied to both periodic and non-periodic systems containing over 400 atoms and the results are in very good agreement with both theory and experiment. Example applications include a vacancy in diamond, an isolated C60 molecule, and a 64-atom cell of GaN with the Ga d-electrons in valence which required a 250 Ry cutoff. A particular strength of a real-space multigrid approach is its ready adaptability to massively parallel computer architectures. The compact representation of the Hamiltonian is especially well suited to such machines. Tests on the Cray-T3D have shown nearly linear scaling of the execution time up to the maximum number of processors (512). The MPP implementation has been used for studies of a large Amyloid Beta Peptide (C_146O_45N_42H_210) found in the brains of Alzheimers disease patients. Further applications of the multigrid method will also be described. (in collaboration D. J. Sullivan and J. Bernholc)
Probing Actinide Electronic Structure through Pu Cluster Calculations
Ryzhkov, Mickhail V.; Mirmelstein, Alexei; Yu, Sung-Woo; ...
2013-02-26
The calculations for the electronic structure of clusters of plutonium have been performed, within the framework of the relativistic discrete-variational method. Moreover, these theoretical results and those calculated earlier for related systems have been compared to spectroscopic data produced in the experimental investigations of bulk systems, including photoelectron spectroscopy. Observation of the changes in the Pu electronic structure as a function of size provides powerful insight for aspects of bulk Pu electronic structure.
Basis functions for electronic structure calculations on spheres
Gill, Peter M. W. Loos, Pierre-François Agboola, Davids
2014-12-28
We introduce a new basis function (the spherical Gaussian) for electronic structure calculations on spheres of any dimension D. We find general expressions for the one- and two-electron integrals and propose an efficient computational algorithm incorporating the Cauchy-Schwarz bound. Using numerical calculations for the D = 2 case, we show that spherical Gaussians are more efficient than spherical harmonics when the electrons are strongly localized.
Electronic-structure calculation for metals by local optimization
NASA Astrophysics Data System (ADS)
Woodward, C.; Min, B. I.; Benedek, R.; Garner, J.
1989-03-01
Recent work by Car and Parrinello has generated considerable interest in the calculation of electronic structure by nonlinear optimization. The technique introduced by these authors, dynamical simulated annealing, is designed for problems that involve energy barriers. When local optimization suffices to determine the energy minimum, more direct methods are available. In this paper we apply the algorithm suggested by Williams and Soler to calculate the electronic structure of metals, using a plane-wave expansion for the electronic orbitals and an electron-ion pseudopotential of the Kleinman-Bylander form. Radial pseudopotentials were taken from the compilation of Bachelet, Hamann, and Schlüter. Calculations are performed to optimize the electronic structure (i) with fixed atomic configuration, or (ii) with the atomic volume being optimized simultaneously. It is found that the dual optimization (ii) converges in essentially the same number of steps as the static lattice optimization (i). Numerical results are presented for Li, K, Al, and simple-cubic P.
Parallel adaptive mesh refinement for electronic structure calculations
Kohn, S.; Weare, J.; Ong, E.; Baden, S.
1996-12-01
We have applied structured adaptive mesh refinement techniques to the solution of the LDA equations for electronic structure calculations. Local spatial refinement concentrates memory resources and numerical effort where it is most needed, near the atomic centers and in regions of rapidly varying charge density. The structured grid representation enables us to employ efficient iterative solver techniques such as conjugate gradients with multigrid preconditioning. We have parallelized our solver using an object-oriented adaptive mesh refinement framework.
Structural and electronic properties of perylene from first principles calculations.
Fedorov, I A; Zhuravlev, Y N; Berveno, V P
2013-03-07
The electronic structure of crystalline perylene has been investigated within the framework of density functional theory including van der Waals interactions. The computations of the lattice parameters and cohesive energy have good agreement with experimental values. We have also calculated the binding distance and energy of perylene dimers, using different schemes, which include van der Waals interactions.
Electronic structure calculations of ESR parameters of melanin units.
Batagin-Neto, Augusto; Bronze-Uhle, Erika Soares; Graeff, Carlos Frederico de Oliveira
2015-03-21
Melanins represent an important class of natural pigments present in plants and animals that are currently considered to be promising materials for applications in optic and electronic devices. Despite their interesting properties, some of the basic features of melanins are not satisfactorily understood, including the origin of their intrinsic paramagnetism. A number of experiments have been performed to investigate the electron spin resonance (ESR) response of melanin derivatives, but until now, there has been no consensus regarding the real structure of the paramagnetic centers involved. In this work, we have employed electronic structure calculations to evaluate the ESR parameters of distinct melanin monomers and dimers in order to identify the possible structures associated with unpaired spins in this biopolymer. The g-factors and hyperfine constants of the cationic, anionic and radicalar structures were investigated. The results confirm the existence of at least two distinct paramagnetic centers in melanin structure, identifying the chemical species associated with them and their roles in electrical conductivity.
Electronic structure calculation by nonlinear optimization: Application to metals
NASA Astrophysics Data System (ADS)
Benedek, R.; Min, B. I.; Woodward, C.; Garner, J.
1988-04-01
There is considerable interest in the development of novel algorithms for the calculation of electronic structure (e.g., at the level of the local-density approximation of density-functional theory). In this paper we consider a first-order equation-of-motion method. Two methods of solution are described, one proposed by Williams and Soler, and the other base on a Born-Dyson series expansion. The extension of the approach to metallic systems is outlined and preliminary numerical calculations for Zintl-phase NaTl are presented.
Real-time feedback from iterative electronic structure calculations.
Vaucher, Alain C; Haag, Moritz P; Reiher, Markus
2016-04-05
Real-time feedback from iterative electronic structure calculations requires to mediate between the inherently unpredictable execution times of the iterative algorithm used and the necessity to provide data in fixed and short time intervals for real-time rendering. We introduce the concept of a mediator as a component able to deal with infrequent and unpredictable reference data to generate reliable feedback. In the context of real-time quantum chemistry, the mediator takes the form of a surrogate potential that has the same local shape as the first-principles potential and can be evaluated efficiently to deliver atomic forces as real-time feedback. The surrogate potential is updated continuously by electronic structure calculations and guarantees to provide a reliable response to the operator for any molecular structure. To demonstrate the application of iterative electronic structure methods in real-time reactivity exploration, we implement self-consistent semiempirical methods as the data source and apply the surrogate-potential mediator to deliver reliable real-time feedback.
Approximate ab initio calculations of electronic structure of amorphous silicon
NASA Astrophysics Data System (ADS)
Durandurdu, M.; Drabold, D. A.; Mousseau, N.
2000-12-01
We report on ab initio calculations of electronic states of two large and realistic models of amorphous silicon generated using a modified version of the Wooten-Winer-Weaire algorithm and relaxed, in both cases, with a Keating and a modified Stillinger-Weber potentials. The models have no coordination defects and a very narrow bond-angle distribution. We compute the electronic density-of-states and pay particular attention to the nature of the band-tail states around the electronic gap. All models show a large and perfectly clean optical gap and realistic Urbach tails. Based on these results and the extended quasi-one-dimensional stringlike structures observed for certain eigenvalues in the band tails, we postulate that the generation of model a-Si without localized states might be achievable under certain circumstances.
Electronic Structure of Silicon Nanowires Matrix from Ab Initio Calculations
NASA Astrophysics Data System (ADS)
Monastyrskii, Liubomyr S.; Boyko, Yaroslav V.; Sokolovskii, Bogdan S.; Potashnyk, Vasylyna Ya.
2016-01-01
An investigation of the model of porous silicon in the form of periodic set of silicon nanowires has been carried out. The electronic energy structure was studied using a first-principle band method—the method of pseudopotentials (ultrasoft potentials in the basis of plane waves) and linearized mode of the method of combined pseudopotentials. Due to the use of hybrid exchange-correlation potentials (B3LYP), the quantitative agreement of the calculated value of band gap in the bulk material with experimental data is achieved. The obtained results show that passivation of dangling bonds with hydrogen atoms leads to substantial transformation of electronic energy structure. At complete passivation of the dangling silicon bonds by hydrogen atoms, the band gap value takes the magnitude which substantially exceeds that for bulk silicon. The incomplete passivation gives rise to opposite effect when the band gap value decreases down the semimetallic range.
Electronic Structure of Silicon Nanowires Matrix from Ab Initio Calculations.
Monastyrskii, Liubomyr S; Boyko, Yaroslav V; Sokolovskii, Bogdan S; Potashnyk, Vasylyna Ya
2016-12-01
An investigation of the model of porous silicon in the form of periodic set of silicon nanowires has been carried out. The electronic energy structure was studied using a first-principle band method-the method of pseudopotentials (ultrasoft potentials in the basis of plane waves) and linearized mode of the method of combined pseudopotentials. Due to the use of hybrid exchange-correlation potentials (B3LYP), the quantitative agreement of the calculated value of band gap in the bulk material with experimental data is achieved. The obtained results show that passivation of dangling bonds with hydrogen atoms leads to substantial transformation of electronic energy structure. At complete passivation of the dangling silicon bonds by hydrogen atoms, the band gap value takes the magnitude which substantially exceeds that for bulk silicon. The incomplete passivation gives rise to opposite effect when the band gap value decreases down the semimetallic range.
A Discontinuous Galerkin Framework for Electronic Structure Calculations
NASA Astrophysics Data System (ADS)
Baczewski, Andrew; Shanker, Balasubramaniam; Mahanti, Subhendra; Levine, Benjamin
2012-02-01
It is generally accepted that a good basis set for any calculation should possess a number of salient features, including systematic improvability, adaptive resolution of multiscale features, and fidelity in capturing the pertinent physics. Considering the progenitors of most modern electronic structure basis sets to be Gaussian-type orbitals or planewaves, descendants of these methods have inherited features that address either systematic improvability (planewaves) or adaptive resolution (Gaussians) separately, and use a variety of tricks to differentiate the core and valence physics. Discontinuous Galerkin methods provide a framework for defining adaptive local basis sets, that may be based on these canonical basis sets, that can be mixed and matched to simultaneously achieve all of these goals. Our group is presently developing a new electronic structure code to enable Density Functional and Hartree-Fock calculations within this framework, particularly in the context of all-electron formulations wherein the accurate resolution of both core and valence states is necessary. Numerous implementation details will be addressed, including the incorporation of hardware- and software-based acceleration, such as GPU-based parallelism, and fast electrostatics solvers.
Multi-million atom electronic structure calculations for quantum dots
NASA Astrophysics Data System (ADS)
Usman, Muhammad
Quantum dots grown by self-assembly process are typically constructed by 50,000 to 5,000,000 structural atoms which confine a small, countable number of extra electrons or holes in a space that is comparable in size to the electron wavelength. Under such conditions quantum dots can be interpreted as artificial atoms with the potential to be custom tailored to new functionality. In the past decade or so, these nanostructures have attracted significant experimental and theoretical attention in the field of nanoscience. The new and tunable optical and electrical properties of these artificial atoms have been proposed in a variety of different fields, for example in communication and computing systems, medical and quantum computing applications. Predictive and quantitative modeling and simulation of these structures can help to narrow down the vast design space to a range that is experimentally affordable and move this part of nanoscience to nano-Technology. Modeling of such quantum dots pose a formidable challenge to theoretical physicists because: (1) Strain originating from the lattice mismatch of the materials penetrates deep inside the buffer surrounding the quantum dots and require large scale (multi-million atom) simulations to correctly capture its effect on the electronic structure, (2) The interface roughness, the alloy randomness, and the atomistic granularity require the calculation of electronic structure at the atomistic scale. Most of the current or past theoretical calculations are based on continuum approach such as effective mass approximation or k.p modeling capturing either no or one of the above mentioned effects, thus missing some of the essential physics. The Objectives of this thesis are: (1) to model and simulate the experimental quantum dot topologies at the atomistic scale; (2) to theoretically explore the essential physics i.e. long range strain, linear and quadratic piezoelectricity, interband optical transition strengths, quantum confined
Comparison of optimization methods for electronic-structure calculations
NASA Astrophysics Data System (ADS)
Garner, J.; Das, S. G.; Min, B. I.; Woodward, C.; Benedek, R.
1989-06-01
The performance of several local-optimization methods for calculating electronic structure is compared. The fictitious first-order equation of motion proposed by Williams and Soler is integrated numerically by three procedures: simple finite-difference integration, approximate analytical integration (the Williams-Soler algorithm), and the Born perturbation series. These techniques are applied to a model problem for which exact solutions are known, the Mathieu equation. The Williams-Soler algorithm and the second Born approximation converge equally rapidly, but the former involves considerably less computational effort and gives a more accurate converged solution. Application of the method of conjugate gradients to the Mathieu equation is discussed.
Supersampling method for efficient grid-based electronic structure calculations.
Ryu, Seongok; Choi, Sunghwan; Hong, Kwangwoo; Kim, Woo Youn
2016-03-07
The egg-box effect, the spurious variation of energy and force due to the discretization of continuous space, is an inherent vexing problem in grid-based electronic structure calculations. Its effective suppression allowing for large grid spacing is thus crucial for accurate and efficient computations. We here report that the supersampling method drastically alleviates it by eliminating the rapidly varying part of a target function along both radial and angular directions. In particular, the use of the sinc filtering function performs best because as an ideal low pass filter it clearly cuts out the high frequency region beyond allowed by a given grid spacing.
Efficient Execution of Electronic Structure Calculations on SMP Clusters
Ustemirov, Nurzhan
2006-01-01
Applications augmented with adaptive capabilities are becoming common in parallel computing environments. For large-scale scientific applications, dynamic adjustments to a computationally-intensive part may lead to a large pay-off in facilitating efficient execution of the entire application while aiming at avoiding resource contention. Application-specific knowledge, often best revealed during the run-time, is required to initiate and time these adjustments. In particular, General Atomic and Molecular Electronic Structure System (GAMESS) is a program for ab initio quantum chemistry that places significant demands on the high-performance computing platforms. Certain electronic structure calculations are characterized by high consumption of a particular resource, such as CPU, main memory, or disk I/O. This may lead to resource contention among concurrent GAMESS jobs and other programs in the dynamically changing environment. Thus, it is desirable to improve GAMESS calculations by means of dynamic adaptations. In this thesis, we show how an application- or algorithm-specific knowledge may play a significant role in achieving this goal. The choice of implementation is facilitated by a module-driven middleware easily integrated with GAMESS that assesses resource consumption and invokes GAMESS adaptations to the system environment. We show that the throughput of GAMESS jobs may be improved greatly as a result of such adaptations.
Electronic structure calculations toward new potentially AChE inhibitors
NASA Astrophysics Data System (ADS)
de Paula, A. A. N.; Martins, J. B. L.; Gargano, R.; dos Santos, M. L.; Romeiro, L. A. S.
2007-10-01
The main purpose of this study was the use of natural non-isoprenoid phenolic lipid of cashew nut shell liquid from Anacardium occidentale as lead material for generating new potentially candidates of acetylcholinesterase inhibitors. Therefore, we studied the electronic structure of 15 molecules derivatives from the cardanol using the following groups: methyl, acetyl, N, N-dimethylcarbamoyl, N, N-dimethylamine, N, N-diethylamine, piperidine, pyrrolidine, and N-benzylamine. The calculations were performed at RHF level using 6-31G, 6-31G(d), 6-31+G(d) and 6-311G(d,p) basis functions. Among the proposed compounds we found that the structures with substitution by acetyl, N, N-dimethylcarbamoyl, N, N-dimethylamine, and pyrrolidine groups were better correlated to rivastigmine indicating possible activity.
Electronic structure and conductivity of ferroelectric hexaferrite: Ab initio calculations
NASA Astrophysics Data System (ADS)
Knížek, K.; Novák, P.; Küpferling, M.
2006-04-01
Ba0.5Sr1.5Zn2Fe12O22 is a promising multiferroic compound in which the electric polarization is intimately connected to the magnetic state. In principle, ferroelectrity might exist above the room temperature, but the electrical conductivity that increases with increasing temperature limits it to temperatures below ≈130K . We present results of an ab initio electronic structure calculation of the (BaSr)Zn2Fe12O22 system. To improve the description of strongly correlated 3d electrons of iron, the GGA+U method is used. The results show that the electrical conductivity strongly depends on relative fractions of iron and zinc in the tetrahedral sublattice that belongs to the spinel block of the hexaferrite structure. If this sublattice is fully occupied by zinc, the system is an insulator with a gap of ≈1.5eV . If it is occupied equally by Fe and Zn the gap decreases by a factor of 2, and the system is metallic when this sublattice is filled by iron only.
Auxiliary basis expansions for large-scale electronic structure calculations.
Jung, Yousung; Sodt, Alex; Gill, Peter M W; Head-Gordon, Martin
2005-05-10
One way to reduce the computational cost of electronic structure calculations is to use auxiliary basis expansions to approximate four-center integrals in terms of two- and three-center integrals, usually by using the variationally optimum Coulomb metric to determine the expansion coefficients. However, the long-range decay behavior of the auxiliary basis expansion coefficients has not been characterized. We find that this decay can be surprisingly slow. Numerical experiments on linear alkanes and a toy model both show that the decay can be as slow as 1/r in the distance between the auxiliary function and the fitted charge distribution. The Coulomb metric fitting equations also involve divergent matrix elements for extended systems treated with periodic boundary conditions. An attenuated Coulomb metric that is short-range can eliminate these oddities without substantially degrading calculated relative energies. The sparsity of the fit coefficients is assessed on simple hydrocarbon molecules and shows quite early onset of linear growth in the number of significant coefficients with system size using the attenuated Coulomb metric. Hence it is possible to design linear scaling auxiliary basis methods without additional approximations to treat large systems.
Adaptations in Electronic Structure Calculations in Heterogeneous Environments
Talamudupula, Sai
2011-01-01
Modern quantum chemistry deals with electronic structure calculations of unprecedented complexity and accuracy. They demand full power of high-performance computing and must be in tune with the given architecture for superior e ciency. To make such applications resourceaware, it is desirable to enable their static and dynamic adaptations using some external software (middleware), which may monitor both system availability and application needs, rather than mix science with system-related calls inside the application. The present work investigates scienti c application interlinking with middleware based on the example of the computational chemistry package GAMESS and middleware NICAN. The existing synchronous model is limited by the possible delays due to the middleware processing time under the sustainable runtime system conditions. Proposed asynchronous and hybrid models aim at overcoming this limitation. When linked with NICAN, the fragment molecular orbital (FMO) method is capable of adapting statically and dynamically its fragment scheduling policy based on the computing platform conditions. Signi cant execution time and throughput gains have been obtained due to such static adaptations when the compute nodes have very di erent core counts. Dynamic adaptations are based on the main memory availability at run time. NICAN prompts FMO to postpone scheduling certain fragments, if there is not enough memory for their immediate execution. Hence, FMO may be able to complete the calculations whereas without such adaptations it aborts.
Large Scale Electronic Structure Calculations using Quantum Chemistry Methods
NASA Astrophysics Data System (ADS)
Scuseria, Gustavo E.
1998-03-01
This talk will address our recent efforts in developing fast, linear scaling electronic structure methods for large scale applications. Of special importance is our fast multipole method( M. C. Strain, G. E. Scuseria, and M. J. Frisch, Science 271), 51 (1996). (FMM) for achieving linear scaling for the quantum Coulomb problem (GvFMM), the traditional bottleneck in quantum chemistry calculations based on Gaussian orbitals. Fast quadratures(R. E. Stratmann, G. E. Scuseria, and M. J. Frisch, Chem. Phys. Lett. 257), 213 (1996). combined with methods that avoid the Hamiltonian diagonalization( J. M. Millam and G. E. Scuseria, J. Chem. Phys. 106), 5569 (1997) have resulted in density functional theory (DFT) programs that can be applied to systems containing many hundreds of atoms and ---depending on computational resources or level of theory-- to many thousands of atoms.( A. D. Daniels, J. M. Millam and G. E. Scuseria, J. Chem. Phys. 107), 425 (1997). Three solutions for the diagonalization bottleneck will be analyzed and compared: a conjugate gradient density matrix search (CGDMS), a Hamiltonian polynomial expansion of the density matrix, and a pseudo-diagonalization method. Besides DFT, our near-field exchange method( J. C. Burant, G. E. Scuseria, and M. J. Frisch, J. Chem. Phys. 105), 8969 (1996). for linear scaling Hartree-Fock calculations will be discussed. Based on these improved capabilities, we have also developed programs to obtain vibrational frequencies (via analytic energy second derivatives) and excitation energies (through time-dependent DFT) of large molecules like porphyn or C_70. Our GvFMM has been extended to periodic systems( K. N. Kudin and G. E. Scuseria, Chem. Phys. Lett., in press.) and progress towards a Gaussian-based DFT and HF program for polymers and solids will be reported. Last, we will discuss our progress on a Laplace-transformed \\cal O(N^2) second-order pertubation theory (MP2) method.
Computational method for general multicenter electronic structure calculations.
Batcho, P F
2000-06-01
Here a three-dimensional fully numerical (i.e., chemical basis-set free) method [P. F. Batcho, Phys. Rev. A 57, 6 (1998)], is formulated and applied to the calculation of the electronic structure of general multicenter Hamiltonian systems. The numerical method is presented and applied to the solution of Schrödinger-type operators, where a given number of nuclei point singularities is present in the potential field. The numerical method combines the rapid "exponential" convergence rates of modern spectral methods with the multiresolution flexibility of finite element methods, and can be viewed as an extension of the spectral element method. The approximation of cusps in the wave function and the formulation of multicenter nuclei singularities are efficiently dealt with by the combination of a coordinate transformation and a piecewise variational spectral approximation. The complete system can be efficiently inverted by established iterative methods for elliptical partial differential equations; an application of the method is presented for atomic, diatomic, and triatomic systems, and comparisons are made to the literature when possible. In particular, local density approximations are studied within the context of Kohn-Sham density functional theory, and are presented for selected subsets of atomic and diatomic molecules as well as the ozone molecule.
Muchová, Eva; Slavícek, Petr; Sobolewski, Andrzej L; Hobza, Pavel
2007-06-21
The goal of this study is to explore the photochemical processes following optical excitation of the glycine molecule into its two low-lying excited states. We employed electronic structure methods at various levels to map the PES of the ground state and the two low-lying excited states of glycine. It follows from our calculations that the photochemistry of glycine can be regarded as a combination of photochemical behavior of amines and carboxylic acid. The first channel (connected to the presence of amino group) results in ultrafast decay, while the channels characteristic for the carboxylic group occur on a longer time scale. Dynamical calculations provided the branching ratio for these channels. We also addressed the question whether conformationally dependent photochemistry can be observed for glycine. While electronic structure calculations favor this possibility, the ab initio multiple spawning (AIMS) calculations showed only minor relevance of the reaction path resulting in conformationally dependent dynamics.
Grid-based electronic structure calculations: The tensor decomposition approach
Rakhuba, M.V.; Oseledets, I.V.
2016-05-01
We present a fully grid-based approach for solving Hartree–Fock and all-electron Kohn–Sham equations based on low-rank approximation of three-dimensional electron orbitals. Due to the low-rank structure the total complexity of the algorithm depends linearly with respect to the one-dimensional grid size. Linear complexity allows for the usage of fine grids, e.g. 8192{sup 3} and, thus, cheap extrapolation procedure. We test the proposed approach on closed-shell atoms up to the argon, several molecules and clusters of hydrogen atoms. All tests show systematical convergence with the required accuracy.
Calculation of 2D electronic band structure using matrix mechanics
NASA Astrophysics Data System (ADS)
Pavelich, R. L.; Marsiglio, F.
2016-12-01
We extend previous work, applying elementary matrix mechanics to one-dimensional periodic arrays (to generate energy bands), to two-dimensional arrays. We generate band structures for the square-lattice "2D Kronig-Penney model" (square wells), the "muffin-tin" potential (circular wells), and Gaussian wells. We then apply the method to periodic arrays of more than one atomic site in a unit cell, specifically to the case of materials with hexagonal lattices like graphene. These straightforward extensions of undergraduate-level calculations allow students to readily determine band structures of current research interest.
Kohn, S.; Weare, J.; Ong, E.; Baden, S.
1997-05-01
We have applied structured adaptive mesh refinement techniques to the solution of the LDA equations for electronic structure calculations. Local spatial refinement concentrates memory resources and numerical effort where it is most needed, near the atomic centers and in regions of rapidly varying charge density. The structured grid representation enables us to employ efficient iterative solver techniques such as conjugate gradient with FAC multigrid preconditioning. We have parallelized our solver using an object- oriented adaptive mesh refinement framework.
Order-N Electronic Structure Calculation of n-TYPE GaAs Quantum Dots
NASA Astrophysics Data System (ADS)
Nomura, S.; Iitaka, T.
2008-10-01
A linear scale method for calculating electronic properties of large and complex systems is introduced within a local density approximation. The method is based on the Chebyshev polynomial expansion and the time-dependent method, which is tested in calculating the electronic structure of a model n-type GaAs quantum dot.
Symmetry and equivalence restrictions in electronic structure calculations
NASA Technical Reports Server (NTRS)
Bauschlicher, Charles W., Jr.; Taylor, Peter R.
1988-01-01
A simple method for obtaining MCSCF orbitals and CI natural orbitals adapted to degenerate point groups, with full symmetry and equivalnece restrictions, is described. Among several advantages accruing from this method are the ability to perform atomic SCF calculations on states for which the SCF energy expression cannot be written in terms of Coulomb and exchange integrals over real orbitals, and the generation of symmetry-adapted atomic natural orbitals for use in a recently proposed method for basis set contraction.
Spectral-Product Methods for Electronic Structure Calculations (Postprint)
2007-06-12
and electronically excited potential energy surfaces in Monte Carlo and molecular dynamics simula- tions of singly doped inert-gas clusters [27], the...Matsunaga N, Nguyen KA, Su SJ, Windus TL, Dupuis M, Montgomery JA (1993) J Comput Chem 14:1347 3. Pauli W (1925) Z Physik 31:765 4. Heisenberg W (1926) Z...Physik 38:411 5. Heisenberg W (1926) Z Physik 39:499 6. Heisenberg W (1926) Z Physik 40:501 7. Dirac PAM (1926) Proc R Soc (London) A 112:661 8
Havu, V. Blum, V.; Havu, P.; Scheffler, M.
2009-12-01
We consider the problem of developing O(N) scaling grid-based operations needed in many central operations when performing electronic structure calculations with numeric atom-centered orbitals as basis functions. We outline the overall formulation of localized algorithms, and specifically the creation of localized grid batches. The choice of the grid partitioning scheme plays an important role in the performance and memory consumption of the grid-based operations. Three different top-down partitioning methods are investigated, and compared with formally more rigorous yet much more expensive bottom-up algorithms. We show that a conceptually simple top-down grid partitioning scheme achieves essentially the same efficiency as the more rigorous bottom-up approaches.
Comparison of optimization methods for electronic-structure calculations
Garner, J.; Das, S. G.; Min, B. I.; Woodward, C.; Benedek, R.
1989-06-15
The performance of several local-optimization methods for calculatingelectronic structure is compared. The fictitious first-order equation of motionproposed by Williams and Soler is integrated numerically by three procedures:simple finite-difference integration, approximate analytical integration (theWilliams-Soler algorithm), and the Born perturbation series. These techniquesare applied to a model problem for which exact solutions are known, the Mathieuequation. The Williams-Soler algorithm and the second Born approximationconverge equally rapidly, but the former involves considerably lesscomputational effort and gives a more accurate converged solution. Applicationof the method of conjugate gradients to the Mathieu equation is discussed.
Larsen, Ross E.
2016-04-12
In this study, we introduce two simple tight-binding models, which we call fragment frontier orbital extrapolations (FFOE), to extrapolate important electronic properties to the polymer limit using electronic structure calculations on only a few small oligomers. In particular, we demonstrate by comparison to explicit density functional theory calculations that for long oligomers the energies of the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and of the first electronic excited state are accurately described as a function of number of repeat units by a simple effective Hamiltonian parameterized from electronic structure calculations on monomers, dimers and, optionally, tetramers. For the alternating copolymer materials that currently comprise some of the most efficient polymer organic photovoltaic devices one can use these simple but rigorous models to extrapolate computed properties to the polymer limit based on calculations on a small number of low-molecular-weight oligomers.
Larsen, Ross E.
2016-04-12
In this study, we introduce two simple tight-binding models, which we call fragment frontier orbital extrapolations (FFOE), to extrapolate important electronic properties to the polymer limit using electronic structure calculations on only a few small oligomers. In particular, we demonstrate by comparison to explicit density functional theory calculations that for long oligomers the energies of the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and of the first electronic excited state are accurately described as a function of number of repeat units by a simple effective Hamiltonian parameterized from electronic structure calculations on monomers, dimers and, optionally,more » tetramers. For the alternating copolymer materials that currently comprise some of the most efficient polymer organic photovoltaic devices one can use these simple but rigorous models to extrapolate computed properties to the polymer limit based on calculations on a small number of low-molecular-weight oligomers.« less
Regularizing the molecular potential in electronic structure calculations. II. Many-body methods
Bischoff, Florian A.
2014-11-14
In Paper I of this series [F. A. Bischoff, “Regularizing the molecular potential in electronic structure calculations. I. SCF methods,” J. Chem. Phys. 141, 184105 (2014)] a regularized molecular Hamilton operator for electronic structure calculations was derived and its properties in SCF calculations were studied. The regularization was achieved using a correlation factor that models the electron-nuclear cusp. In the present study we extend the regularization to correlated methods, in particular the exact solution of the two-electron problem, as well as second-order many body perturbation theory. The nuclear and electronic correlation factors lead to computations with a smaller memory footprint because the singularities are removed from the working equations, which allows coarser grid resolution while maintaining the precision. Numerical examples are given.
Band like Electronic Structures in Square Hollow Quantum Dots by 3D-MHFKS Calculation
NASA Astrophysics Data System (ADS)
Takizawa, Tokihiro; Okada, Hoshihito; Matsuse, Takehiro
To find novel aspects of the electronic structures in quantum dots (QD) from a view point of spatial broken symmetry, 3-dimensional-mesh Hartree-Fock-Kohn-Sham (3D-MHFKS) calculations1 are applied to the interacting electron system of electron number N in a symmetry broken hollow QD. For the case of a square hollow quantum dot confined in square hard wall (HW) potential (SSHQD), the magnetic (B) field dependence of the obtained single particle energy levels and chemical potentials in B-N diagram are shown to have a band like electronic structures over the wide B-field range up to 20T. To clarify the origin of the band like electronic structures in SSHQD, 3D-MHFKS calculations are also applied for the mixed symmetry QD's with a circular hollow in square HW potential (SCHQD) and with a square hollow in circular HW potential (CSHQD).
NASA Astrophysics Data System (ADS)
Ghosh, Binita; Halder, Saswata; Das, Sayantani; Sinha, T. P.
2016-05-01
Europium-doped luminescent barium samarium tantalum oxide Ba2SmTaO6 (BST) has been investigated by first-principles calculation, and the crystal structure, electronic structure, and optical properties of pure BST and Eu-doped BST have been examined and compared. Based on the calculated results, the luminescence properties and mechanism of Eu-doped BST has been discussed. In the case of Eu-doped BST, there is an impurity energy band at the Fermi level, which is formed by seven spin up energy levels of Eu and act as the luminescent centre, which is evident from the band structure calculations.
Sun, Shih-Jye; Lin, Ken-Huang; Li, Jia-Yun; Ju, Shin-Pon
2014-10-07
The simulated annealing basin-hopping method incorporating the penalty function was used to predict the lowest-energy structures for ultrathin tungsten nanowires and nanotubes of different sizes. These predicted structures indicate that tungsten one-dimensional structures at this small scale do not possess B.C.C. configuration as in bulk tungsten material. In order to analyze the relationship between multi-shell geometries and electronic transfer, the electronic and structural properties of tungsten wires and tubes including partial density of state and band structures which were determined and analyzed by quantum chemistry calculations. In addition, in order to understand the application feasibility of these nanowires and tubes on nano-devices such as field emitters or chemical catalysts, the electronic stability of these ultrathin tungsten nanowires was also investigated by density functional theory calculations.
Ab Initio Calculations of the Electronic Structures and Biological Functions of Protein Molecules
NASA Astrophysics Data System (ADS)
Zheng, Haoping
The self-consistent cluster-embedding (SCCE) calculation method reduces the computational effort from M3 to about M1 (M is the number of atoms in the system) with precise calculations. Thus the ab initio, all-electron calculation of the electronic structure and biological function of protein molecule has become a reality, which will promote new proteomics considerably. The calculated results of two real protein molecules, the trypsin inhibitor from the seeds of squash Cucurbita maxima (CMTI-I, 436 atoms) and the ascaris trypsin inhibitor (912 atoms, two three-dimensional structures), will be presented in this paper. The reactive sites of the inhibitors are determined and explained. The accuracy of structure determination of the inhibitors are tested theoretically.
Ab Initio Calculations of the Electronic Structures and Biological Functions of Protein Molecules
NASA Astrophysics Data System (ADS)
Zheng, Haoping
2003-04-01
The self-consistent cluster-embedding (SCCE) calculation method reduces the computational effort from M3 to about M1 (M is the number of atoms in the system) with unchanged calculation precision. So the ab initio, all-electron calculation of the electronic structure and biological function of protein molecule becomes a reality, which will promote new proteomics considerably. The calculated results of two real protein molecules, the trypsin inhibitor from the seeds of squash Cucurbita maxima (CMTI-I, 436 atoms) and the Ascaris trypsin inhibitor (912 atoms, two three-dimensional structures), are presented. The reactive sites of the inhibitors are determined and explained. The precision of structure determination of inhibitors are tested theoretically.
Ogura, M; Takahashi, C; Akai, H
2007-09-12
The possibility of half-metallic diluted antiferromagnetic semiconductors of II-VI compounds is investigated on the basis of first-principles electronic structure calculation. The electronic structures of ZnS, ZnSe, ZnO, CdS and CdSe doped with two kinds of 3d transition metal ions are calculated using the Korringa-Kohn-Rostoker (KKR) method and their magnetic transition temperatures are determined using a cluster-type approximation. It is predicted that II-VI compound semiconductors doped with two kinds of magnetic ions might be good candidates for half-metallic antiferromagnets.
Ab-initio calculations of electronic structure and optical properties of TiAl alloy
NASA Astrophysics Data System (ADS)
Hussain, Altaf; Sikandar Hayat, Sardar; Choudhry, M. A.
2011-05-01
The electronic structures and optical properties of TiAl intermetallic alloy system are studied by the first-principle orthogonalized linear combination of atomic orbitals method. Results on the band structure, total and partial density of states, localization index, effective atomic charges, and optical conductivity are presented and discussed in detail. Total density of states spectra reveal that (near the Fermi level) the majority of the contribution is from Ti-3d states. The effective charge calculations show an average charge transfer of 0.52 electrons from Ti to Al in primitive cell calculations of TiAl alloy. On the other hand, calculations using supercell approach reveal an average charge transfer of 0.48 electrons from Ti to Al. The localization index calculations, of primitive cell as well as of supercell, show the presence of relatively localized states even above the Fermi level for this alloy. The calculated optical conductivity spectra of TiAl alloy are rich in structures, showing the highest peak at 5.73 eV for supercell calculations. Calculations of the imaginary part of the linear dielectric function show a prominent peak at 5.71 eV and a plateau in the range 1.1-3.5 eV.
Structure and properties of electronic and hole centers in CsBr from theoretical calculations
Halliday, Matthew T.; Hess, Wayne P.; Shluger, Alexander L.
2015-06-24
The electronic structure, geometry, diffusion barriers and optical properties of fundamental defects of CsBr are calculated using hybrid functional DFT and TD- DFT methods. The B3LYP functional with a modified exchange contribution has been used in an embedded cluster scheme to model the structure and spectroscopic properties of self-trapped triplet exciton, interstitial Br atoms and ions, self-trapped holes and Br vacancies. The calculated migration barriers and positions of maxima of optical absorption bands are in good agreement with experiment, justifying the obtained defect geometries. The o*-center triplet exciton luminescence energy is also accurately calculated.
Self-consistent GW calculation of the electronic structure of co-doped ZnO
NASA Astrophysics Data System (ADS)
Kim, Maengsuk; Park, Chul Hong
2012-01-01
The electronic structure of Co-doped ZnO is presented using a first-principles self-consistent GW calculation based on the screened hybrid HSE06 functional and is compared to the structure calculated using the generalized gradient density approximation plus U (GGA+U) method. The obtained energy splittings between unoccupied Co t 2 and the occupied Co e states are about 3.0 eV and 5.1 eV for the GGA+U and the HSE06 calculations, respectively. Through a correction of the self-consistent GW calculations on the top of HSE06, the electronic energy levels of the occupied Co e band states are moved downward slightly while those at the unoccupied Co t 2 bands are shifted upward, and the occupied Co e and the empty Co t 2 levels of the minority spin are located, respectively, far below and far above the conduction band minimum.
Dave, Mudra R.; Sharma, A. C.
2015-06-24
The structural, electronic and magnetic properties of free standing Au-Pd bimetallic atomic chain is studied using ab-initio method. It is found that electronic and magnetic properties of chains depend on position of atoms and number of atoms. Spin polarization factor for different atomic configuration of atomic chain is calculated predicting a half metallic behavior. It suggests a total spin polarised transport in these chains.
Kurova, N. V. Burdov, V. A.
2013-12-15
The results of ab initio calculations of the electronic structure of Si nanocrystals doped with shallow donors (Li, P) are reported. It is shown that phosphorus introduces much more significant distortions into the electronic structure of the nanocrystal than lithium, which is due to the stronger central cell potential of the phosphorus ion. It is found that the Li-induced splitting of the ground state in the conduction band of the nanocrystal into the singlet, doublet, and triplet retains its inverse structure typical for bulk silicon.
Aguiar, J; Asta, M; Gronbech-Jensen, N; Perlov, A; Milman, V; Gao, S; Pickard, C; Browning, N
2009-06-05
Energy loss spectra from a variety of cubic oxides are compared with ab-initio calculations based on the density functional plane wave method (CASTEP). In order to obtain agreement between experimental and theoretical spectra, unique material specific considerations were taken into account. The spectra were calculated using various approximations to describe core-hole effects and electronic correlations. All the calculations are based on the local spin density approximation to show qualitative agreement with the sensitive oxygen K-edge spectra in ceria, zirconia, and urania. Comparison of experimental and theoretical results let us characterize the main electronic interactions responsible for both the electronic structure and the resulting EEL spectra of the compounds in question.
Electronic Structure Calculations for Heavy Elements: Radon (Z=86) and Francium (Z=87)
NASA Astrophysics Data System (ADS)
Koufos, Alexander; Papaconstantopoulos, Dimitrios
2010-03-01
Electronic structure calculations allow scientists to predict the properties of solids without the use of physical material. Although the ability to manipulate matter has improved dramatically within the past couple decades, some matter is still hard to study. Modern computers not only let us study this matter, but allow us to do it more quickly and just as accurately. The electronic structure of two rare and mostly unstudied elements, Radon (Z=86) and Francium (Z=87), has been calculated. The augmented plane wave (APW) method with local density approximation (LDA) functional as well as the linearized augmented plane wave (LAPW) method with both LDA and generalized gradient approximation (GGA) functionals were used to perform the calculations. Francium total energy calculations gave the fcc structure slightly below the bcc structure with a minimal energy difference of δE=0.33mRy. The difference found is consistent with other alkali metal total energy calculations which do not verify the bcc structure to be the ground state. Radon was predicted to be an insulator with a gap of 0.931 Ry similar to the other noble gases.
Ab Initio Electronic Structure Calculations of Cytochrome P450 -- Ligand Interactions
NASA Astrophysics Data System (ADS)
Segall, M. D.; Payne, M. C.; Ellis, S. W.; Tucker, G. T.
1997-03-01
The Cytochrome P450 superfamily of enzymes are of great interest in pharmacology as they participate in an enormous range of physiological processes including drug deactivation and xenobiotic detoxification. We apply ab initio electronic structure calculations to model the interactions of the haem molecule at the P450 active site with substrate and inhibitor ligands. These calculations, based on density function theory, were performed with the CETEP code which uses a plane wave basis set and pseudopotentials to perform efficient LDA, GGA and spin dependent calculations. A change in the spin state of the haem iron atom is observed on binding of a substrate molecule, consistent with the accepted reaction mechanism.
NASA Astrophysics Data System (ADS)
Takaba, Hiromitsu; Kimura, Shou; Alam, Md. Khorshed
2017-03-01
Durability of organo-lead halide perovskite are important issue for its practical application in a solar cells. In this study, using density functional theory (DFT) and molecular dynamics, we theoretically investigated a crystal structure, electronic structure, and ionic diffusivity of the partially substituted cubic MA0.5X0.5PbI3 (MA = CH3NH3+, X = NH4+ or (NH2)2CH+ or Cs+). Our calculation results indicate that a partial substitution of MA induces a lattice distortion, resulting in preventing MA or X from the diffusion between A sites in the perovskite. DFT calculations show that electronic structures of the investigated partially substituted perovskites were similar with that of MAPbI3, while their bandgaps slightly decrease compared to that of MAPbI3. Our results mean that partial substitution in halide perovskite is effective technique to suppress diffusion of intrinsic ions and tune the band gap.
Relativistic atomic structure calculations and electron impact excitations of Fe23+
NASA Astrophysics Data System (ADS)
El-Maaref, A. A.
2016-02-01
Relativistic calculations using the multiconfiguration Dirac-Fock method for energy levels, oscillator strengths, and electronic dipole transition probabilities of Li-like iron (Fe23+) are presented. A configuration state list with the quantum numbers nl, where n = 2 - 7 and l = s , p , d , f , g , h , i has been considered. Excitations up to three electrons and correlation contributions from higher orbitals up to 7 l have been included. Contributions from core levels have been taken into account, EOL (extended optimal level) type calculations have been applied, and doubly excited levels are considered. The calculations have been executed by using the fully relativistic atomic structure package GRASP2K. The present calculations have been compared with the available experimental and theoretical sources, the comparisons show a good agreement between the present results of energy levels and oscillator strengths with the literature. In the second part of the present study, the atomic data (energy levels, and radiative parameters) have been used to calculate the excitation and deexcitation rates of allowed transitions by electron impact, as well as the population densities of some excited levels at different electron temperatures.
Density functional calculation of the structural and electronic properties of germanium quantum dots
Anas, M. M.; Gopir, G.
2015-04-24
We apply first principles density functional computational methods to study the structures, densities of states (DOS), and higher occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) gaps of selected free-standing Ge semiconductor quantum dots up to 1.8nm. Our calculations are performed using numerical atomic orbital approach where linear combination of atomic orbital was applied. The surfaces of the quantum dots was passivized by hydrogen atoms. We find that surface passivation does affect the electronic properties associated with the changes of surface state, electron localization, and the energy gaps of germanium nanocrystals as well as the confinement of electrons inside the quantum dots (QDs). Our study shows that the energy gaps of germanium quantum dots decreases with the increasing dot diameter. The size-dependent variations of the computed HOMO-LUMO gaps in our quantum dots model were found to be consistent with the effects of quantum confinement reported in others theoretical and experimental calculation.
Density functional calculation of the structural and electronic properties of germanium quantum dots
NASA Astrophysics Data System (ADS)
Anas, M. M.; Gopir, G.
2015-04-01
We apply first principles density functional computational methods to study the structures, densities of states (DOS), and higher occupied molecular orbital (HOMO) - lowest unoccupied molecular orbital (LUMO) gaps of selected free-standing Ge semiconductor quantum dots up to 1.8nm. Our calculations are performed using numerical atomic orbital approach where linear combination of atomic orbital was applied. The surfaces of the quantum dots was passivized by hydrogen atoms. We find that surface passivation does affect the electronic properties associated with the changes of surface state, electron localization, and the energy gaps of germanium nanocrystals as well as the confinement of electrons inside the quantum dots (QDs). Our study shows that the energy gaps of germanium quantum dots decreases with the increasing dot diameter. The size-dependent variations of the computed HOMO-LUMO gaps in our quantum dots model were found to be consistent with the effects of quantum confinement reported in others theoretical and experimental calculation.
NASA Astrophysics Data System (ADS)
Xu, C.; Li, Q.; Liu, C. M.; Duan, M. Y.; Wang, H. K.
2016-05-01
First-principles calculations are employed to investigate the structural and elastic properties, formation enthalpies and chemical bonding features as well as hardness values of chromium tetraboride (CrB4) with different structures. The lattice parameters, Poisson’s ratio and B/G ratio are also derived. Our calculations indicate that the orthorhombic structure with Pnnm symmetry is the most energetically stable one for CrB4. Except for WB4P63/mmc structure with imaginary frequencies, another six new structures are investigated through the full phonon dispersion calculations. Their mechanical and thermodynamic stabilities are also studied by calculating the elastic constants and formation enthalpies. Our calculations show that the thermodynamic stabilities of all these CrB4 phases can be enhanced under high pressure. The large shear moduli, Young’s moduli and hardness values indicate that these CrB4 phases are potential hard materials. Analyses of the densities of states (DOSs) and electron localization functions (ELFs) provide further understandings of the chemical and physical properties of these CrB4 phases. It is observed that the large occupations and high strengths of the B-B covalent bonds are important for the stabilities, incompressibility and hardnesses of these CrB4 phases.
Davis, Sergio; Gutiérrez, Gonzalo
2011-12-14
First-principles molecular dynamics calculations of the structural, elastic, vibrational and electronic properties of amorphous Al(2)O(3), in a system consisting of a supercell of 80 atoms, are reported. A detailed analysis of the interatomic correlations allows us to conclude that the short-range order is mainly composed of AlO(4) tetrahedra, but, in contrast with previous results, also an important number of AlO(6) octahedra and AlO(5) units are present. The vibrational density of states presents two frequency bands, related to bond-bending and bond-stretching modes. It also shows other recognizable features present in similar amorphous oxides. We also present the calculation of elastic properties (bulk modulus and shear modulus). The calculated electronic structure of the material, including total and partial electronic density of states, charge distribution, electron localization function and the ionicity for each species, gives evidence of correlation between the ionicity and the coordination for each Al atom.
The structural and electronic properties of amorphous HgCdTe from first-principles calculations
NASA Astrophysics Data System (ADS)
Zhao, Huxian; Chen, Xiaoshuang; Lu, Jianping; Shu, Haibo; Lu, Wei
2014-01-01
Amorphous mercury cadmium telluride (a-MCT) model structures, with x being 0.125 and 0.25, are obtained from first-principles calculations. We generate initial structures by computation alchemy method. It is found that most atoms in the network of amorphous structures tend to be fourfold and form tetrahedral structures, implying that the chemical ordered continuous random network with some coordination defects is the ideal structure for a-MCT. The electronic structure is also concerned. The gap is found to be 0.30 and 0.26 eV for a-Hg0.875Cd0.125Te and a-Hg0.75Cd0.25Te model structures, independent of the composition. By comparing with the properties of crystalline MCT with the same composition, we observe a blue-shift of energy band gap. The localization of tail states and its atomic origin are also discussed.
Efficient electronic structure calculation for molecular ionization dynamics at high x-ray intensity
Hao, Yajiang; Inhester, Ludger; Hanasaki, Kota; Son, Sang-Kil; Santra, Robin
2015-01-01
We present the implementation of an electronic-structure approach dedicated to ionization dynamics of molecules interacting with x-ray free-electron laser (XFEL) pulses. In our scheme, molecular orbitals for molecular core-hole states are represented by linear combination of numerical atomic orbitals that are solutions of corresponding atomic core-hole states. We demonstrate that our scheme efficiently calculates all possible multiple-hole configurations of molecules formed during XFEL pulses. The present method is suitable to investigate x-ray multiphoton multiple ionization dynamics and accompanying nuclear dynamics, providing essential information on the chemical dynamics relevant for high-intensity x-ray imaging. PMID:26798806
Wills, John M; Mattsson, Ann E
2012-06-06
Brooks, Johansson, and Skriver, using the LMTO-ASA method and considerable insight, were able to explain many of the ground state properties of the actinides. In the many years since this work was done, electronic structure calculations of increasing sophistication have been applied to actinide elements and compounds, attempting to quantify the applicability of DFT to actinides and actinide compounds and to try to incorporate other methodologies (i.e. DMFT) into DFT calculations. Through these calculations, the limits of both available density functionals and ad hoc methodologies are starting to become clear. However, it has also become clear that approximations used to incorporate relativity are not adequate to provide rigorous tests of the underlying equations of DFT, not to mention ad hoc additions. In this talk, we describe the result of full-potential LMTO calculations for the elemental actinides, comparing results obtained with a full Dirac basis with those obtained from scalar-relativistic bases, with and without variational spin-orbit. This comparison shows that the scalar relativistic treatment of actinides does not have sufficient accuracy to provide a rigorous test of theory and that variational spin-orbit introduces uncontrolled errors in the results of electronic structure calculations on actinide elements.
A novel Gaussian-Sinc mixed basis set for electronic structure calculations
Jerke, Jonathan L.; Lee, Young; Tymczak, C. J.
2015-08-14
A Gaussian-Sinc basis set methodology is presented for the calculation of the electronic structure of atoms and molecules at the Hartree–Fock level of theory. This methodology has several advantages over previous methods. The all-electron electronic structure in a Gaussian-Sinc mixed basis spans both the “localized” and “delocalized” regions. A basis set for each region is combined to make a new basis methodology—a lattice of orthonormal sinc functions is used to represent the “delocalized” regions and the atom-centered Gaussian functions are used to represent the “localized” regions to any desired accuracy. For this mixed basis, all the Coulomb integrals are definable and can be computed in a dimensional separated methodology. Additionally, the Sinc basis is translationally invariant, which allows for the Coulomb singularity to be placed anywhere including on lattice sites. Finally, boundary conditions are always satisfied with this basis. To demonstrate the utility of this method, we calculated the ground state Hartree–Fock energies for atoms up to neon, the diatomic systems H{sub 2}, O{sub 2}, and N{sub 2}, and the multi-atom system benzene. Together, it is shown that the Gaussian-Sinc mixed basis set is a flexible and accurate method for solving the electronic structure of atomic and molecular species.
Svane, A.; Trygg, J.; Johansson, B.; Eriksson, O. |
1997-09-01
Electronic-structure calculations of elemental praseodymium are presented. Several approximations are used to describe the Pr f electrons. It is found that the low-pressure, trivalent phase is well described using either the self-interaction corrected (SIC) local-spin-density (LSD) approximation or the generalized-gradient approximation (GGA) with spin and orbital polarization (OP). In the SIC-LSD approach the Pr f electrons are treated explicitly as localized with a localization energy given by the self-interaction of the f orbital. In the GGA+OP scheme the f-electron localization is described by the onset of spin and orbital polarization, the energetics of which is described by spin-moment formation energy and a term proportional to the total orbital moment, L{sub z}{sup 2}. The high-pressure phase is well described with the f electrons treated as band electrons, in either the LSD or the GGA approximations, of which the latter describes more accurately the experimental equation of state. The calculated pressure of the transition from localized to delocalized behavior is 280 kbar in the SIC-LSD approximation and 156 kbar in the GGA+OP approach, both comparing favorably with the experimentally observed transition pressure of 210 kbar. {copyright} {ital 1997} {ital The American Physical Society}
NASA Astrophysics Data System (ADS)
Makode, Chandrabhan; Sanyal, Sankar P.
2011-09-01
We have investigated the structural and electronic properties of monophospides of thorium, uranium and neptunium. The total energy as a function of volume is obtained by means of the self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From the present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl-type structure at ambient pressure. The structural stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B 1-phase) structure to CsCl-type (B 2-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP-NpP). We also calculate lattice parameter ( a0), bulk modulus ( B0), band structure and density of states. From energy band diagram it is observed that ThP, UP and NpP exhibit metallic behavior. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.
Ab initio calculation of structural stability, electronic and optical properties of Ag2Se
NASA Astrophysics Data System (ADS)
Rameshkumar, S.; Jaiganesh, G.; Jayalakshmi, V.; Palanivel, B.
2015-06-01
The structural stability, electronic and optical properties of Ag2Se compound is studied using ab initio packages. Ag2Se is found to crystallize in orthorhombic structure with two different space groups i.e. P212121 (No. 19) and P2221 (No. 17). For this compound in these two space groups, the total energy has been computed as a function of volume. Our calculated results suggest that the P212121-phase is more stable than that of the P2221-phase. The band structure calculation show that Ag2Se is semimetallic with an overlap of about 0.014 eV in P212121-phase whereas is metallic in nature in P2221-phase. Moreover, the optical properties including the dielectric fuction, energy loss spectrum are obtained and analysed.
Electronic structure and excitations in oxygen deficient CeO2-δ from DFT calculations
NASA Astrophysics Data System (ADS)
Jarlborg, T.; Barbiellini, B.; Lane, C.; Wang, Yung Jui; Markiewicz, R. S.; Liu, Zhi; Hussain, Zahid; Bansil, A.
2014-04-01
The electronic structures of supercells of CeO2-δ have been calculated within the density functional theory (DFT). The equilibrium properties such as lattice constants, bulk moduli, and magnetic moments are well reproduced by the generalized gradient approximation (GGA). Electronic excitations are simulated by robust total-energy calculations for constrained states with atomic core holes or valence holes. Pristine ceria CeO2 is found to be a nonmagnetic insulator with magnetism setting in as soon as oxygens are removed from the structure. In the ground state of defective ceria, the Ce-f majority band resides near the Fermi level but appears at about 2 eV below the Fermi level in photoemission spectroscopy experiments due to final-state effects. We also tested our computational method by calculating threshold energies in Ce-M5 and O-K x-ray absorption spectroscopy and comparing theoretical predictions with the corresponding measurements. Our result that f electrons reside near the Fermi level in the ground state of oxygen-deficient ceria is crucial for understanding the catalytic properties of CeO2 and related materials.
Electronic structure and defect properties of Tl6SeI4: Density functional calculations
NASA Astrophysics Data System (ADS)
Biswas, Koushik; Du, Mao-Hua; Singh, David J.
2012-10-01
We report density functional calculations of electronic structure, phase diagram, and dielectric, optical, and defect properties of Tl6SeI4. We discuss how electronic structure and defect properties affect resistivity and carrier mobility-lifetime (μτ) products in Tl6SeI4. We find large Born effective charges due to covalency involving Tl-6p states. High Born charges generally enhance the static dielectric constant. This provides a mechanism for effective screening of charged defects and impurities. We find that high resistivity can be obtained under near-stoichiometric growth conditions via Fermi level pinning near the middle of the band gap by shallow donors and acceptors, as opposed to deep traps that can give high resistivity, but at the expense of short carrier drift lengths. Defect calculations also reveal the presence of deep native donors that may cause electron trapping. The experimentally observed good μτ products may be explained by a combination of small effective masses and effective screening of charged defects. High resistivity and good μτ products make Tl6SeI4 a promising room-temperature radiation detector material. We also show the calculated defect diffusion barriers, which affect defect migration under external bias in a detector.
NASA Astrophysics Data System (ADS)
Petrova, N. V.; Yakovkin, I. N.
The electronic band structure, density of states (DOS) and interlayer interaction in Li-intercalated graphene bilayers are studied by means of density functional theory (DFT) calculations. It has been found that for a pristine bilayer, the relative shift of graphene layers from AB stacking configuration, pertinent to a bulk graphite, to AA configuration results in the opening of the bandgap at Fermi level, so that the bilayer becomes a semiconductor. The Li intercalation of the graphene bilayer significantly increases the density of states at Fermi level, which can be considered as an increased metallicity. The electronic density in the space between graphene layers also substantially increases and leads to related increase of the interlayer interaction. We hope that the obtained results of calculations will be useful for various applications of Li-intercalated graphene layers in nanoelectronics.
Near-edge structures from first principles all-electron Bethe-Salpeter equation calculations.
Olovsson, W; Tanaka, I; Puschnig, P; Ambrosch-Draxl, C
2009-03-11
We obtain x-ray absorption near-edge structures (XANES) by solving the equation of motion for the two-particle Green's function for the electron-hole pair, the Bethe-Salpeter equation (BSE), within the all-electron full-potential linearized augmented plane wave method (FPLAPW). The excited states are calculated for the Li K-edge in the insulating solids LiF, Li(2)O and Li(2)S, and absorption spectra are compared with independent particle results using the random phase approximation (RPA), as well as supercell calculations using the core-hole approximation within density functional theory (DFT). The binding energies of strongly bound excitations are determined in the materials, and core-exciton wavefunctions are demonstrated for LiF.
Ab initio calculations on twisted graphene/hBN: Electronic structure and STM image simulation
NASA Astrophysics Data System (ADS)
Correa, J. D.; Cisternas, E.
2016-09-01
By performing ab initio calculations we obtained theoretical scanning tunneling microscopy (STM) images and studied the electronic properties of graphene on a hexagonal boron-nitrite (hBN) layer. Three different stack configurations and four twisted angles were considered. All calculations were performed using density functional theory, including van der Waals interactions as implemented in the SIESTA ab initio package. Our results show that the electronic structure of graphene is preserved, although some small changes are induced by the interaction with the hBN layer, particularly in the total density of states at 1.5 eV under the Fermi level. When layers present a twisted angle, the density of states shows several van Hove singularities under the Fermi level, which are associated to moiré patterns observed in theoretical STM images.
The Study of Surface Diffusion and Growth Phenomena Using Electronic Structure Calculations.
NASA Astrophysics Data System (ADS)
Kaxiras, Efthimios
1996-03-01
Diffusion and growth phenomena on semiconductor surfaces represent one of the most challenging problems in the theory of materials. At the core of these phenomena are issues of kinetics and of the thermodynamic stability of surface structures. Such complex issues can be addressed accurately only through the use of first-principles electronic structure calculations in the framework of density functional theory. For realistic systems, these calculations are computationally demanding, but they provide a reliable description of the energetics and the electronic properties. In addition to prototypical systems like Si or Ge, the calculations can also handle successfully variations in the chemical composition, such as the presence of adsorbates (which can affect significantly both the kinetics and the equilibrium geometries on a surface). The results of these calculations can be combined with stochastic simulations and simple phenomenological models to provide direct comparison to experiment. We will illustrate the ability of this theoretical approach to tackle realistic problems of technological importance and to make predictions on the behavior of complicated systems, through several examples, including passivation of surfaces, surfactant mediated growth, and electromigration on stepped surfaces ( In collaboration with D. Kandel. This work was supported by ONR, Contract#N00014-95-1-0350. ).
Cao Jun; Fang Weihai; Fang Qiu
2011-01-28
In the present paper, different electronic structure methods have been used to determine stationary and intersection structures on the ground (S{sub 0}) and {sup 1}{pi}{pi}* (S{sub 2}) states of 4-methylpyridine, which is followed by adiabatic and nonadiabatic dynamics simulations to explore the mechanistic photoisomerization of 4-methylpyridine. Photoisomerization starts from the S{sub 2}({sup 1}{pi}{pi}*) state and overcomes a small barrier, leading to formation of the prefulvene isomer in the S{sub 0} state via a S{sub 2}/S{sub 0} conical intersection. The ultrafast S{sub 2}{yields} S{sub 0} nonradiative decay and low quantum yield for the photoisomerization reaction were well reproduced by the combined electronic structure calculation and dynamics simulation. The prefulvene isomer was assigned as a long-lived intermediate and suggested to isomerize to 4-methylpyridine directly in the previous study, which is not supported by the present calculation. The nonadiabatic dynamics simulation and electronic structure calculation reveal that the prefulvene isomer is a short-lived intermediate and isomerizes to benzvalene form very easily. The benzvalene form was predicted as the stable isomer in the present study and is probably the long-lived intermediate observed experimentally. A consecutive light and thermal isomerization cycle via Dewar isomer was determined and this cycle mechanism is different from that reported in the previous study. It should be pointed out that formation of Dewar isomer from the S{sub 2}({sup 1}{pi}{pi}*) state is not in competition with the isomerization to the prefulvene form. The Dewar structure observed experimentally may originate from other excited states.
NASA Astrophysics Data System (ADS)
Roondhe, Basant; Upadhyay, Deepak; Som, Narayan; Pillai, Sharad B.; Shinde, Satyam; Jha, Prafulla K.
2017-01-01
The structural, electronic, dynamical and thermodynamical properties of CmX (X = N, P, As, Sb, and Bi) compounds are studied using first principles calculations within density functional theory. The Perdew-Burke-Ernzerhof spin polarized generalized gradient approximation and Perdew-Wang (PW) spin polarized local density approximation as the exchange correlational functionals are used in these calculations. There is a good agreement between the present and previously reported data. The calculated electronic density of states suggests that the curium monopnictides are metallic in nature, which is consistent with earlier studies. The significant values of magnetic moment suggest their magnetic nature. The phonon dispersion curves and phonon density of states are also calculated, which depict the dynamical stability of these compounds. There is a significant separation between the optical and acoustical phonon branches. The temperature dependence of the thermodynamical functions are also calculated and discussed. Internal energy and vibrational contribution to the Helmholtz free energy increases and decreases, respectively, with temperature. The entropy increases with temperature. The specific heat at constant volume and Debye temperature obey Debye theory. The temperature variation of the considered thermodynamical functions is in line with those of other crystalline solids.
NASA Astrophysics Data System (ADS)
Roondhe, Basant; Upadhyay, Deepak; Som, Narayan; Pillai, Sharad B.; Shinde, Satyam; Jha, Prafulla K.
2017-03-01
The structural, electronic, dynamical and thermodynamical properties of CmX (X = N, P, As, Sb, and Bi) compounds are studied using first principles calculations within density functional theory. The Perdew-Burke-Ernzerhof spin polarized generalized gradient approximation and Perdew-Wang (PW) spin polarized local density approximation as the exchange correlational functionals are used in these calculations. There is a good agreement between the present and previously reported data. The calculated electronic density of states suggests that the curium monopnictides are metallic in nature, which is consistent with earlier studies. The significant values of magnetic moment suggest their magnetic nature. The phonon dispersion curves and phonon density of states are also calculated, which depict the dynamical stability of these compounds. There is a significant separation between the optical and acoustical phonon branches. The temperature dependence of the thermodynamical functions are also calculated and discussed. Internal energy and vibrational contribution to the Helmholtz free energy increases and decreases, respectively, with temperature. The entropy increases with temperature. The specific heat at constant volume and Debye temperature obey Debye theory. The temperature variation of the considered thermodynamical functions is in line with those of other crystalline solids.
First principles calculations of structural, electronic and optical properties of InN compound
NASA Astrophysics Data System (ADS)
Graine, R.; Chemam, R.; Gasmi, F. Z.; Nouri, R.; Meradji, H.; Khenata, R.
2015-11-01
We carried out ab initio calculations of structural, electronic and optical properties of Indium nitride (InN) compound in both zinc blende and wurtzite phases, using the full-potential linearized augmented plane wave method (FP-LAPW), within the framework of density functional theory (DFT). For the exchange and correlation potential, local density approximation (LDA) and generalized gradient approximation (GGA) were used. Moreover, the alternative form of GGA proposed by Engel and Vosko (EV-GGA) and modified Becke-Johnson schemes (mBJ) were also applied for band structure calculations. Ground state properties such as lattice parameter, bulk modulus and its pressure derivative are calculated. Results obtained for band structure of these compounds have been compared with experimental results as well as other first principle computations. Our results show good agreement with the available data. The calculated band structure shows a direct band gap Γ → Γ. In the optical properties section, several optical quantities are investigated; in particular we have deduced the interband transitions from the imaginary part of the dielectric function.
NASA Astrophysics Data System (ADS)
Pardo, Victor; Lado, Jose L.
2015-03-01
Ab initio calculations have been performed in 5d5-electron-based oxides in the large spin-orbit coupling limit. Our work tries to analyze the effects of strain and dimensionality in the electronic structure properties of iridates with Ir4+:5d5 electronic configuration in order to understand the different set of properties these materials present: they can be either metals or insulators, e.g. We focus on studying how close to the fully ionic jeff=1/2 limit the system is by analyzing the Lz/Sz ratio. We observe that it varies continuously as a function of strain or pressure, changing drastically with relatively small variations. We also analyze what effects on the band structure accompany this variation. In order to do this, we needed to include a full non-collinearity in the calculation of spin-orbit interaction. We have explored SrIrO3, Sr2IrO4, Sr3Ir2O7, thin films of SrIrO3 so as to analyze the dimensionality effects and the structural implications. We acknowledge support of the MINECO through the Ramon y Cajal Program, Xunta de Galicia through Project No. EM2013/037 and the EU through the Marie-Curie ITN ``Spinograph''.
Electronic structure of cubic ScF3 from first-principles calculations
NASA Astrophysics Data System (ADS)
Bocharov, D.; Žguns, P.; Piskunov, S.; Kuzmin, A.; Purans, J.
2016-07-01
The ground state properties of cubic scandium trifluoride (ScF3) perovskite were studied using first-principles calculations. The electronic structure of ScF3 was determined by linear combination of atomic orbital (LCAO) and plane wave projector augmented-wave (PAW) methods using modified hybrid exchange-correlation functionals within the density functional theory (DFT). The comprehensive comparison of the results obtained by two methods is presented. Both methods allowed us to reproduce the lattice constant found experimentally in ScF3 at low temperatures and to predict its electronic structure in good agreement with known experimental valence-band photoelectron and F 1s x-ray absorption spectra.
Structural and electronic phase transitions of ThS2 from first-principles calculations
NASA Astrophysics Data System (ADS)
Guo, Yongliang; Wang, Changying; Qiu, Wujie; Ke, Xuezhi; Huai, Ping; Cheng, Cheng; Zhu, Zhiyuan; Chen, Changfeng
2016-10-01
Thorium and its compounds have received considerable attention in recent years due to the renewed interest in developing the thorium fuel cycle as an alternative nuclear energy technology. There is pressing current need to explore the physical properties essential to the fundamental understanding and practical application of these materials. Here we report on a computational study of thorium disulfide (ThS2), which plays an important role in the thorium fuel reprocessing cycle. We have employed the density functional theory and evolutionary structure search methods to determine the crystal structures, electronic band structures, phonon dispersions and density of states, and thermodynamic properties of ThS2 under various pressure and temperature conditions. Our calculations identify several crystalline phases of ThS2 and a series of structural phase transitions induced by pressure and temperature. The calculated results also reveal electronic phase transitions from the semiconducting state in the low-pressure phases of ThS2 in the P n m a and F m 3 ¯m symmetry to the metallic state in the high-pressure phases of ThS2 in the P n m a and I 4 /m m m symmetry. These results explain the experimental observation of the thermodynamic stability of the P n m a phase of ThS2 at the ambient conditions and a pressure-induced structural phase transition in ThS2 around 40 GPa. Moreover, the present study reveals considerable additional information on the structural and electronic properties of ThS2 in a wide range of pressure and temperature. Such information provides key insights into the fundamental material behavior and the underlying mechanisms that lay the foundation for further exploration and application of ThS2.
Genovese, Luigi; Deutsch, Thierry
2015-12-21
Discretizing an analytic function on a uniform real-space grid is often done via a straightforward collocation method. This is ubiquitous in all areas of computational physics and quantum chemistry. An example in density functional theory (DFT) is given by the external potential or the pseudo-potential describing the interaction between ions and electrons. The accuracy of the collocation method used is therefore very important for the reliability of subsequent treatments like self-consistent field solutions of the electronic structure problems. By construction, the collocation method introduces numerical artifacts typical of real-space treatments, like the so-called egg-box error, which may spoil the numerical stability of the description when the real-space grid is too coarse. As the external potential is an input of the problem, even a highly precise computational treatment cannot cope this inconvenience. We present in this paper a new quadrature scheme that is able to exactly preserve the moments of a given analytic function even for large grid spacings, while reconciling with the traditional collocation method when the grid spacing is small enough. In the context of real-space electronic structure calculations, we show that this method improves considerably the stability of the results for large grid spacings, opening up the path towards reliable low-accuracy DFT calculations with a reduced number of degrees of freedom.
Vibrational structure of defect luminescence bands in GaN from electronic structure calculations
NASA Astrophysics Data System (ADS)
Alkauskas, Audrius; van de Walle, Chris G.
2012-02-01
Optical methods are among the most powerful to characterize defects in materials. The study of optical signatures based on state-of-the-art electronic structure methods is therefore very important. In this work we investigate the vibrational structure of luminescence bands pertaining to deep defect levels in GaN. Since luminescence lineshapes depend sensitively on defect geometries and vibrational frequencies, these should be described accurately. The latter is achieved through the use of hybrid density functionals. Both quasi-localized and bulk phonons are included in our description. In the case of transitions accompanied by very large lattice relaxations, anharmonic effects become sizeable, and these are also accounted for. For the defects studied a very good agreement with available experimental data is achieved. For instance, in the case of wide luminescence bands the resulting line widths are within 0.05 eV of the experimental values. This work was supported by the Swiss NSF and by NSF.
A Linear Scaling Three Dimensional Fragment Method for Large ScaleElectronic Structure Calculations
Wang, Lin-Wang; Zhao, Zhengji; Meza, Juan
2007-07-26
We present a novel linear scaling ab initio total energyelectronic structure calculation method, which is simple to implement,easily to parallelize, and produces essentially thesame results as thedirect ab initio method, while it could be thousands of times faster.Using this method, we have studied the dipole moments of CdSe quantumdots, and found both significant bulk and surface contributions. The bulkdipole contribution cannot simply be estimated from the bulk spontaneouspolarization value by a proportional volume factor. Instead it has ageometry dependent screening effect. The dipole moment also produces astrong internal electric field which induces a strong electron holeseparation.
NASA Astrophysics Data System (ADS)
Seiler, Christian; Evers, Ferdinand
2016-10-01
A formalism for electronic-structure calculations is presented that is based on the functional renormalization group (FRG). The traditional FRG has been formulated for systems that exhibit a translational symmetry with an associated Fermi surface, which can provide the organization principle for the renormalization group (RG) procedure. We here advance an alternative formulation, where the RG flow is organized in the energy-domain rather than in k space. This has the advantage that it can also be applied to inhomogeneous matter lacking a band structure, such as disordered metals or molecules. The energy-domain FRG (ɛ FRG) presented here accounts for Fermi-liquid corrections to quasiparticle energies and particle-hole excitations. It goes beyond the state of the art G W -BSE , because in ɛ FRG the Bethe-Salpeter equation (BSE) is solved in a self-consistent manner. An efficient implementation of the approach that has been tested against exact diagonalization calculations and calculations based on the density matrix renormalization group is presented. Similar to the conventional FRG, also the ɛ FRG is able to signalize the vicinity of an instability of the Fermi-liquid fixed point via runaway flow of the corresponding interaction vertex. Embarking upon this fact, in an application of ɛ FRG to the spinless disordered Hubbard model we calculate its phase boundary in the plane spanned by the interaction and disorder strength. Finally, an extension of the approach to finite temperatures and spin S =1 /2 is also given.
Linearly Scaling 3D Fragment Method for Large-Scale Electronic Structure Calculations
Wang, Lin-Wang; Lee, Byounghak; Shan, Hongzhang; Zhao, Zhengji; Meza, Juan; Strohmaier, Erich; Bailey, David H.
2008-07-01
We present a new linearly scaling three-dimensional fragment (LS3DF) method for large scale ab initio electronic structure calculations. LS3DF is based on a divide-and-conquer approach, which incorporates a novel patching scheme that effectively cancels out the artificial boundary effects due to the subdivision of the system. As a consequence, the LS3DF program yields essentially the same results as direct density functional theory (DFT) calculations. The fragments of the LS3DF algorithm can be calculated separately with different groups of processors. This leads to almost perfect parallelization on tens of thousands of processors. After code optimization, we were able to achieve 35.1 Tflop/s, which is 39percent of the theoretical speed on 17,280 Cray XT4 processor cores. Our 13,824-atom ZnTeO alloy calculation runs 400 times faster than a direct DFTcalculation, even presuming that the direct DFT calculation can scale well up to 17,280 processor cores. These results demonstrate the applicability of the LS3DF method to material simulations, the advantage of using linearly scaling algorithms over conventional O(N3) methods, and the potential for petascale computation using the LS3DF method.
NASA Astrophysics Data System (ADS)
Jin, Na; Yang, Yanqing; Luo, Xian; Li, Jian; Huang, Bin; Liu, Shuai; Xiao, Zhiyuan
2014-09-01
The β-SiC(1 1 1)/α-W(1 1 0) interfaces were studied by first-principles calculations based on density functional theory (DFT). The ideal work of adhesion (Wad) and interface energy (γint) were calculated for six different interfacial structures, taking into account both Si- and C-terminations of β-SiC(1 1 1) surfaces, and three different stacking sequences. The interfacial electronic structures including charge density distribution and difference, and density of states (DOS) were simulated to determine the nature of SiC/W bonding. The results show that the Si-terminated top-site interface is the most stable interface, yielding the highest Wad and the lowest γint. During the optimization, the Si-terminated top-site interface will transform into the center-site structure, resulting in the interaction among the interfacial W and Si atoms, and subinterfacial C atoms. In addition, the calculated interface energies show that an interdiffusion layer will form on the SiC/W interface. The experimental results also have verified the existence of an interdiffusion layer on the SiC/W interface in a CVD-SiC fiber.
Daubechies wavelets for high performance electronic structure calculations: The BigDFT project
NASA Astrophysics Data System (ADS)
Genovese, Luigi; Videau, Brice; Ospici, Matthieu; Deutsch, Thierry; Goedecker, Stefan; Méhaut, Jean-François
2011-02-01
In this contribution we will describe in detail a Density Functional Theory method based on a Daubechies wavelets basis set, named BigDFT. We will see that, thanks to wavelet properties, this code shows high systematic convergence properties, very good performances and an excellent efficiency for parallel calculations. BigDFT code operation are also well-suited for GPU acceleration. We will discuss how the problematic of fruitfully benefit of this new technology can be match with the needs of robustness and flexibility of a complex code like BigDFT. This work may be of interest not only for expert in electronic structure calculations, but may also provide feedback to the wider community of high performance scientific computing.
NASA Astrophysics Data System (ADS)
Cao, Jun; Xie, Zhi-Zhong; Yu, Xiaodong
2016-08-01
In the present work, the combined electronic structure calculations and surface hopping simulations have been performed to investigate the excited-state decay of the parent oxazole in the gas phase. Our calculations show that the S2 state decay of oxazole is an ultrafast process characterized by the ring-opening and ring-closure of the five-membered oxazole ring, in which the triplet contribution is minor. The ring-opening involves the Osbnd C bond cleavage affording the nitrile ylide and airine intermediates, while the ring-closure gives rise to a bicyclic species through a 2sbnd 5 bond formation. The azirine and bicyclic intermediates in the S0 state are very likely involved in the phototranspositions of oxazoles. This is different from the previous mechanism in which these intermediates in the T1 state have been proposed for these phototranspositions.
Cai, Yunfeng; Bai, Zhaojun; Pask, John E.; Sukumar, N.
2013-12-15
The iterative diagonalization of a sequence of large ill-conditioned generalized eigenvalue problems is a computational bottleneck in quantum mechanical methods employing a nonorthogonal basis for ab initio electronic structure calculations. We propose a hybrid preconditioning scheme to effectively combine global and locally accelerated preconditioners for rapid iterative diagonalization of such eigenvalue problems. In partition-of-unity finite-element (PUFE) pseudopotential density-functional calculations, employing a nonorthogonal basis, we show that the hybrid preconditioned block steepest descent method is a cost-effective eigensolver, outperforming current state-of-the-art global preconditioning schemes, and comparably efficient for the ill-conditioned generalized eigenvalue problems produced by PUFE as the locally optimal block preconditioned conjugate-gradient method for the well-conditioned standard eigenvalue problems produced by planewave methods.
NASA Astrophysics Data System (ADS)
Fritsch, Daniel; Morgan, Benjamin J.; Walsh, Aron
2017-01-01
The development of new exchange-correlation functionals within density functional theory means that increasingly accurate information is accessible at moderate computational cost. Recently, a newly developed self-consistent hybrid functional has been proposed (Skone et al., Phys. Rev. B 89:195112, 2014), which allows for a reliable and accurate calculation of material properties using a fully ab initio procedure. Here, we apply this new functional to wurtzite ZnO, rutile SnO2, and rocksalt MgO. We present calculated structural, electronic, and optical properties, which we compare to results obtained with the PBE and PBE0 functionals. For all semiconductors considered here, the self-consistent hybrid approach gives improved agreement with experimental structural data relative to the PBE0 hybrid functional for a moderate increase in computational cost, while avoiding the empiricism common to conventional hybrid functionals. The electronic properties are improved for ZnO and MgO, whereas for SnO2 the PBE0 hybrid functional gives the best agreement with experimental data.
Brgoch, Jakoah; Goerens, Christian; Fokwa, Boniface P T; Miller, Gordon J
2011-05-04
The electronic structures of "Ti(9-n)Fe(2+n)Ru(18)B(8)" (n=0, 0.5, 1, 2, 3), in connection to the recently synthesized Ti(9-n)Fe(2+n)Ru(18)B(8) (n=1, 2), have been investigated and analyzed using LSDA tight-binding calculations to elucidate the distribution of Fe and Ti, to determine the maximum Fe content, and to explore possible magnetic structures to interpret experimental magnetization results. Through a combination of calculations on specific models and using the rigid band approximation, which is validated by the DOS curves for "Ti(9-n)Fe(2+n)Ru(18)B(8)" (n=0, 0.5, 1, 2, 3), mixing of Fe and Ti is anticipated at both the 2b- and 4h-chain sites. The model "Ti(8.5)Fe(2.5)Ru(18)B(8)" (n=0.5) revealed that both Brewer-type Ti-Ru interactions as well as ligand field splitting of the Fe 3d orbitals regulated the observed valence electron counts between 220 and 228 electrons/formula unit. Finally, models of magnetic structures were created using "Ti(6)Fe(5)Ru(18)B(8)" (n=3). A rigid band analysis of the LSDA DOS curves concluded preferred ferromagnetic ordering at low Fe content (n≤0.75) and ferrimagnetic ordering at higher Fe content (n>0.75). Ferrimagnetism arises from antiferromagnetic exchange coupling in the scaffold of Fe1-ladder and 4h-chain sites.
Density functional calculations of the vibronic structure of electronic absorption spectra.
Dierksen, Marc; Grimme, Stefan
2004-02-22
Calculations of the vibronic structure in electronic spectra of large organic molecules based on density functional methods are presented. The geometries of the excited states are obtained from time-dependent density functional (TDDFT) calculations employing the B3LYP hybrid functional. The vibrational functions and transition dipole moment derivatives are calculated within the harmonic approximation by finite difference of analytical gradients and the transition dipole moment, respectively. Normal mode mixing is taken into account by the Duschinsky transformation. The vibronic structure of strongly dipole-allowed transitions is calculated within the Franck-Condon approximation. Weakly dipole-allowed and dipole-forbidden transitions are treated within the Franck-Condon-Herzberg-Teller and Herzberg-Teller approximation, respectively. The absorption spectra of several organic pi systems (anthracene, pentacene, pyrene, octatetraene, styrene, azulene, phenoxyl) are calculated and compared with experimental data. For dipole-allowed transitions in general a very good agreement between theory and experiment is obtained. This indicates the good quality of the optimized geometries and harmonic force fields. Larger errors are found for the weakly dipole-allowed S0 --> S1 transition of pyrene which can tentatively be assigned to TDDFT errors for the relative energies of excited states close to the target state. The weak bands of azulene and phenoxyl are very well described within the Franck-Condon approximation which can be explained by the large energy gap (>1.2 eV) to higher-lying excited states leading to small vibronic couplings. Once corrections are made for the errors in the theoretical 0-0 transition energies, the TDDFT approach to calculate vibronic structure seems to outperform both widely used ab initio methods based on configuration interaction singles or complete active space self-consistent field wave functions and semiempirical treatments regarding accuracy
Domain overlap matrices from plane-wave-based methods of electronic structure calculation
NASA Astrophysics Data System (ADS)
Golub, Pavlo; Baranov, Alexey I.
2016-10-01
Plane waves are one of the most popular and efficient basis sets for electronic structure calculations of solids; however, their delocalized nature makes it difficult to employ for them classical orbital-based methods of chemical bonding analysis. The quantum chemical topology approach, introducing chemical concepts via partitioning of real space into chemically meaningful domains, has no difficulties with plane-wave-based basis sets. Many popular tools employed within this approach, for instance delocalization indices, need overlap integrals over these domains—the elements of the so called domain overlap matrices. This article reports an efficient algorithm for evaluation of domain overlap matrix elements for plane-wave-based calculations as well as evaluation of its implementation for one of the most popular projector augmented wave (PAW) methods on the small set of simple and complex solids. The stability of the obtained results with respect to PAW calculation parameters has been investigated, and the comparison of the results with the results from other calculation methods has also been made.
Electronic Structure Calculations of an Oxygen Vacancy in KH2PO4
Liu, C S; Hou, C J; Kioussis, N; Demos, S; Radousky, H
2005-02-18
We present first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of an oxygen vacancy in KH{sub 2}PO{sub 4} (KDP). Even though the overall DOS profiles for the defective KDP are quite similar to those of the perfect KDP, the oxygen vacancy in the neutral and +1 charge states induces defect states in the band gap. For the neutral oxygen vacancy, the gap states are occupied by two electrons. The difference between the integral of the total density of states (DOS) and the sum of the DOS projected on the atoms of 0.98 |e|, indicates that one of the two electrons resulting from the removal of the oxygen atom is trapped in the vacancy, while the other tends to delocalize in the neighboring atoms. For the +1 charge oxygen vacancy, the addition of the hole reduces the occupation of the filled gap-states in the neutral case from two to one electron and produces new empty states in the gap. The new empty gap states are very close to the highest occupied states, leading to a dramatic decrease of the band gap. The difference between the integral of the total DOS and the sum of the DOS projected on the atoms is 0.56 |e|, which implies that more than 56% of the redundant electron is trapped in the oxygen vacancy, and 44% spreads over the neighboring atoms. In sharp contrast, no defect states appear in the energy gap for the +2 charge O vacancy. Thus, the addition of the two holes completely compensates the two redundant electrons, and removes in turn the occupied gap states in the neutral case.
NASA Astrophysics Data System (ADS)
Brandelik, Andreas
2009-07-01
CALCMIN, an open source Visual Basic program, was implemented in EXCEL™. The program was primarily developed to support geoscientists in their routine task of calculating structural formulae of minerals on the basis of chemical analysis mainly obtained by electron microprobe (EMP) techniques. Calculation programs for various minerals are already included in the form of sub-routines. These routines are arranged in separate modules containing a minimum of code. The architecture of CALCMIN allows the user to easily develop new calculation routines or modify existing routines with little knowledge of programming techniques. By means of a simple mouse-click, the program automatically generates a rudimentary framework of code using the object model of the Visual Basic Editor (VBE). Within this framework simple commands and functions, which are provided by the program, can be used, for example, to perform various normalization procedures or to output the results of the computations. For the clarity of the code, element symbols are used as variables initialized by the program automatically. CALCMIN does not set any boundaries in complexity of the code used, resulting in a wide range of possible applications. Thus, matrix and optimization methods can be included, for instance, to determine end member contents for subsequent thermodynamic calculations. Diverse input procedures are provided, such as the automated read-in of output files created by the EMP. Furthermore, a subsequent filter routine enables the user to extract specific analyses in order to use them for a corresponding calculation routine. An event-driven, interactive operating mode was selected for easy application of the program. CALCMIN leads the user from the beginning to the end of the calculation process.
Electronic structure and Fermi surface of UNZ ( Z=Se and Te) by ab initio calculations
NASA Astrophysics Data System (ADS)
Samsel-Czekała, M.
2010-05-01
The electronic structures of ferromagnetic (FM) UNTe and its nonmagnetically ordered (NMO) isostructural (tetragonal P4/nmm ) and isoelectronic counterpart, UNSe, have been calculated from first principles in the framework of the fully relativistic and full-potential local-orbital band-structure code within local-spin density approximation (LSDA) including also an orbital polarization correction by Eriksson, Brooks, and Johansson (OPB). The results predict that both ternaries have a covalently metallic character and solely uranium atoms, located in (001) planes, form a metallic bond due to the U5f-6d electrons. The U5f electrons contribute also to a covalent bond with the ligand N and Te or Se atoms and they reveal a dual character, i.e., partly localized and itinerant. Contrary to UNSe, UNTe is a collinear FM with the magnetic moment alignment along the c axis, as observed experimentally in the past and now is well reproduced by the LSDA+OPB calculations. In NMO states of both systems, band pseudogaps are opening merely ˜0.25eV below the Fermi level, which cause an instability of the metallic state under small perturbations leading to a semiconducting behavior. The two-band Fermi surfaces (FSs) of both compounds (in NMO state) have similar quasi-two-dimensional (Q2D) properties with nesting vectors along the [100] direction. In turn, UNTe in the FM state possesses three-band FS with also Q2D properties and nesting features along the [100] and [110] directions, being important, e.g., in arising such collective phenomena as superconductivity.
Ab initio calculation of structural stability, electronic and optical properties of Ag{sub 2}Se
Rameshkumar, S.; Jayalakshmi, V.; Jaiganesh, G.; Palanivel, B.
2015-06-24
The structural stability, electronic and optical properties of Ag{sub 2}Se compound is studied using ab initio packages. Ag{sub 2}Se is found to crystallize in orthorhombic structure with two different space groups i.e. P2{sub 1}2{sub 1}2{sub 1} (No. 19) and P222{sub 1} (No. 17). For this compound in these two space groups, the total energy has been computed as a function of volume. Our calculated results suggest that the P2{sub 1}2{sub 1}2{sub 1}–phase is more stable than that of the P222{sub 1}–phase. The band structure calculation show that Ag{sub 2}Se is semimetallic with an overlap of about 0.014 eV in P2{sub 1}2{sub 1}2{sub 1}–phase whereas is metallic in nature in P222{sub 1}–phase. Moreover, the optical properties including the dielectric function, energy loss spectrum are obtained and analysed.
Electron beam dose calculations.
Hogstrom, K R; Mills, M D; Almond, P R
1981-05-01
Electron beam dose distributions in the presence of inhomogeneous tissue are calculated by an algorithm that sums the dose distribution of individual pencil beams. The off-axis dependence of the pencil beam dose distribution is described by the Fermi-Eyges theory of thick-target multiple Coulomb scattering. Measured square-field depth-dose data serve as input for the calculations. Air gap corrections are incorporated and use data from'in-air' measurements in the penumbra of the beam. The effective depth, used to evaluate depth-dose, and the sigma of the off-axis Gaussian spread against depth are calculated by recursion relations from a CT data matrix for the material underlying individual pencil beams. The correlation of CT number with relative linear stopping power and relative linear scattering power for various tissues is shown. The results of calculations are verified by comparison with measurements in a 17 MeV electron beam from the Therac 20 linear accelerator. Calculated isodose lines agree nominally to within 2 mm of measurements in a water phantom. Similar agreement is observed in cork slabs simulating lung. Calculations beneath a bone substitute illustrate a weakness in the calculation. Finally a case of carcinoma in the maxillary antrum is studied. The theory suggests an alternative method for the calculation of depth-dose of rectangular fields.
Electronic structure and thermoelectricity of filled skutterudite EuRu4As12: a DFT calculation
NASA Astrophysics Data System (ADS)
Shankar, A.; Rai, D. P.; Sandeep; Ghimire, M. P.; Thapa, R. K.
2017-01-01
On the basis of the first-principles density functional theory (DFT), we have investigated the electronic structure, elastic and magnetic properties of the filled skutterudite EuRu4As12 using the full-potential linearized augmented plane wave (FP-LAPW) method. The calculations presented here are carried out within the framework of the local spin density approximation (LSDA) approach. The effects of the localized electrons are treated by including Hubbard's U term in the calculation. The study of the elastic properties suggests the brittle nature of the material with covalent contribution in the atomic bonding. EuRu4As12 is a semi-metal with the presence of the bands originating from the bottom of the conduction region crossing the Fermi energy level (EF) more than twice. The existence of the high density of states at EF implies the large thermopower of the material and likely to be an effective candidate for the thermoelectric application. The exchange-splitting of Eu- f states are analyzed to explain the ferromagnetic ground state of the material. The analysis of the thermal transport properties suggests the high value of Seebeck coefficient of the material with figure of merit ( ZT) value of 0.55, which is consistent with the values obtained for the analogous compounds.
Thürmer, Stephan; Seidel, Robert; Winter, Bernd; Ončák, Milan; Slavíček, Petr
2011-06-16
The effect of hydration on the electronic structure of H(2)O(2) is investigated by liquid-jet photoelectron spectroscopy measurements and ab initio calculations. Experimental valence electron binding energies of the H(2)O(2) orbitals in water are, on average, 1.9 eV red-shifted with respect to the gas-phase molecule. A smaller width of the first peak was observed in the photoelectron spectrum from the solution. Our experiment is complemented by simulated photoelectron spectra, calculated at the ab initio level of theory (with EOM-IP-CCSD and DFT methods), and using path-integral sampling of the ground-state density. The observed shift in ionization energy upon solvation is attributed to a combination of nonspecific electrostatic effects (long-range polarization) and of the specific interactions between H(2)O(2) and H(2)O molecules in the first solvation shell. Changes in peak widths are found to result from merging of the two lowest ionized states of H(2)O(2) in water due to conformational changes upon solvation. Hydration effects on H(2)O(2) are stronger than on the H(2)O molecule. In addition to valence spectra, we report oxygen 1s core-level photoelectron spectra from H(2)O(2)(aq), and observed energies and spectral intensities are discussed qualitatively.
Model creation and electronic structure calculation of amorphous hydrogenated boron carbide
NASA Astrophysics Data System (ADS)
Belhadj Larbi, Mohammed
Boron-rich solids are of great interest for many applications, particularly, amorphous hydrogenated boron carbide (a-BC:H) thin films are a leading candidate for numerous applications such as: heterostructure materials, neutron detectors, and photovoltaic energy conversion. Despite this importance, the local structural properties of these materials are not well-known, and very few theoretical studies for this family of disordered solids exist in the literature. In order to optimize this material for its potential applications the structure property relationships need to be discovered. We use a hybrid method in this endeavor---which is to the best of our knowledge the first in the literature---to model and calculate the electronic structure of amorphous hydrogenated boron carbide (a-BC:H). A combination of classical molecular dynamics using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) and ab initio quantum mechanical simulations using the Vienna ab initio simulation package (VASP) have been conducted to create geometry optimized models that consist of a disordered hydrogenated twelve-vertex boron carbide icosahedra, with hydrogenated carbon cross-linkers. Then, the density functional theory (DFT) based orthogonalized linear combination of atomic orbitals (OLCAO) method was used to calculate the total and partial density of states (TDOS, PDOS), the complex dielectric function epsilon, and the radial pair distribution function (RPDF). The RPDF data stand as predictions that may be compared with future experimental electron or neutron diffraction data. The electronic structure simulations were not able to demonstrate a band gap of the same nature as that seen in prior experimental work, a general trend of the composition-properties relationship was established. The content of hydrogen and boron was found to be directly proportional to the decrease in the number of available states near the fermi energy, and inversely proportional to the
NASA Astrophysics Data System (ADS)
Wang, Neng-Ping
2011-01-01
I present a method to calculate the ballistic transport properties of atomic-scale structures under bias. The electronic structure of the system is calculated using the Kohn-Sham scheme of density functional theory (DFT). The DFT eigenvectors are then transformed into a set of maximally localized Wannier functions (MLWFs) [N. Marzari and D. Vanderbilt, Phys. Rev. B 56 (1997) 12847]. The MLWFs are used as a minimal basis set to obtain the Hamitonian matrices of the scattering region and the adjacent leads, which are needed for transport calculation using the nonequilibrium Green's function formalism. The coupling of the scattering region to the semi-infinite leads is described by the self-energies of the leads. Using the nonequilibrium Green's function method, one calculates self-consistently the charge distribution of the system under bias and evaluates the transmission and current through the system. To solve the Poisson equation within the scheme of MLWFs I introduce a computationally efficient method. The method is applied to a molecular hydrogen contact in two transition metal monatomic wires (Cu and Pt). It is found that for Pt the I-V characteristics is approximately linear dependence, however, for Cu the I-V characteristics manifests a linear dependence at low bias voltages and exhibits apparent nonlinearity at higher bias voltages. I have also calculated the transmission in the zero bias voltage limit for a single CO molecule adsorbed on Cu and Pt monatomic wires. While a chemical scissor effect occurs for the Cu monatomic wire with an adsorbed CO molecule, it is absent for the Pt monatomic wire due to the contribution of d-orbitals at the Fermi energy.
Faraji, Shirin; Köppel, Horst
2008-08-21
The multimode multistate vibronic interactions between the five lowest electronic states of all three isomers of the difluorobenzene radical cation are investigated theoretically, based on ab initio electronic structure data, and employing a well-established vibronic coupling model. The approach rests on the linear vibronic coupling scheme, augmented by quadratic coupling terms for the totally symmetric modes. The underlying ionization potentials and coupling constants are obtained from ab initio coupled-cluster calculations. Low-energy conical intersections and strong vibronic couplings are found to prevail within the sets of X-A and B-C-D cationic states, while the interactions between these two sets of states are found to be weaker and depend on the isomer. The inclusion of the aforementioned quadratic couplings is found to be essential to correctly reproduce the lowest-energy conical intersections between the two different sets of electronic states. Differences between the three isomers regarding these quantities are pointed out. The results will be used as basis for multidimensional wave-packet dynamical simulations for these coupled potential energy surfaces to be presented in the following paper (Paper II).
NASA Astrophysics Data System (ADS)
Saikia, Nabanita; Pati, Swapan K.; Deka, Ramesh C.
2012-09-01
One-dimensional nanostructures such as nanowires and nanotubes are stimulating tremendous research interest due to their structural, electronic and magnetic properties. We perform first principles calculation using density functional theory on the structural, and electronics properties of BNNTs adsorbed with isoniazid (INH) drug via noncovalent functionalization using the GGA/PBE functional and DZP basis set implemented in SIESTA program. The band structure, density of states and projected density of states (PDOS) plots suggest that isoniazid prefers to get adsorbed at the hollow site in case of (5,5) BNNT, whereas in (10,0) BNNT it favours the bridge site. The adsorption energy of INH onto (5,5) BNNT is smaller than in (10,0) BNNT which proposes that (10,0) BNNT with a larger radius compared to (5,5) BNNT is more favourable for INH adsorption as the corresponding distortion energy will also be quite lower. Functionalization of (5,5) and (10,0) BNNTs with isoniazid displays the presence of new impurity states (dispersionless bands) within the HOMO-LUMO energy gap of pristine BNNT leading to an increase in reactivity of the INH/BNNT system and lowering of the energy gap of the BNNTs. The PDOS plots show the major contribution towards the dispersionless impurity states is from INH molecule itself rather than from BNNT near the Fermi energy region. To summarize, noncovalent functionalization of BNNTs with isoniazid drug modulates the electronic properties of the pristine BNNT by lowering its energy gap with respect to the Fermi level, as well as demonstrating the preferential site selectivity for adsorption of isoniazid onto the nanotube sidewalls of varying chirality.
Lin, Lin; Chen, Mohan; Yang, Chao; He, Lixin
2012-02-10
We describe how to apply the recently developed pole expansion plus selected inversion (PEpSI) technique to Kohn-Sham density function theory (DFT) electronic structure calculations that are based on atomic orbital discretization. We give analytic expressions for evaluating charge density, total energy, Helmholtz free energy and atomic forces without using the eigenvalues and eigenvectors of the Kohn-Sham Hamiltonian. We also show how to update the chemical potential without using Kohn-Sham eigenvalues. The advantage of using PEpSI is that it has a much lower computational complexity than that associated with the matrix diagonalization procedure. We demonstrate the performance gain by comparing the timing of PEpSI with that of diagonalization on insulating and metallic nanotubes. For these quasi-1D systems, the complexity of PEpSI is linear with respect to the number of atoms. This linear scaling can be observed in our computational experiments when the number of atoms in a nanotube is larger than a few hundreds. Both the wall clock time and the memory requirement of PEpSI is modest. This makes it even possible to perform Kohn-Sham DFT calculations for 10,000-atom nanotubes on a single processor. We also show that the use of PEpSI does not lead to loss of accuracy required in a practical DFT calculation.
NASA Astrophysics Data System (ADS)
Belyakov, Alexander V.; Nikolaenko, Kirill O.; Davidovich, Pavel B.; Ivanov, Anatolii D.; Garabadzhiu, Alexander V.; Rykov, Anatolii N.; Shishkov, Igor F.
2017-03-01
The molecular structure of isatin, indole-2,3-dione, was studied by gas-phase electron diffraction (GED) and quantum chemical calculations (M062X and MP2 methods with aug-cc-pVTZ basis set). The best fit of the experimental scattering intensities (R-factor = 4.4%) was obtained for a molecular model of Cs symmetry. The structure of the benzene ring deviates from a regular hexagon due to the adjacent pyrrole heterocycle. The small differences between similar geometric parameters were constrained at the values calculated at the M062X level. The experimental structural parameters agree well with the results of theoretical calculations. The bonds in the benzene moiety are in agreement with their standard values. The (Odbnd)Csbnd C(dbnd O) carbon-carbon bond of the pyrrole moiety (1.573(7) Å) is remarkably lengthened in comparison with standard C(sp2)sbnd C(sp2) value, 1.425(11) Å for N-methylpyrrole. According to NBO analysis of isatin, glyoxal and pyrrole-2,3-dione molecules this lengthening cannot be attributed to the steric interactions of Cdbnd O bonds alone and is, mainly, due to the electrostatic repulsion and hyperconjugation that is delocalization of oxygen lone pairs of π-type into the corresponding carbon-carbon antibonding orbital, nπ(O) → σ∗(Csbnd C). Deletion of σ∗(Csbnd C) orbital followed by subsequent geometry optimization led to shortening of the corresponding Csbnd C bond by 0.06 Å. According to different aromaticity descriptors, aromaticity of benzene moiety of isatin is smaller in comparison with benzene molecule. External magnetic field induces diatropic ring current in benzene moiety of isatin.
Grant, Daniel J; Dixon, David A; Kemeny, Andre E; Francisco, Joseph S
2008-04-28
High level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set limit have been performed on the PNO, NOP, and NPO isomers and their corresponding anions and cations. Geometries for all species were optimized up through the aug-cc-pV(Q+d)Z level and vibrational frequencies were calculated with the aug-cc-pV(T+d)Z basis set. The most stable of the three isomers is NPO and it is predicted to have a heat of formation of 23.3 kcal/mol. PNO is predicted to be only 1.7 kcal/mol higher in energy. The calculated adiabatic ionization potential of NPO is 12.07 eV and the calculated adiabatic electron affinity is 2.34 eV. The calculated adiabatic ionization potential of PNO is 10.27 eV and the calculated adiabatic electron affinity is only 0.24 eV. NOP is predicted to be much higher in energy by 29.9 kcal/mol. The calculated rotational constants for PNO and NPO should allow for these species to be spectroscopically distinguished. The adiabatic bond dissociation energies for the P[Single Bond]N, P[Single Bond]O, and N[Single Bond]O bonds in NPO and PNO are the same within approximately 10 kcal/mol and fall in the range of 72-83 kcal/mol.
NASA Astrophysics Data System (ADS)
Grant, Daniel J.; Dixon, David A.; Kemeny, Andre E.; Francisco, Joseph S.
2008-04-01
High level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set limit have been performed on the PNO, NOP, and NPO isomers and their corresponding anions and cations. Geometries for all species were optimized up through the aug-cc-pV(Q +d)Z level and vibrational frequencies were calculated with the aug-cc-pV(T +d)Z basis set. The most stable of the three isomers is NPO and it is predicted to have a heat of formation of 23.3kcal/mol. PNO is predicted to be only 1.7kcal/mol higher in energy. The calculated adiabatic ionization potential of NPO is 12.07eV and the calculated adiabatic electron affinity is 2.34eV. The calculated adiabatic ionization potential of PNO is 10.27eV and the calculated adiabatic electron affinity is only 0.24eV. NOP is predicted to be much higher in energy by 29.9kcal/mol. The calculated rotational constants for PNO and NPO should allow for these species to be spectroscopically distinguished. The adiabatic bond dissociation energies for the P N, P O, and N O bonds in NPO and PNO are the same within ˜10kcal/mol and fall in the range of 72-83kcal/mol.
Yuan, H. K.; Chen, H. Tian, C. L.; Kuang, A. L.; Wang, J. Z.
2014-04-21
Gadolinium-oxide clusters in various sizes and stoichiometries have been systematically studied by employing the density functional theory with the generalized gradient approximation. The clusters in bulk stoichiometry are relatively more stable and their binding energies increase with the increasing size. Stoichiometric (Gd{sub 2}O{sub 3}){sub n} clusters of n = 1–3 prefer cage-like structures, whereas the clusters of n = 4–30 prefer compact structures layered by wedge-like units and exhibit a rough feature toward the bulk-like arrangement with small disorders of atomic positions. The polyhedral-cages analogous to carbon-fullerenes are stable isomers yet not the minimum energy configurations. Their stabilities can be improved by embedding one oxygen atom or a suitable cage to form core-shell configurations. The mostly favored antiferromagnetic couplings between adjacent Gd atoms are nearly degenerated in energy with their ferromagnetic couplings, resulting in super-paramagnetic characters of gadolinium-oxide clusters. The Ruderman-Kittel-Kasuya-Yosida (RKKY)-type mechanism together with the superexchange-type mechanism plays cooperation role for the magnetic interactions in clusters. We present, as a function of n, calculated binding energies, ionization potential, electron affinity, and electronic dipole moment.
GPAW - massively parallel electronic structure calculations with Python-based software.
Enkovaara, J.; Romero, N.; Shende, S.; Mortensen, J.
2011-01-01
Electronic structure calculations are a widely used tool in materials science and large consumer of supercomputing resources. Traditionally, the software packages for these kind of simulations have been implemented in compiled languages, where Fortran in its different versions has been the most popular choice. While dynamic, interpreted languages, such as Python, can increase the effciency of programmer, they cannot compete directly with the raw performance of compiled languages. However, by using an interpreted language together with a compiled language, it is possible to have most of the productivity enhancing features together with a good numerical performance. We have used this approach in implementing an electronic structure simulation software GPAW using the combination of Python and C programming languages. While the chosen approach works well in standard workstations and Unix environments, massively parallel supercomputing systems can present some challenges in porting, debugging and profiling the software. In this paper we describe some details of the implementation and discuss the advantages and challenges of the combined Python/C approach. We show that despite the challenges it is possible to obtain good numerical performance and good parallel scalability with Python based software.
NASA Astrophysics Data System (ADS)
Wu, Wenxia; Xue, Zhiyong; Hong, Xing; Li, Xiumei; Guo, Yongquan
2009-06-01
The valence electronic structures of Fe, Co and Ni have been investigated with Empirical Electron Theory of Solids and Molecules. The magnetic moments, Curie temperature, cohesive energy and melting point have been calculated according to the valence electronic structure. These calculations fit the experimental data very well. Based on the calculations, the magnetic moments are proportional to the number of 3d magnetic electrons. Curie temperatures are related to the magnetic electrons and the bond lengths between magnetic atoms. Cohesive energies increase with the increase of the number of covalent electrons, and the decrease of the number of magnetic and dumb pair electrons. The melting point is mainly related to the number of covalent electron pairs distributed in the strongest bond. The contribution from the lattice electrons is very small, the dumb pair electrons weaken the melting point; however, the contribution to melting point of the magnetic electrons can be neglected. It reveals that the magnetic and thermal properties are closely related to the valence electronic structures, and the changes or transitions between the electrons obviously affect the physical properties.
Is C50 a superaromat? Evidence from electronic structure and ring current calculations.
Matías, Ana Sanz; Havenith, Remco W A; Alcamí, Manuel; Ceulemans, Arnout
2016-04-28
The fullerene-50 is a 'magic number' cage according to the 2(N + 1)(2) rule. For the three lowest isomers of C50 with trigonal and pentagonal symmetries, we calculate the sphericity index, the spherical parentage of the occupied π-orbitals, and the current density in an applied magnetic field. The minimal energy isomer, with D3 symmetry, comes closest to a spherical aromat or a superaromat. In the D5h bond-stretch isomers the electronic structure shows larger deviations from the ideal spherical shells, with hybridisation or even reversal of spherical parentages. It is shown that relative stabilities of fullerene cages do not correlate well with aromaticity, unlike the magnetic properties which are very sensitive indicators of spherical aromaticity. Superaromatic diamagnetism in the D3 cage is characterized by global diatropic currents, which encircle the whole cage. The breakdown of sphericity in the D5h cages gives rise to local paratropic countercurrents.
NASA Astrophysics Data System (ADS)
Yelgel, Celal
2016-04-01
We present an extensive density functional theory (DFT) based investigation of the electronic structures of ABC-stacked N-layer graphene. It is found that for such systems the dispersion relations of the highest valence and the lowest conduction bands near the K point in the Brillouin zone are characterised by a mixture of cubic, parabolic, and linear behaviours. When the number of graphene layers is increased to more than three, the separation between the valence and conduction bands decreases up until they touch each other. For five and six layer samples these bands show flat behaviour close to the K point. We note that all states in the vicinity of the Fermi energy are surface states originated from the top and/or bottom surface of all the systems considered. For the trilayer system, N = 3, pronounced trigonal warping of the bands slightly above the Fermi level is directly obtained from DFT calculations.
Brown, David M. L.; Cho, Herman; de Jong, Wibe A.
2016-02-09
Here, the testing of theoretical models with experimental data is an integral part of the scientific method, and a logical place to search for new ways of stimulating scientific productivity. Often experiment/theory comparisons may be viewed as a workflow comprised of well-defined, rote operations distributed over several distinct computers, as exemplified by the way in which predictions from electronic structure theories are evaluated with results from spectroscopic experiments. For workflows such as this, which may be laborious and time consuming to perform manually, software that could orchestrate the operations and transfer results between computers in a seamless and automated fashion would offer major efficiency gains. Such tools also promise to alter how researchers interact with data outside their field of specialization by, e.g., making raw experimental results more accessible to theorists, and the outputs of theoretical calculations more readily comprehended by experimentalists.
Brown, David M. L.; Cho, Herman; de Jong, Wibe A.
2016-02-09
Here, the testing of theoretical models with experimental data is an integral part of the scientific method, and a logical place to search for new ways of stimulating scientific productivity. Often experiment/theory comparisons may be viewed as a workflow comprised of well-defined, rote operations distributed over several distinct computers, as exemplified by the way in which predictions from electronic structure theories are evaluated with results from spectroscopic experiments. For workflows such as this, which may be laborious and time consuming to perform manually, software that could orchestrate the operations and transfer results between computers in a seamless and automated fashionmore » would offer major efficiency gains. Such tools also promise to alter how researchers interact with data outside their field of specialization by, e.g., making raw experimental results more accessible to theorists, and the outputs of theoretical calculations more readily comprehended by experimentalists.« less
Electronic Structure Calculations and Adaptation Scheme in Multi-core Computing Environments
Seshagiri, Lakshminarasimhan; Sosonkina, Masha; Zhang, Zhao
2009-05-20
Multi-core processing environments have become the norm in the generic computing environment and are being considered for adding an extra dimension to the execution of any application. The T2 Niagara processor is a very unique environment where it consists of eight cores having a capability of running eight threads simultaneously in each of the cores. Applications like General Atomic and Molecular Electronic Structure (GAMESS), used for ab-initio molecular quantum chemistry calculations, can be good indicators of the performance of such machines and would be a guideline for both hardware designers and application programmers. In this paper we try to benchmark the GAMESS performance on a T2 Niagara processor for a couple of molecules. We also show the suitability of using a middleware based adaptation algorithm on GAMESS on such a multi-core environment.
Hydrogen trapping in δ-Pu: insights from electronic structure calculations.
Taylor, Christopher D; Hernandez, Sarah C; Francis, Michael F; Schwartz, Daniel S; Ray, Asok K
2013-07-03
Density functional theory calculations have been performed to provide details of the structural and charge-transfer details related to the solid solution of hydrogen in (δ)-plutonium. We follow the Flanagan model that outlines the process by which hydrogen interacts with a metal to produce hydride phases, via a sequence of surface, interstitial and defect-bound (trapped) states. Due to the complexities of the electronic structure in plutonium solid-state systems, we take the pragmatic approach of adopting the 'special quasirandom structure' to disperse the atomic magnetic moments. We find that this approach produces sound structural and thermodynamic properties in agreement with the available experimental data. In δ-Pu, hydrogen has an exothermic binding energy to all of the states relevant in the Flanagan model, and, furthermore, is anionic in all these states. The charge transfer is maximized (i.e. most negative for hydrogen) in the hydride phase. The pathway from surface to hydride is sequentially exothermic, in the order surface < interstitial < grain boundary < vacancy < hydride (hydride being the most exothermic state). Thus, we find that there is no intermediate state that involves an endothermic increase in energy, consistent with the general experimental observations that the hydriding reaction in plutonium metal can proceed with zero apparent activation barrier.
Kemnitz, C.R.; Ellison, G.B.; Karney, W.L.; Borden, W.T.
2000-02-16
(12/11)CASSCF and (12/11)CASPT2 ab initio electronic structure calculations with both the cc-pVDZ and cc-pVTZ basis sets find that there is a barrier to the very exothermic hydrogen shift that converts singlet methylnitrene, CH{sub 3}N, to methyleneimine, H{sub 2}C{double{underscore}bond}NH. These two energy minima are connected by a transition structure of C{sub s} symmetry, which is computed to lie 3.8 kcal/mol above the reactant at the (12/11)CASPT2/cc-pVTZ//(12/11)CASSCF/cc-pVTZ level of theory. The (12/11)CASSCF/cc-pVTZ value for the lowest frequency vibration in the transition structure is 854 cm{sup {minus}1}, and CASPT2 calculations concur that this a{double{underscore}prime} vibration does indeed have a positive force constant. Thus, there is no evidence that this geometry is actually a mountain top, rather than a transition structure, on the global potential energy surface or that a C{sub 1} pathway of lower energy connects the reactant to the product. Therefore, computational results indicate that the bands seen for singlet methylnitrene in the negative ion photoelectron spectrum of CH{sub 3}N{sup {minus}} are due to singlet methylnitrene being an energy minimum, rather than a transition state. These results also lead to the prediction that, at least in principle, singlet methylnitrene should be an observable intermediate in the formation of methyleneimine.
NASA Astrophysics Data System (ADS)
Imachi, Hiroto; Yokoyama, Seiya; Kaji, Takami; Abe, Yukiya; Tada, Tomofumi; Hoshi, Takeo
2016-12-01
One-hundred-nm-scale electronic structure calculations were carried out on the K supercomputer by our original simulation code ELSES (http://www.elses.jp/) The present paper reports preliminary results of transport calculations for condensed organic polymers. Large-scale calculations are realized by novel massively parallel order-N algorithms. The transport calculations were carried out as a theoretical extension for the quantum wavepacket dynamics simulation. The method was applied to a single polymer chain and condensed polymers.
Electronic structure, rovibrational, and dipole moment calculations for the AsCl molecule.
Mourad, Khaled A; Abdulal, Saleh N; Korek, Mahmoud
2016-02-01
The potential energy curves of the 19 lowest-lying singlet and triplet electronic states in the (2S+1)Λ((+/-)) representation of the AsCl molecule have been investigated using the complete active space self-consistent field (CASSCF) with multireference configuration interaction (MRCI+Q) method including single and double excitations and with the Davidson correction. The harmonic frequency ω e, the internuclear distance R e, the dipole moment, and the electronic energy with respect to the ground state T e were calculated for the electronic states considered. By using the canonical functions approach, the eigenvalue E v, the rotational constant B v, and the abscissae of the turning points R min and R max were calculated for the electronic states up to the vibrational level v = 60. The values obtained in the present work agree well with corresponding values available in the literature for several electronic states. Fifteen new electronic states were investigated here for the first time.
Wang, Lin-Wang
2006-12-01
Quantum mechanical ab initio calculation constitutes the biggest portion of the computer time in material science and chemical science simulations. As a computer center like NERSC, to better serve these communities, it will be very useful to have a prediction for the future trends of ab initio calculations in these areas. Such prediction can help us to decide what future computer architecture can be most useful for these communities, and what should be emphasized on in future supercomputer procurement. As the size of the computer and the size of the simulated physical systems increase, there is a renewed interest in using the real space grid method in electronic structure calculations. This is fueled by two factors. First, it is generally assumed that the real space grid method is more suitable for parallel computation for its limited communication requirement, compared with spectrum method where a global FFT is required. Second, as the size N of the calculated system increases together with the computer power, O(N) scaling approaches become more favorable than the traditional direct O(N{sup 3}) scaling methods. These O(N) methods are usually based on localized orbital in real space, which can be described more naturally by the real space basis. In this report, the author compares the real space methods versus the traditional plane wave (PW) spectrum methods, for their technical pros and cons, and the possible of future trends. For the real space method, the author focuses on the regular grid finite different (FD) method and the finite element (FE) method. These are the methods used mostly in material science simulation. As for chemical science, the predominant methods are still Gaussian basis method, and sometime the atomic orbital basis method. These two basis sets are localized in real space, and there is no indication that their roles in quantum chemical simulation will change anytime soon. The author focuses on the density functional theory (DFT), which is the
Goldsmith, Zachary K.; Harshan, Aparna K.; Gerken, James B.; Galli, Giulia; Stahl, Shannon S.
2017-01-01
NiFe oxyhydroxide materials are highly active electrocatalysts for the oxygen evolution reaction (OER), an important process for carbon-neutral energy storage. Recent spectroscopic and computational studies increasingly support iron as the site of catalytic activity but differ with respect to the relevant iron redox state. A combination of hybrid periodic density functional theory calculations and spectroelectrochemical experiments elucidate the electronic structure and redox thermodynamics of Ni-only and mixed NiFe oxyhydroxide thin-film electrocatalysts. The UV/visible light absorbance of the Ni-only catalyst depends on the applied potential as metal ions in the film are oxidized before the onset of OER activity. In contrast, absorbance changes are negligible in a 25% Fe-doped catalyst up to the onset of OER activity. First-principles calculations of proton-coupled redox potentials and magnetizations reveal that the Ni-only system features oxidation of Ni2+ to Ni3+, followed by oxidation to a mixed Ni3+/4+ state at a potential coincident with the onset of OER activity. Calculations on the 25% Fe-doped system show the catalyst is redox inert before the onset of catalysis, which coincides with the formation of Fe4+ and mixed Ni oxidation states. The calculations indicate that introduction of Fe dopants changes the character of the conduction band minimum from Ni-oxide in the Ni-only to predominantly Fe-oxide in the NiFe electrocatalyst. These findings provide a unified experimental and theoretical description of the electrochemical and optical properties of Ni and NiFe oxyhydroxide electrocatalysts and serve as an important benchmark for computational characterization of mixed-metal oxidation states in heterogeneous catalysts. PMID:28265083
NASA Astrophysics Data System (ADS)
Sæterli, Ragnhild; Flage-Larsen, Espen; Prytz, Øystein; Taftø, Johan; Marthinsen, Knut; Holmestad, Randi
2009-08-01
In this study we report the results of experiments and theoretical calculations on the phosphorus L2,3 edges of the skutterudites CoP3 , LaFe4P12 , NiP3 , RhP3 , and IrP3 . Phosphorus s and d density of states above the Fermi level was studied by transmission electron energy loss spectroscopy while theoretical calculations were performed using both a real-space multiple-scattering procedure and density-functional theory. Generally, there are good agreements between both types of calculations and the experimental results. The near-edge structure of all the examined compounds shows the same overall features, including the metallic NiP3 and the metallic filled skutterudite LaFeP12 , and is well explained by comparison to phosphorus density of states. We also discuss the similarities to previously reported results on SiL2,3 edges and interpret the differences of the various skutterudites in terms of the electronegativities of the involved atom species.
Hydrogen trapping in δ-Pu: insights from electronic structure calculations
NASA Astrophysics Data System (ADS)
Taylor, Christopher D.; Hernandez, Sarah C.; Francis, Michael F.; Schwartz, Daniel S.; Ray, Asok K.
2013-07-01
Density functional theory calculations have been performed to provide details of the structural and charge-transfer details related to the solid solution of hydrogen in (δ)-plutonium. We follow the Flanagan model that outlines the process by which hydrogen interacts with a metal to produce hydride phases, via a sequence of surface, interstitial and defect-bound (trapped) states. Due to the complexities of the electronic structure in plutonium solid-state systems, we take the pragmatic approach of adopting the ‘special quasirandom structure’ to disperse the atomic magnetic moments. We find that this approach produces sound structural and thermodynamic properties in agreement with the available experimental data. In δ-Pu, hydrogen has an exothermic binding energy to all of the states relevant in the Flanagan model, and, furthermore, is anionic in all these states. The charge transfer is maximized (i.e. most negative for hydrogen) in the hydride phase. The pathway from surface to hydride is sequentially exothermic, in the order surface < interstitial < grain boundary < vacancy < hydride (hydride being the most exothermic state). Thus, we find that there is no intermediate state that involves an endothermic increase in energy, consistent with the general experimental observations that the hydriding reaction in plutonium metal can proceed with zero apparent activation barrier.
NASA Astrophysics Data System (ADS)
Jin, Na; Yang, Yanqing; Luo, Xian; Liu, Shuai; Xiao, Zhiyuan; Guo, Pengfei; Huang, Bin
2015-01-01
The structural, adhesive, and electronic properties of α-W(1 1 0)/α-WC(0 0 0 1) interfaces are studied by first-principles calculation based on density functional theory (DFT). Six different W/WC interface geometries are considered in this study, including two terminations of WC(0 0 0 1) surface, and each of them involves three different stacking sequences. It is demonstrated that whatever stacking sequence is, the interfacial separations of C-terminated interfaces decrease after optimization, and the lateral movement of the interfacial W atoms will bring three nearest neighbor C atoms around it. Therefore, the C-terminated interfaces are stable geometries, and yield larger adhesion energy, Wad. Using several analytic techniques including charge density distribution and its difference, and density of states, we characterized the electronic properties and determined the interfacial bonding of W-terminated hollow-site interface to be of metallic nature while the interfacial bonding of C-terminated hollow-site interface to be of a mixed covalent-ionic nature.
The linearly scaling 3D fragment method for large scale electronic structure calculations
Zhao, Zhengji; Meza, Juan; Lee, Byounghak; Shan, Hongzhang; Strohmaier, Erich; Bailey, David; Wang, Lin-Wang
2009-07-28
The Linearly Scaling three-dimensional fragment (LS3DF) method is an O(N) ab initio electronic structure method for large-scale nano material simulations. It is a divide-and-conquer approach with a novel patching scheme that effectively cancels out the artificial boundary effects, which exist in all divide-and-conquer schemes. This method has made ab initio simulations of thousand-atom nanosystems feasible in a couple of hours, while retaining essentially the same accuracy as the direct calculation methods. The LS3DF method won the 2008 ACM Gordon Bell Prize for algorithm innovation. Our code has reached 442 Tflop/s running on 147,456 processors on the Cray XT5 (Jaguar) at OLCF, and has been run on 163,840 processors on the Blue Gene/P (Intrepid) at ALCF, and has been applied to a system containing 36,000 atoms. In this paper, we will present the recent parallel performance results of this code, and will apply the method to asymmetric CdSe/CdS core/shell nanorods, which have potential applications in electronic devices and solar cells.
The Linearly Scaling 3D Fragment Method for Large Scale Electronic Structure Calculations
Zhao, Zhengji; Meza, Juan; Lee, Byounghak; Shan, Hongzhang; Strohmaier, Erich; Bailey, David; Wang, Lin-Wang
2009-06-26
The Linearly Scaling three-dimensional fragment (LS3DF) method is an O(N) ab initio electronic structure method for large-scale nano material simulations. It is a divide-and-conquer approach with a novel patching scheme that effectively cancels out the artificial boundary effects, which exist in all divide-and-conquer schemes. This method has made ab initio simulations of thousand-atom nanosystems feasible in a couple of hours, while retaining essentially the same accuracy as the direct calculation methods. The LS3DF method won the 2008 ACM Gordon Bell Prize for algorithm innovation. Our code has reached 442 Tflop/s running on 147,456 processors on the Cray XT5 (Jaguar) at OLCF, and has been run on 163,840 processors on the Blue Gene/P (Intrepid) at ALCF, and has been applied to a system containing 36,000 atoms. In this paper, we will present the recent parallel performance results of this code, and will apply the method to asymmetric CdSe/CdS core/shell nanorods, which have potential applications in electronic devices and solar cells.
NASA Astrophysics Data System (ADS)
Briggs, Emil; Hodak, Miroslav; Lu, Wenchang; Bernholc, Jerry; Li, Yan
RMG is a cross platform open source package for ab initio electronic structure calculations that uses real-space grids, multigrid pre-conditioning, and subspace diagonalization to solve the Kohn-Sham equations. The code has been successfully used for a wide range of problems ranging from complex bulk materials to multifunctional electronic devices and biological systems. RMG makes efficient use of GPU accelerators, if present, but does not require them. Recent work has extended GPU support to systems with multiple GPU's per computational node, as well as optimized both CPU and GPU memory usage to enable large problem sizes, which are no longer limited by the memory of the GPU board. Additional enhancements include increased portability, scalability and performance. New versions of the code are regularly released at sourceforge.net/projects/rmgdft/. The releases include binaries for Linux, Windows and MacIntosh systems, automated builds for clusters using cmake, as well as versions adapted to the major supercomputing installations and platforms.
The linearly scaling 3D fragment method for large scale electronic structure calculations
NASA Astrophysics Data System (ADS)
Zhao, Zhengji; Meza, Juan; Lee, Byounghak; Shan, Hongzhang; Strohmaier, Erich; Bailey, David; Wang, Lin-Wang
2009-07-01
The linearly scaling three-dimensional fragment (LS3DF) method is an O(N) ab initio electronic structure method for large-scale nano material simulations. It is a divide-and-conquer approach with a novel patching scheme that effectively cancels out the artificial boundary effects, which exist in all divide-and-conquer schemes. This method has made ab initio simulations of thousand-atom nanosystems feasible in a couple of hours, while retaining essentially the same accuracy as the direct calculation methods. The LS3DF method won the 2008 ACM Gordon Bell Prize for algorithm innovation. Our code has reached 442 Tflop/s running on 147,456 processors on the Cray XT5 (Jaguar) at OLCF, and has been run on 163,840 processors on the Blue Gene/P (Intrepid) at ALCF, and has been applied to a system containing 36,000 atoms. In this paper, we will present the recent parallel performance results of this code, and will apply the method to asymmetric CdSe/CdS core/shell nanorods, which have potential applications in electronic devices and solar cells.
Bringing about matrix sparsity in linear-scaling electronic structure calculations.
Rubensson, Emanuel H; Rudberg, Elias
2011-05-01
The performance of linear-scaling electronic structure calculations depends critically on matrix sparsity. This article gives an overview of different strategies for removal of small matrix elements, with emphasis on schemes that allow for rigorous control of errors. In particular, a novel scheme is proposed that has significantly smaller computational overhead compared with the Euclidean norm-based truncation scheme of Rubensson et al. (J Comput Chem 2009, 30, 974) while still achieving the desired asymptotic behavior required for linear scaling. Small matrix elements are removed while ensuring that the Euclidean norm of the error matrix stays below a desired value, so that the resulting error in the occupied subspace can be controlled. The efficiency of the new scheme is investigated in benchmark calculations for water clusters including up to 6523 water molecules. Furthermore, the foundation of matrix sparsity is investigated. This includes a study of the decay of matrix element magnitude with distance between basis function centers for different molecular systems and different methods. The studied methods include Hartree–Fock and density functional theory using both pure and hybrid functionals. The relation between band gap and decay properties of the density matrix is also discussed.
Electronic structure of O-doped SiGe calculated by DFT + U method
NASA Astrophysics Data System (ADS)
Zhao, Zong-Yan; Yang, Wen; Yang, Pei-Zhi
2016-12-01
To more in depth understand the doping effects of oxygen on SiGe alloys, both the micro-structure and properties of O-doped SiGe (including: bulk, (001) surface, and (110) surface) are calculated by DFT + U method in the present work. The calculated results are as follows. (i) The (110) surface is the main exposing surface of SiGe, in which O impurity prefers to occupy the surface vacancy sites. (ii) For O interstitial doping on SiGe (110) surface, the existences of energy states caused by O doping in the band gap not only enhance the infrared light absorption, but also improve the behaviors of photo-generated carriers. (iii) The finding about decreased surface work function of O-doped SiGe (110) surface can confirm previous experimental observations. (iv) In all cases, O doing mainly induces the electronic structures near the band gap to vary, but is not directly involved in these variations. Therefore, these findings in the present work not only can provide further explanation and analysis for the corresponding underlying mechanism for some of the experimental findings reported in the literature, but also conduce to the development of μc-SiGe-based solar cells in the future. Project supported by the Natural Science Foundation of Yunnan Province, China (Grant No. 2015FB123), the 18th Yunnan Province Young Academic and Technical Leaders Reserve Talent Project, China (Grant No. 2015HB015), and the National Natural Science Foundation of China (Grant No. U1037604).
Aguiar, J A; Ramasse, Q M; Asta, M; Browning, N D
2012-07-25
Energy loss spectra from fluorite-structured ZrO(2), CeO(2), and UO(2) compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.
Aguiar, Jeff; Ramasse, Q. M.; Asta, Mark D.; Browning, Nigel D.
2012-06-27
Energy loss spectra from fluorite-structured ZrO2, CeO2, and UO2 compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.
NASA Technical Reports Server (NTRS)
Luo, D.; Pradhan, A. K.
1990-01-01
The new R-matrix package for comprehensive close-coupling calculations for electron scattering with the first three ions in the boron isoelectronic sequence, the astrophysically significant C(+), N(2+), and O(3+), is presented. The collision strengths are calculated in the LS coupling approximation, as well as in pair-coupling scheme, for the transitions among the fine-structure sublevels. Calculations are carried out at a large number of energies in order to study the detailed effects of autoionizing resonances.
Structural and electronic phase transitions of ThS2 from first-principles calculations
Guo, Yongliang; Wang, Changying; Qiu, Wujie; ...
2016-10-07
Performed a systematic study using first-principles methods of the pressure-induced structural and electronic phase transitions in ThS2, which may play an important role in the next generation nuclear energy fuel technology.
Lin, Lin; Yang, Chao; Lu, Jiangfeng; Ying, Lexing; E, Weinan
2009-09-25
We present an efficient parallel algorithm and its implementation for computing the diagonal of $H^-1$ where $H$ is a 2D Kohn-Sham Hamiltonian discretized on a rectangular domain using a standard second order finite difference scheme. This type of calculation can be used to obtain an accurate approximation to the diagonal of a Fermi-Dirac function of $H$ through a recently developed pole-expansion technique \\cite{LinLuYingE2009}. The diagonal elements are needed in electronic structure calculations for quantum mechanical systems \\citeHohenbergKohn1964, KohnSham 1965,DreizlerGross1990. We show how elimination tree is used to organize the parallel computation and how synchronization overhead is reduced by passing data level by level along this tree using the technique of local buffers and relative indices. We analyze the performance of our implementation by examining its load balance and communication overhead. We show that our implementation exhibits an excellent weak scaling on a large-scale high performance distributed parallel machine. When compared with standard approach for evaluating the diagonal a Fermi-Dirac function of a Kohn-Sham Hamiltonian associated a 2D electron quantum dot, the new pole-expansion technique that uses our algorithm to compute the diagonal of $(H-z_i I)^-1$ for a small number of poles $z_i$ is much faster, especially when the quantum dot contains many electrons.
Liu, Ming-Yang; Huang, Yang; Chen, Qing-Yuan; Cao, Chao; He, Yao
2016-01-01
We study the equilibrium geometry and electronic structure of alloyed and doped arsenene sheets based on the density functional theory calculations. AsN, AsP and SbAs alloys possess indirect band gap and BiAs is direct band gap. Although AsP, SbAs and BiAs alloyed arsenene sheets maintain the semiconducting character of pure arsenene, they have indirect-direct and semiconducting-metallic transitions by applying biaxial strain. We find that B- and N-doped arsenene render p-type semiconducting character, while C- and O-doped arsenene are metallic character. Especially, the C-doped arsenene is spin-polarization asymmetric and can be tuned into the bipolar spin-gapless semiconductor by the external electric field. Moreover, the doping concentration can effectively affect the magnetism of the C-doped system. Finally, we briefly study the chemical molecule adsorbed arsenene. Our results may be valuable for alloyed and doped arsenene sheets applications in mechanical sensors and spintronic devices in the future. PMID:27373712
NASA Astrophysics Data System (ADS)
Liu, Ming-Yang; Huang, Yang; Chen, Qing-Yuan; Cao, Chao; He, Yao
2016-07-01
We study the equilibrium geometry and electronic structure of alloyed and doped arsenene sheets based on the density functional theory calculations. AsN, AsP and SbAs alloys possess indirect band gap and BiAs is direct band gap. Although AsP, SbAs and BiAs alloyed arsenene sheets maintain the semiconducting character of pure arsenene, they have indirect-direct and semiconducting-metallic transitions by applying biaxial strain. We find that B- and N-doped arsenene render p-type semiconducting character, while C- and O-doped arsenene are metallic character. Especially, the C-doped arsenene is spin-polarization asymmetric and can be tuned into the bipolar spin-gapless semiconductor by the external electric field. Moreover, the doping concentration can effectively affect the magnetism of the C-doped system. Finally, we briefly study the chemical molecule adsorbed arsenene. Our results may be valuable for alloyed and doped arsenene sheets applications in mechanical sensors and spintronic devices in the future.
DFT-Based Electronic Structure Calculations on Hybrid and Massively Parallel Computer Architectures
NASA Astrophysics Data System (ADS)
Briggs, Emil; Hodak, Miroslav; Lu, Wenchang; Bernholc, Jerry
2014-03-01
The latest generation of supercomputers is capable of multi-petaflop peak performance, achieved by using thousands of multi-core CPU's and often coupled with thousands of GPU's. However, efficient utilization of this computing power for electronic structure calculations presents significant challenges. We describe adaptations of the Real-Space Multigrid (RMG) code that enable it to scale well to thousands of nodes. A hybrid technique that uses one MPI process per node, rather than on per core was adopted with OpenMP and POSIX threads used for intra-node parallelization. This reduces the number of MPI process's by an order of magnitude or more and improves individual node memory utilization. GPU accelerators are also becoming common and are capable of extremely high performance for vector workloads. However, they typically have much lower scalar performance than CPU's, so achieving good performance requires that the workload is carefully partitioned and data transfer between CPU and GPU is optimized. We have used a hybrid approach utilizing MPI/OpenMP/POSIX threads and GPU accelerators to reach excellent scaling to over 100,000 cores on a Cray XE6 platform as well as a factor of three performance improvement when using a Cray XK7 system with CPU-GPU nodes.
NASA Astrophysics Data System (ADS)
El-Kork, Nayla; Abu el kher, Nariman; Korjieh, Farah; Chtay, John Anwar; Korek, Mahmoud
2017-04-01
A theoretical investigation for the feasibility of laser-cooling is performed through the calculation of accurate potential energy curves, static dipole moments, spectroscopic constants and rovibrational calculations for 24, 26 and 27 highly excited electronic states for BeF, CaF and MgF molecules respectively. In order to understand the electronic structure of their lowest lying electronic states and to learn the characteristic behavior of their chemical bonding, a high level of calculation is realized by using the complete active space self-consistent field (CASSCF) with multi-reference configuration interaction MRCI method including single and double excitations with Davidson correction (+ Q) for the three considered molecules. The comparison between the values of the present work and those available in the literature for several electronic states shows a good agreement. Fifty new excited electronic states have been investigated, in the present work, for the first time for the three studied molecules.
El-Kork, Nayla; Abu El Kher, Nariman; Korjieh, Farah; Chtay, John Anwar; Korek, Mahmoud
2017-04-15
A theoretical investigation for the feasibility of laser-cooling is performed through the calculation of accurate potential energy curves, static dipole moments, spectroscopic constants and rovibrational calculations for 24, 26 and 27 highly excited electronic states for BeF, CaF and MgF molecules respectively. In order to understand the electronic structure of their lowest lying electronic states and to learn the characteristic behavior of their chemical bonding, a high level of calculation is realized by using the complete active space self-consistent field (CASSCF) with multi-reference configuration interaction MRCI method including single and double excitations with Davidson correction (+Q) for the three considered molecules. The comparison between the values of the present work and those available in the literature for several electronic states shows a good agreement. Fifty new excited electronic states have been investigated, in the present work, for the first time for the three studied molecules.
NASA Astrophysics Data System (ADS)
M, Shakil; Muhammad, Zafar; Shabbir, Ahmed; Muhammad Raza-ur-rehman, Hashmi; M, A. Choudhary; T, Iqbal
2016-07-01
The plane wave pseudo-potential method was used to investigate the structural, electronic, and elastic properties of CdSe1-x Te x in the zinc blende phase. It is observed that the electronic properties are improved considerably by using LDA+U as compared to the LDA approach. The calculated lattice constants and bulk moduli are also comparable to the experimental results. The cohesive energies for pure CdSe and CdTe binary and their mixed alloys are calculated. The second-order elastic constants are also calculated by the Lagrangian theory of elasticity. The elastic properties show that the studied material has a ductile nature.
NASA Astrophysics Data System (ADS)
Egawa, Toru; Matsumoto, Rui; Yamamoto, Daisuke; Takeuchi, Hiroshi
2008-12-01
The molecular structure of trans-cinnamaldehyde (( E)-3-phenyl-2-propenal) was determined by means of gas electron diffraction. The nozzle temperature was 165 °C. The results of B3LYP calculations with the 6-31G ∗∗ basis set were used as Supporting information. It was found that this molecule has two stable conformers, s- cis and s- trans, which differ in the orientation of the sbnd CH dbnd O group. Their abundances at 165 °C were determined to be 25 ± 19% and 75%, for the s- cis and s- trans, respectively. This conformational composition is consistent with the prediction by the theoretical calculations. The determined structural parameters ( rg and ∠ α) of the more abundant conformer, s- trans, of trans-cinnamaldehyde are as follows: < r(C sbnd C) ring> = 1.398(1) Å; r(C dbnd C) = 1.348 (←) Å; r(C 1sbnd C) = 1.470(8) Å; r(C sbnd C( dbnd O)) = 1.473(←) Å; r(C dbnd O) = 1.225(6) Å; < r(C sbnd H)> = 1.116(6) Å; ∠C 6sbnd C 1sbnd C 2 = 118.6(3)°; ∠C 1sbnd C 2sbnd C = 121.0(←)°; ∠C sbnd C 6sbnd C 1 = 121.4(←)°; ∠C 2sbnd C 1sbnd C( dbnd C) = 122.0(26)°; ∠C 1sbnd C dbnd C = 128.3(26)°; ∠C dbnd C sbnd CO = 115.3(27)°; ∠C sbnd C dbnd O = 126.6(19)°. The C 1, C 2 and C 6 atoms are on the ring with the C 1 attached to the sbnd CH dbnd CH sbnd CHO group, and the C 2 and C 6 are on the cis and trans sides to the C dbnd C bond, respectively. Angle brackets denote average values; parenthesized values are the estimated limits of error (3 σ) referring to the last significant digit; left arrows in the parentheses mean that the differences to the preceding parameters are fixed.
Ab-initio calculations of electronic, transport, and structural properties of boron phosphide
Ejembi, J. I.; Nwigboji, I. H.; Franklin, L.; Malozovsky, Y.; Zhao, G. L.; Bagayoko, D.
2014-09-14
We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a=4.5383 Å, is in excellent agreement with the experimental value of 2.02±0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.
NASA Astrophysics Data System (ADS)
Hu, Zuowei; Li, Yun; Zhang, Chuanyu; Ao, Bingyun
2016-11-01
The first-principles calculations are performed within the density functional theory to investigate the crystal structure, energy band structure, density of states, optical properties, and bonding properties of strontianite. The optimized structure parameters and bonding results with the generalized gradient approximation (GGA) functional and the localized density approximation (LDA) functional are in good agreement with the earlier experimental data. The band structure, density of states and chemical bonding of strontianite have been calculated and analyzed. The indirect band gap of strontianite is estimated to be ~4.45 eV (GGA) or ~4.24 eV (LDA). The absorption, reflectivity, refractive index and extinction coefficient have been calculated using the imaginary part of the dielectric function. The calculated results of the optical properties show that strontianite has an optical anisotropy along [100] (or [010]) and [010] polarization directions of incoming light. Furthermore, the calculated results of the density of states and Mulliken population indicate that the interactions among atoms are both ionic and covalent bonding in strontianite.
NASA Astrophysics Data System (ADS)
Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.
2017-04-01
We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.
NASA Astrophysics Data System (ADS)
Çiftci, Yasemin Ö.; Evecen, Meryem; Aldırmaz, Emine
2017-01-01
First-principles calculations for the structural, elastic, electronic and vibrational properties of BeGeAs2 with chalcopyrite structure have been reported in the frame work of the density functional theory. The calculated ground state properties are in good agreement with the available data. By considering the electronic band structure and electronic density of states calculation, it is found that this compound is a semiconductor which confirmed the previous work. Single-crystal elastic constants and related properties such as Young's modulus, Poisson ratio, shear modulus and bulk modulus have been predicted using the stress-finite strain technique. It can be seen from the calculated elastic constants that this compound is mechanically stable in the chalcopyrite structure. Pressure dependences of elastic constants and band gap are also reported. Finally, the phonon dispersion curves and total and partial density of states were calculated and discussed. The calculated phonon frequencies BeGeAs2 are positive, indicating the dynamical stability of the studied compound.
Vazart, Fanny; Latouche, Camille; Skouteris, Dimitrios; Barone, Vincenzo; Balucani, Nadia
2015-09-10
New insights into the formation of interstellar cyanomethanimine, a species of great relevance in prebiotic chemistry, are provided by electronic structure and kinetic calculations for the reaction CN + CH{sub 2} = NH. This reaction is a facile formation route of Z,E-C-cyanomethanimine, even under the extreme conditions of density and temperature typical of cold interstellar clouds. E-C-cyanomethanimine has been recently identified in Sgr B2(N) in the Green Bank Telescope (GBT) PRIMOS survey by P. Zaleski et al. and no efficient formation routes have been envisaged so far. The rate coefficient expression for the reaction channel leading to the observed isomer E-C-cyanomethanimine is 3.15 × 10-10 × (T/300){sup 0.152} × e{sup (−0.0948/T)}. According to the present study, the more stable Z-C-cyanomethanimine isomer is formed with a slightly larger yield (4.59 × 10{sup −10} × (T/300){sup 0.153} × e{sup (−0.0871/T)}. As the detection of E-isomer is favored due to its larger dipole moment, the missing detection of the Z-isomer can be due to the sensitivity limit of the GBT PRIMOS survey and the detection of the Z-isomer should be attempted with more sensitive instrumentation. The CN + CH{sub 2} = NH reaction can also play a role in the chemistry of the upper atmosphere of Titan where the cyanomethanimine products can contribute to the buildup of the observed nitrogen-rich organic aerosols that cover the moon.
Pratapa, Phanisri P.; Suryanarayana, Phanish; Pask, John E.
2015-12-02
We present the Clenshaw–Curtis Spectral Quadrature (SQ) method for real-space O(N) Density Functional Theory (DFT) calculations. In this approach, all quantities of interest are expressed as bilinear forms or sums over bilinear forms, which are then approximated by spatially localized Clenshaw–Curtis quadrature rules. This technique is identically applicable to both insulating and metallic systems, and in conjunction with local reformulation of the electrostatics, enables the O(N) evaluation of the electronic density, energy, and atomic forces. The SQ approach also permits infinite-cell calculations without recourse to Brillouin zone integration or large supercells. We employ a finite difference representation in order tomore » exploit the locality of electronic interactions in real space, enable systematic convergence, and facilitate large-scale parallel implementation. In particular, we derive expressions for the electronic density, total energy, and atomic forces that can be evaluated in O(N) operations. We demonstrate the systematic convergence of energies and forces with respect to quadrature order as well as truncation radius to the exact diagonalization result. In addition, we show convergence with respect to mesh size to established O(N3) planewave results. In conclusion, we establish the efficiency of the proposed approach for high temperature calculations and discuss its particular suitability for large-scale parallel computation.« less
NASA Astrophysics Data System (ADS)
Pratapa, Phanisri P.; Suryanarayana, Phanish; Pask, John E.
2016-03-01
We present the Clenshaw-Curtis Spectral Quadrature (SQ) method for real-space O(N) Density Functional Theory (DFT) calculations. In this approach, all quantities of interest are expressed as bilinear forms or sums over bilinear forms, which are then approximated by spatially localized Clenshaw-Curtis quadrature rules. This technique is identically applicable to both insulating and metallic systems, and in conjunction with local reformulation of the electrostatics, enables the O(N) evaluation of the electronic density, energy, and atomic forces. The SQ approach also permits infinite-cell calculations without recourse to Brillouin zone integration or large supercells. We employ a finite difference representation in order to exploit the locality of electronic interactions in real space, enable systematic convergence, and facilitate large-scale parallel implementation. In particular, we derive expressions for the electronic density, total energy, and atomic forces that can be evaluated in O(N) operations. We demonstrate the systematic convergence of energies and forces with respect to quadrature order as well as truncation radius to the exact diagonalization result. In addition, we show convergence with respect to mesh size to established O(N3) planewave results. Finally, we establish the efficiency of the proposed approach for high temperature calculations and discuss its particular suitability for large-scale parallel computation.
Nonrelativistic structure calculations of two-electron ions in a strongly coupled plasma environment
Bhattacharyya, S.; Saha, J. K.; Mukherjee, T. K.
2015-04-01
In this work, the controversy between the interpretations of recent measurements on dense aluminum plasma created with the Linac coherent light source (LCLS) x-ray free electron laser (FEL) and the Orion laser has been addressed. In both kinds of experiments, heliumlike and hydrogenlike spectral lines are used for plasma diagnostics. However, there exist no precise theoretical calculations for He-like ions within a dense plasma environment. The strong need for an accurate theoretical estimate for spectral properties of He-like ions in a strongly coupled plasma environment leads us to perform ab initio calculations in the framework of the Rayleigh-Ritz variation principle in Hylleraas coordinates where an ion-sphere potential is used. An approach to resolve the long-drawn problem of numerical instability for evaluating two-electron integrals with an extended basis inside a finite domain is presented here. The present values of electron densities corresponding to the disappearance of different spectral lines obtained within the framework of an ion-sphere potential show excellent agreement with Orion laser experiments in Al plasma and with recent theories. Moreover, this method is extended to predict the critical plasma densities at which the spectral lines of H-like and He-like carbon and argon ions disappear. Incidental degeneracy and level-crossing phenomena are being reported for two-electron ions embedded in strongly coupled plasma. Thermodynamic pressure experienced by the ions in their respective ground states inside the ion spheres is also reported.
First principle calculations of structural phase transition and electronic properties in AmTe
NASA Astrophysics Data System (ADS)
Pataiya, Jagdeesh; Aynyas, Mahendra; Makode, C.; Singh, A.; Sanyal, S. P.
2015-06-01
The tight-binding linear muffin-tin orbital (TB-LMTO) with in the local density approximation is used to calculate total energy, lattice parameters, bulk modulus, density of states and energy band structure of americium telluride at ambient as well as at high pressure. It is found that AmTe is stable in NaCl - type structure under ambient pressure. The phase transition pressure was found to be 15.0 GPa from NaCl-type (B1-phase) structure to CsCl-type (B2-phase) structure for this compound. From energy band diagram it is observed that AmTe exhibit metallic behaviour. The calculated ground state properties such as lattice parameters and bulk modulus are in general good agreement with the available results.
First principle calculations of structural phase transition and electronic properties in AmTe
Pataiya, Jagdeesh Makode, C.; Aynyas, Mahendra; Singh, A.; Sanyal, S. P.
2015-06-24
The tight-binding linear muffin-tin orbital (TB-LMTO) with in the local density approximation is used to calculate total energy, lattice parameters, bulk modulus, density of states and energy band structure of americium telluride at ambient as well as at high pressure. It is found that AmTe is stable in NaCl – type structure under ambient pressure. The phase transition pressure was found to be 15.0 GPa from NaCl-type (B{sub 1}-phase) structure to CsCl-type (B{sub 2}-phase) structure for this compound. From energy band diagram it is observed that AmTe exhibit metallic behaviour. The calculated ground state properties such as lattice parameters and bulk modulus are in general good agreement with the available results.
Electron correlation and relativistic effects in atomic structure calculations of the thorium atom.
Roy, S K; Prasad, Rajendra; Chandra, P
2011-06-21
Relativistic two-component ab initio calculations have been performed for the Th atom. The spin free low lying states have been calculated at state-averaged complete active space self-consistent field (SA-CASSCF) and multi-state complete active space second-order perturbation (MS-CASPT2) level of theories using different sets of active orbitals. The spin-orbit states have been computed using Douglas-Kroll type of atomic mean-field integral approach. The effects of dynamic electron correlation have been studied at the MS-CASPT2 level. The energy levels of spin-orbit states below 30,000 cm(-1) obtained by the inclusion of dynamic electron correlation are in very good agreement with the experimental values. The radiative properties such as weighted transition probabilities (gA) and oscillator strengths (gf) among several spin-orbit states have been calculated at the SA-CASSCF and MS-CASPT2 levels and are expected to be very helpful for future experiments.
NASA Astrophysics Data System (ADS)
Takeshima, Tsuguhide; Takeuchi, Hiroshi; Egawa, Toru; Konaka, Shigehiro
2007-09-01
The molecular structure of cotinine (( S)-1-methyl-5-(3-pyridinyl)-2-pyrrolidinone), the major metabolite of nicotine, has been determined at about 182 °C by gas electron diffraction combined with MP2 and DFT calculations. The diffraction data are consistent with the existence of the (ax, sc), (ax, ap), (eq, sp) and (eq, ap) conformers, where ax and eq indicate the configuration of the pyrrolidinone ring by means of the position (axial and equatorial) of the pyridine ring, and sc, sp and ap distinguish the isomers arising from the internal rotation around the bond connecting the two rings. The (CH 3)NCCC(N) dihedral angles, ϕ, of the (ax, sc) and (eq, sp) conformers were determined independently to be 158(12)° and 129(13)°, respectively, where the numbers in parentheses are three times the standard errors, 3 σ. According to the MP2 calculations, the corresponding dihedral angles for the (ax, ap) and (eq, ap) conformers were assumed to differ by 180° from their syn counterparts. The ratios x(ax, sc)/ x(ax, ap) and x(eq, sp)/ x(eq, ap) were taken from the theoretically estimated free energy differences, Δ G, where x is the abundance of the conformer. The resultant abundances of (ax, sc), (ax, ap), (eq, sp) and (eq, ap) conformers are 34(6)%, 21% (d.p.), 28% (d.p.), and 17% (d.p.), respectively, where d.p. represents dependent parameters. The determined structural parameters ( rg (Å) and ∠ α (°)) of the most abundant conformer, (ax, sc), are as follows: r(N sbnd C) pyrrol = 1.463(5); r(N sbnd C methyl) = 1.457(←); r(N sbnd C( dbnd O)) = 1.384(12); r(C dbnd O) = 1.219(5); < r(C sbnd C) pyrrol> = 1.541(3); r(C pyrrolsbnd C pyrid) = 1.521(←); < r(C sbnd C) pyrid> = 1.396(2); < r(C sbnd N) pyrid> = 1.343(←); ∠(CNC) pyrrol = 113.9(11); ∠CCC pyrrol(-C pyrid) = 103.6(←); ∠NCO = 124.1(13); ∠NC pyrrolC pyrid = 113.1(12); ∠C pyrrolC pyrrolC pyrid = 113.3(←); ∠(CNC) pyrid = 117.1(2); <∠(NCC) pyrid> = 124.4(←); ∠C methylNC( dbnd O) =
Hegde, Ganesh Bowen, R. Chris
2015-10-15
The accuracy of a single s-orbital representation of Cu towards enabling multi-thousand atom ab initio calculations of electronic structure is evaluated in this work. If an electrostatic compensation charge of 0.3 electron per atom is used in this basis representation, the electronic transmission in bulk and nanocrystalline Cu can be made to compare accurately to that obtained with a Double Zeta Polarized basis set. The use of this representation is analogous to the use of single band effective mass representation for semiconductor electronic structure. With a basis of just one s-orbital per Cu atom, the representation is extremely computationally efficient and can be used to provide much needed ab initio insight into electronic transport in nanocrystalline Cu interconnects at realistic dimensions of several thousand atoms.
First-principles calculations of the electronic structure of one-dimensional C60 polymers
NASA Astrophysics Data System (ADS)
Beu, Titus A.; Onoe, Jun; Hida, Akira
2005-10-01
The geometrical and electronic properties of two dimers (one with C2h symmetry) from the Stone-Wales rearrangement sequence of C60 dimers [described by E. Osawa and K. Honda, Full Sci. Technol. 4, 939 (1996)] are investigated by density functional and tight-binding calculations. The trimer and the infinite periodic polymer derived from the C2h symmetry dimer are shown to continue a decreasing trend of the energy gap between the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals to values smaller than 0.1 eV. The very small energy gap, in conjunction with the extension of the HOMO orbital over the whole cross-linkage region, provides an explanation for the observed conducting properties of electron beam irradiated C60 films.
NASA Astrophysics Data System (ADS)
Mishra, Rashmi; Srivastava, Anubha; Sharma, Anamika; Tandon, Poonam; Baraldi, Cecilia; Gamberini, Maria Christina
2013-01-01
The global problem of advancing bacterial resistance to newer drugs has led to renewed interest in the use of Chloramphenicol Palmitate (C27H42Cl2N2O6) [Palmitic acid alpha ester with D-threo-(-),2-dichloro-N-(beta-hydroxy-alpha-(hydroxymethyl)-p-nitrophenethyl)acetamide also known as Detereopal]. The characterization of the three polymorphic forms of Chloramphenicol Palmitate (CPP) was done spectroscopically by employing FT-IR and FT-Raman techniques. The equilibrium geometry, various bonding features, and harmonic wavenumbers have been investigated for most stable form A with the help of DFT calculations and a good correlation was found between experimental data and theoretical values. Electronic properties have been analyzed employing TD-DFT for both gaseous and solvent phase. The theoretical calculation of thermodynamical properties along with NBO analysis has also been performed to have a deep insight into the molecule for further applications.
Kraemer, W.P.; Hazi, A.U.
1988-06-17
Laboratory measurements of the dissociative recombination of electrons with HCO/sup /plus// ions were performed using the stationary microwave afterglow technique as well as in a flowing afterglow Langmuir probe (FALP) experiment. CASSCF calculations suggest that the recombination of vibrationally cold HCO/sup /plus// ions with low-energy electrons can only proceed via an indirect reaction mechanism. Three different dissociation channels are in principle available to stabilize the intermediate states formed by electron capture. Dissociation can occur along the repulsive potentials of the X /sup 2/..sigma../sup /plus// and of the first excited /sup 2/..sigma../sup /plus// and /sup 2//Pi/ states of HCO. Different electronic states of CO are produced in the three different dissociation channels and their exothermicities vary from ..delta..E/sub e/ = 7.1 eV for CO (X /sup 1/..sigma../sup /plus//) to ..delta..E/sub e/ = 1.1 eV for CO (a /sup 3//Pi/) and finally to ..delta..E/sub e/ = 0.2 eV for CO (a' /sup 3/..sigma../sup /plus//). 11 refs., 3 figs.
State-of-the-art eigensolvers for electronic structure calculations of large scale nano-systems
Voemel, Christof Tomov, Stanimire Z. Marques, Osni A.; Canning, A. Wang, L.-W. Dongarra, Jack J.
2008-07-20
The band edge states determine optical and electronic properties of semiconductor nano-structures which can be computed from an interior eigenproblem. We study the reliability and performance of state-of-the-art iterative eigensolvers on large quantum dots and wires, focusing on variants of preconditioned CG, Lanczos, and Davidson methods. One Davidson variant, the GD + k (Olsen) method, is identified to be as reliable as the commonly used preconditioned CG while consistently being between two and three times faster.
NASA Astrophysics Data System (ADS)
Song, T.; Ma, Q.; Sun, X. W.; Liu, Z. J.; Fu, Z. J.; Wei, X. P.; Wang, T.; Tian, J. H.
2016-09-01
The phase transition, electronic band structure, and equation of state (EOS) of cubic TcN are investigated by first-principles pseudopotential method based on density-functional theory. The calculated enthalpies show that TcN has a transformation between zincblende and rocksalt phases and the pressure determined by the relative enthalpy is 32 GPa. The calculated band structure indicates the metallic feature and it might make cubic TcN a better candidate for hard materials. Particular attention is paid to the predictions of volume, bulk modulus and its pressure derivative which play a central role in the formulation of approximate EOSs using the quasi-harmonic Debye model.
NASA Astrophysics Data System (ADS)
Canning, Andrew
2013-03-01
Inorganic scintillation phosphors (scintillators) are extensively employed as radiation detector materials in many fields of applied and fundamental research such as medical imaging, high energy physics, astrophysics, oil exploration and nuclear materials detection for homeland security and other applications. The ideal scintillator for gamma ray detection must have exceptional performance in terms of stopping power, luminosity, proportionality, speed, and cost. Recently, trivalent lanthanide dopants such as Ce and Eu have received greater attention for fast and bright scintillators as the optical 5d to 4f transition is relatively fast. However, crystal growth and production costs remain challenging for these new materials so there is still a need for new higher performing scintillators that meet the needs of the different application areas. First principles calculations can provide a useful insight into the chemical and electronic properties of such materials and hence can aid in the search for better new scintillators. In the past there has been little first-principles work done on scintillator materials in part because it means modeling f electrons in lanthanides as well as complex excited state and scattering processes. In this talk I will give an overview of the scintillation process and show how first-principles calculations can be applied to such systems to gain a better understanding of the physics involved. I will also present work on a high-throughput first principles approach to select new scintillator materials for fabrication as well as present more detailed calculations to study trapping process etc. that can limit their brightness. This work in collaboration with experimental groups has lead to the discovery of some new bright scintillators. Work supported by the U.S. Department of Homeland Security and carried out under U.S. Department of Energy Contract no. DE-AC02-05CH11231 at Lawrence Berkeley National Laboratory.
Iterative diagonalization in augmented plane wave based methods in electronic structure calculations
Blaha, P.; Laskowski, R.; Schwarz, K.
2010-01-20
Due to the increased computer power and advanced algorithms, quantum mechanical calculations based on Density Functional Theory are more and more widely used to solve real materials science problems. In this context large nonlinear generalized eigenvalue problems must be solved repeatedly to calculate the electronic ground state of a solid or molecule. Due to the nonlinear nature of this problem, an iterative solution of the eigenvalue problem can be more efficient provided it does not disturb the convergence of the self-consistent-field problem. The blocked Davidson method is one of the widely used and efficient schemes for that purpose, but its performance depends critically on the preconditioning, i.e. the procedure to improve the search space for an accurate solution. For more diagonally dominated problems, which appear typically for plane wave based pseudopotential calculations, the inverse of the diagonal of (H - ES) is used. However, for the more efficient 'augmented plane wave + local-orbitals' basis set this preconditioning is not sufficient due to large off-diagonal terms caused by the local orbitals. We propose a new preconditioner based on the inverse of (H - {lambda}S) and demonstrate its efficiency for real applications using both, a sequential and a parallel implementation of this algorithm into our WIEN2k code.
Many-body electronic structure calculations of Eu-doped ZnO
NASA Astrophysics Data System (ADS)
Lorke, M.; Frauenheim, T.; da Rosa, A. L.
2016-03-01
The formation energies and electronic structure of europium-doped zinc oxide has been determined using DFT and many-body G W methods. In the absence of intrisic defects, we find that the europium-f states are located in the ZnO band gap with europium possessing a formal charge of 2+. On the other hand, the presence of intrinsic defects in ZnO allows intraband f -f transitions otherwise forbidden in atomic europium. This result corroborates with recently observed photoluminescence in the visible red region S. Geburt et al. [Nano Lett. 14, 4523 (2014), 10.1021/nl5015553].
Ding, Li-Ping; Shao, Peng; Zhang, Fang-Hui; Lu, Cheng; Ding, Lei; Ning, Shu Ya; Huang, Xiao Fen
2016-07-18
On the basis of the first-principles techniques, we perform the structure prediction for MoB2. Accordingly, a new ground-state crystal structure WB2 (P63/mmc, 2 fu/cell) is uncovered. The experimental synthesized rhombohedral R3̅m and hexagonal AlB2, as well as theoretical predicted RuB2 structures, are no longer the most favorite structures. By analyzing the elastic constants, formation enthalpies, and phonon dispersion, we find that the WB2 phase is thermodynamically and mechanically stable. The high bulk modulus B, shear modulus G, low Poisson's ratio ν, and small B/G ratio are benefit to its low compressibility. When the pressure is 10 GPa, a phase transition is observed between the WB2-MoB2 and the rhombohedral R3̅m MoB2 phases. By analyzing the density of states and electron density, we find that the strong covalent is formed in MoB2 compounds, which contributes a great deal to its low compressibility. Furthermore, the low compressibility is also correlated with the local buckled structure.
Tohme, Samir N.; Korek, Mahmoud E-mail: fkorek@yahoo.com; Awad, Ramadan
2015-03-21
Ab initio techniques have been applied to investigate the electronic structure of the LiYb molecule. The potential energy curves have been computed in the Born–Oppenheimer approximation for the ground and 29 low-lying doublet and quartet excited electronic states. Complete active space self-consistent field, multi-reference configuration interaction, and Rayleigh Schrödinger perturbation theory to second order calculations have been utilized to investigate these states. The spectroscopic constants, ω{sub e}, R{sub e}, B{sub e}, …, and the static dipole moment, μ, have been investigated by using the two different techniques of calculation with five different types of basis. The eigenvalues, E{sub v}, the rotational constant, B{sub v}, the centrifugal distortion constant, D{sub v}, and the abscissas of the turning points, R{sub min} and R{sub max}, have been calculated by using the canonical functions approach. The comparison between the values of the present work, calculated by different techniques, and those available in the literature for several electronic states shows a very good agreement. Twenty-one new electronic states have been studied here for the first time.
NASA Astrophysics Data System (ADS)
Weber, Sven-Ulf; Grodzicki, Michael; Lottermoser, Werner; Redhammer, Günther J.; Tippelt, Gerold; Ponahlo, Johann; Amthauer, Georg
2007-09-01
Natural alexandrite Al2BeO4:Cr from Malyshevo near Terem Tschanka, Sverdlovsk, Ural, Russia, has been characterized by 57Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine oxidation state and location of iron. The sample contains 0.3 wt% of total iron oxide. The 57Fe Mössbauer spectrum can be resolved into three doublets. Two of them with hyperfine parameters typical for octahedrally coordinated high-spin Fe3+ and Fe2+, respectively, are assigned to iron substituting for Al in the octahedral M2-site. The third doublet is attributed to Fe3+ in hematite. Electronic structure calculations in the local spin density approximation are in reasonable agreement with experimental data provided that expansion and/or distortion of the coordination octahedra are presumed upon iron substitution. The calculated hyperfine parameters of Fe3+ are almost identical for the M1 and M2 positions, but the calculated ligand-field splitting is by far too large for high-spin Fe3+ on M1.
Viñes, Francesc; Illas, Francesc
2017-03-30
The atomic and electronic structure of stoichiometric and reduced ZnO wurtzite has been studied using a periodic relativistic all electron hybrid density functional (PBE0) approach and numeric atom-centered orbital basis set with quality equivalent to aug-cc-pVDZ. To assess the importance of relativistic effects, calculations were carried out without and with explicit inclusion of relativistic effects through the zero order regular approximation. The calculated band gap is ∼0.2 eV smaller than experiment, close to previous PBE0 results including relativistic calculation through the pseudopotential and ∼0.25 eV smaller than equivalent nonrelativistic all electron PBE0 calculations indicating possible sources of error in nonrelativistic all electron density functional calculations for systems containing elements with relatively high atomic number. The oxygen vacancy formation energy converges rather fast with the supercell size, the predicted value agrees with previously hybrid density functional calculations and analysis of the electronic structure evidences the presence of localized electrons at the vacancy site with a concomitant well localized peak in the density of states ∼0.5 eV above the top of the valence band and a significant relaxation of the Zn atoms near to the oxygen vacancy. Finally, present work shows that accurate results can be obtained in systems involving large supercells containing up to ∼450 atoms using a numeric atomic-centered orbital basis set within a full all electron description including scalar relativistic effects at an affordable cost. © 2017 Wiley Periodicals, Inc.
Electronic Structure of Organic/Inorganic Interfaces: Insights from First Principles Calculations
NASA Astrophysics Data System (ADS)
Segev, Lior
Electronic devices based on molecules draw a lot of attention in both scientific and industrial activities. Molecules in electronic devices can serve as the heart of the device, featuring versatile physical properties i.e. electronical, optical, magnetic, etc. Molecules can also function as an assist mechanism in which the electronic properties of the underlying material are modified in a predictable fashion according to the molecular monolayer properties. But, the route to applications in both these directions lies in answering fundamental questions related to band offsets between two materials, full electronic structure determination of molecule and substrates, work function modifications, etc. To tackle these questions, we chose to study the interface formed by an alkyl monolayer adsorbed on a Si substrate by utilizing two ab initio methods. First, the density functional theory (DFT) utilizing the local density or the B3LYP approximations for the exchange-correlation potential and, second, the many-body perturbation theory based on the GW approximation. We adapted a "divide and conquer" approach to our system by simulating the infinite counterpart, polyethylene, of our finite alkyl chain to test how the band gap of the two molecules changes when moving from an infinite 1D molecule to a finite length molecule. We find excellent agreement between our GW simulation results for polyethylene and experimental results for the bandstructure, ionization potential and band gap values. From DFT simulations, we analyze the ultra-violet photoelectron spectra (UPS) of odd and even number of carbons alkyl chains and identify the origin of their differences in spectral signature. GW simulations of the full alkyl monolayer/Si(111) system reveal that the projected density of states (DOS) of the upper alkyl chain have an excellent agreement to experimental UPS and inverse-photoemission spectra results. Based on this correspondence, we find the band alignment between the alkyl
NASA Astrophysics Data System (ADS)
GALVAN, DONALD H.
To gain insight into the electronic properties of PrFe4P12 filled skutterudite, band electronic structure calculations, total and projected density of states, crystal orbital overlap population and Mulliken population analysis were performed. The energy bands yield a semi-metallic behavior with a direct gap (at Γ) of 0.02 eV. Total and Projected Density of States provided information of the contribution from each orbital of each atom to the total Density of States. Moreover, the bonding strength between some atoms within the unit cell was obtained. Mulliken Population Analysis suggests ionic behavior for this filled skutterudite.
Liang, WanZhen; Head-Gordon, Martin
2004-06-08
A new formulation of the diagonalization step in self-consistent-field (SCF) electronic structure calculations is presented. It exactly replaces the diagonalization of the effective Hamiltonian with the solution of a set of second order nonlinear equations. The density matrix and/or the new set of occupied orbitals can be directly obtained from the resulting solution. This formulation may offer interesting possibilities for new approaches to efficient SCF calculations. The working equations can be derived either from energy minimization with respect to a Cayley-type parametrization of a unitary matrix, or from a similarity transformation approach.
Close-coupling calculations of fine-structure excitation of Ne II due to H and electron collisions
NASA Astrophysics Data System (ADS)
Stancil, Phillip C.; Cumbee, Renata; Wang, Qianxia; Loch, Stuart; Pindzola, Michael; Schultz, David R.; Buenker, Robert; McLaughlin, Brendan; Ballance, Connor
2016-06-01
Fine-structure transitions within the ground term of ions and neutral atoms dominate the cooling in a variety of molecular regions and also provide important density and temperature diagnostics. While fine-structure rates due to electron collisions have been studied for many systems, data are generally sparse for elements larger than oxygen, at low temperatures, and for collisions due to heavy particles. We provide rate coefficients for H collisions for the first time. The calculations were performed using the quantum molecular-orbital close-coupling approach and the elastic approximation. The heavy-particle collisions use new potential energies for the lowest-lying NeH+ states computed with the MRDCI method. The focus of the electron-impact calculations is to provide fine-structure excitation rate coefficients down to 10 K. We compare with previous calculations at higher temperatures (Griffin et al. 2001), and use a range of calculations to provide an estimate of the uncertainty on our recommended rate coefficients. A brief discussion of astrophysical applications is also provided.Griffin, D.C., et al., 2001, J. Phys. B, 34, 4401This work partially supported by NASA grant No. NNX15AE47G.
NASA Astrophysics Data System (ADS)
Labra-Vázquez, Pablo; Palma-Contreras, Miguel; Santillan, Rosa; Farfán, Norberto
2017-03-01
The molecular structure of 1-[2-oxo-2-(2-pyridinyl)ethyl]pyridinium iodide (C12H11IN2O) is discussed using an experimental (FT-IR/ATR, NMR, SXRD) and theoretical (DFT, B3LYP/6-311G**) approach. Compound 2 crystallized in the monoclinic P21/c space group with 4 molecules per unit cell and unit cell dimensions a = 7.5629 Å (3), b = 21.5694 Å (7), c = 7.8166 Å (3). The crystal packing is governed by ion-dipole contacts and π-π stacking. High electrostatic potential at the ethanone hydrogens was derived from DFT calculations, further explaining the acidity and reactivity of the molecule as a Michael donor.
Du, Jincheng; Devanathan, Ramaswami; Corrales, Louis R.; Weber, William J.
2012-05-01
First-principles periodic density functional theory (DFT) calculations have been performed to understand the electronic structure, chemical bonding, phase transition, and physical properties of the mineral zircon (in the chemical composition of ZrSiO4) and its high pressure phase reidite. Temperature effect on phase transition and thermal–mechanical properties such as heat capacity and bulk modulus have been studied by combining the equation of states obtained from DFT calculations with the quasi-harmonic Debye model to take into account the entropy contribution to free energy. Local density approximation (LDA) and generalized gradient approximation (GGA) DFT functionals have been systematically compared in predicting the structure and property of this material. It is found that the LDA functional provides a better description of the equilibrium structure and bulk modulus, while GGA predicts a transition pressure closer to experimental values. Both functionals correctly predict the relative stability of the two phases, with GGA giving slightly larger energy differences. The calculated band structures show that both zircon and reidite have indirect bandgaps and the reidite phase has a narrower bandgap than the zircon phase. The electronic density of states and atomic charges analyses show that bonding in the high-pressure reidite phase has a stronger covalent character.
NASA Astrophysics Data System (ADS)
Kong, Bo; Zhang, Yachao
2016-07-01
The electronic structures of the cubic GdH3 are extensively investigated using the ab initio many-body GW calculations treating the Gd 4f electrons either in the core (4f-core) or in the valence states (4f-val). Different degrees of quasiparticle (QP) self-consistent calculations with the different starting points are used to correct the failures of the GGA/GGA + U/HSE03 calculations. In the 4f-core case, GGA + G0W0 calculations give a fundamental band gap of 1.72 eV, while GGA+ GW0 or GGA + GW calculations present a larger band gap. In the 4f-val case, the nonlocal exchange-correlation (xc) functional HSE03 can account much better for the strong localization of the 4f states than the semilocal or Hubbard U corrected xc functional in the Kohn-Sham equation. We show that the fundamental gap of the antiferromagnetic (AFM) or ferromagnetic (FM) GdH3 can be opened up by solving the QP equation with improved starting point of eigenvalues and wave functions given by HSE03. The HSE03 + G0W0 calculations present a fundamental band gap of 2.73 eV in the AFM configuration, and the results of the corresponding GW0 and GW calculations are 2.89 and 3.03 eV, respectively. In general, for the cubic structure, the fundamental gap from G0W0 calculations in the 4f-core case is the closest to the real result. By G0W0 calculations in the 4f-core case, we find that H or Gd defects can strongly affect the band structure, especially the H defects. We explain the mechanism in terms of the possible electron correlation on the hydrogen site. Under compression, the insulator-to-metal transition in the cubic GdH3 occurs around 40 GPa, which might be a satisfied prediction.
Katsukura, Hirotaka; Miyata, Tomohiro; Tomita, Kota; Mizoguchi, Teruyasu
2016-07-21
The effect of the van der Waals (vdW) interaction on the simulation of the electron energy-loss near edge structure (ELNES) by a first-principles band-structure calculation is reported. The effect of the vdW interaction is considered by the Tkatchenko-Scheffler scheme, and the change of the spectrum profile and the energy shift are discussed. We perform calculations on systems in the solid, liquid and gaseous states. The transition energy shifts to lower energy by approximately 0.1eV in the condensed (solid and liquid) systems by introducing the vdW effect into the calculation, whereas the energy shift in the gaseous models is negligible owing to the long intermolecular distance. We reveal that the vdW interaction exhibits a larger effect on the excited state than the ground state owing to the presence of an excited electron in the unoccupied band. Moreover, the vdW effect is found to depend on the local electron density and the molecular coordination. In addition, this study suggests that the detection of the vdW interactions exhibited within materials is possible by a very stable and high resolution observation.
New Polytypoid SnO2(ZnO:Sn)m Nanowire: Characterization and Calculation of its Electronic Structure
Cao, B.; Shi, T.; Zheng, S.; Ikuhara, Y. H.; Zhou, W.; Wood, D.; Al-Jassim, M.; Yan, Y.
2012-03-01
A new SnO{sub 2}(ZnO:Sn){sub m} polytypoid nanowire has been synthesized through a two-step chemical vapor deposition (CVD) method. Cs-corrected scanning transmission electron microscopy (STEM) studies indicate the crystal structure consists of an alternating stack of a Sn-O octahedral layer and a (Sn/Zn)-O wurtzite slab. First-principles density-functional theory (DFT) calculations suggest Sn atoms in the Sn-doped ZnO slabs have both 4- and 6-fold coordination, forming an inversion boundary of polarity. The SnO{sub 2}(ZnO:Sn){sub m} nanowires may exhibit unique anisotropic electronic properties: poor conductivity along the wire axis, but metallic conductivity along the diametrical directions according to the calculations.
NASA Astrophysics Data System (ADS)
Cimrman, Robert; Tůma, Miroslav; Novák, Matyáš; Čertík, Ondřej; Plešek, Jiří; Vackář, Jiří
2013-10-01
Ab-initio calculations of electronic states within the density-functional framework has been performed by means of the open source finite element package SfePy (Simple Finite Elements in Python, http://sfepy.org). We describe a new robust ab-initio real-space code based on (i) density functional theory, (ii) finite element method and (iii) environment-reflecting pseudopotentials. This approach brings a new quality to solving Kohn-Sham equations, calculating electronic states, total energy, Hellmann-Feynman forces and material properties particularly for non-crystalline, non-periodic structures. The main asset of the above approach is an efficient combination of excellent convergence control of standard, universal basis used in industrially proved finite-element method, high precision of ab-initio environment-reflecting pseudopotentials, and applicability not restricted to electrically neutral periodic environment. We present also numerical examples illustrating the outputs of the method.
First-Principles Calculations for the Structural and Electronic Properties of ScxAl1-xN Alloys
NASA Astrophysics Data System (ADS)
Mohammad, Rezek; Katircioğlu, Şenay
2013-10-01
The first-principles calculations based on Density Functional Theory (DFT) within generalized gradient approximation (GGA) of Engel-Vosko-Perdew-Wang and modified exact exchange potential of Becke-Johnson have been introduced for the structural and electronic properties of the ScxAl1-xN alloys, respectively. The present lattice constants calculated for the ScAlN alloys and the end compounds (AlN and ScN) are found to be in very good agreement with the available experimental and theoretical ones. The stable ground state structures of the ScxAl1-xN alloys are determined to be wurtzite for the Sc concentration less than 0.403 and rock-salt for the higher Sc concentrations. The present electronic band structure calculations within Becke-Johnson scheme are found to be capable of providing energy band gaps of the AlN and ScN compounds very close to the ones of the available experiments and expensive calculations. According to the calculations of Becke-Johnson potential, the ScxAl1-xN alloys in the wurtzite and zinc-blende structures are direct band gap materials for the Sc concentrations in the ranges of (0.056 ≤ x ≤ 0.833) and (0.03125 ≤ x ≤ 0.0625, 0.375 ≤ x ≤ 0.96875), respectively. However, the ScAlN alloys in the rock-salt phase are determined to be direct band gap materials for total range of the Sc concentration considered in this work. While the energy gaps of the RS-AlScN alloys are found to be extending from near ultraviolet to near infrared with a large (negative) bowing, the ones of the WZ-AlScN and ZB-AlScN alloys are determined to be varying in a small energy range around near ultraviolet with a small (negative) bowing.
Chen, Xingqiu; Fu, Chong Long; Morris, James R
2010-01-01
Using an ab initio density functional approach, we report on the ground-state phase stabilities, enthalpies of formation, electronic, and elastic properties of the Ti-Pd alloy system. The calculated enthalpies of formation are in excellent agreement with available calorimetric data. We found a linear dependence between the calculated enthalpies of formation of several intermetallic structures and the Pd-concentration, indicating that each of these compounds has a very limited composition range. The elastic constants for many of these Ti-Pd intermetallics were calculated and analyzed. The B2 TiPd phase is found to be mechanically unstable with respect to the transformation into the monoclinic B19 structure. A series of hydrides, Ti2PdHx (x=1, 1.5, 2, 3, 4), have been investigated in terms of electronic structure, enthalpies of hydrogen absorption, and site preference of H atoms. Our results illustrate the physical mechanism for hydrogen absorption in term of the charge transfer, and explain why TiPd2 does not form a stable hydride.
Wang, Ya-Ting; Gao, Yuan-Jun; Wang, Qian; Cui, Ganglong
2017-02-02
Intramolecularly bridged diarylethenes exhibit improved photocyclization quantum yields because the anti-syn isomerization that originally suppresses photocyclization in classical diarylethenes is blocked. Experimentally, three possible channels have been proposed to interpret experimental observation, but many details of photochromic mechanism remain ambiguous. In this work we have employed a series of electronic structure methods (OM2/MRCI, DFT, TDDFT, RI-CC2, DFT/MRCI, and CASPT2) to comprehensively study excited state properties, photocyclization, and photoreversion dynamics of 1,2-dicyano[2,2]metacyclophan-1-ene. On the basis of optimized stationary points and minimum-energy conical intersections, we have refined experimentally proposed photochromic mechanism. Only an S1/S0 minimum-energy conical intersection is located; thus, we can exclude the third channel experimentally proposed. In addition, we find that both photocyclization and photoreversion processes use the same S1/S0 conical intersection to decay the S1 system to the S0 state, so we can unify the remaining two channels into one. These new insights are verified by our OM2/MRCI nonadiabatic dynamics simulations. The S1 excited-state lifetimes of photocyclization and photoreversion are estimated to be 349 and 453 fs, respectively, which are close to experimentally measured values: 240 ± 60 and 250 fs in acetonitrile solution. The present study not only interprets experimental observations and refines previously proposed mechanism but also provides new physical insights that are valuable for future experiments.
NASA Astrophysics Data System (ADS)
Azhar, N. S.; Taib, M. F. M.; Hassan, O. H.; Yahya, M. Z. A.; Ali, A. M. M.
2017-03-01
Crystal structures of α-Bi2O3 and β-Bi2O3 were calculated using Cambridge serial total energy package (CASTEP) based on the first-principles plane-wave ultrasoft pseudopotential method within local density approximation (LDA) and generalized gradient approximation (GGA) together with Perdew–Burke–Ernzerhof (GGA-PBE) and Perdew–Burke–Ernzerhof revised for solid (GGA-PBEsol). The structural parameter of α-Bi2O3 and β-Bi2O3 are in good agreement with previous experimental and theoretical data. All of the polymorphs were calculated for the total density of states (TDOS) and the partial density of states (PDOS) of Bi, O atoms. Density of states exhibits hybridization of Bi 6s and O 2p orbitals and the calculated charge density profiles exhibit the ionic character in the chemical bonding of this compound. The narrowed band gap (E g) and red-shift of light absorption edge are responsible for the photocatalytic activity of Bi2O3 for water splitting application. The optical properties such as optical absorption and electron energy loss function were calculated to show the best structure among these polymorphs for the photocatalytic water splitting application.
Electronic band structure and specific features of Sm2NiMnO6 compound: DFT calculation
NASA Astrophysics Data System (ADS)
Reshak, A. H.; Azam, Sikander
2013-09-01
The band structure, density of states, electronic charge density, Fermi surface and optical properties of Sm2NiMnO6 compound have been investigated with the support of density functional theory (DFT). The atomic positions of Sm2NiMnO6 compound were optimized by minimizing the forces acting on the atoms, using the full potential linear augmented plane wave method. We employed the local density approximation (LDA), generalized gradient approximation (GGA) and Engel-Vosko GGA (EVGGA) to treat the exchange correlation potential by solving Kohn-Sham equations. The calculation shows that the compound is metallic with strong hybridization near the Fermi energy level (EF). The calculated density of states at the EF is about 21.60, 24.52 and 26.21 states/eV, and the bare linear low-temperature electronic specific heat coefficient (γ) is found to be 3.74, 4.25 and 4.54 mJ/mol K2 for EVGGA, GGA and LDA, respectively. The Fermi surface is composed of two sheets. The bonding features of the compounds are analyzed using the electronic charge density in the (011) crystallographic plane. The dispersion of the optical constants was calculated and discussed.
NASA Technical Reports Server (NTRS)
Long, E. R., Jr.
1979-01-01
The Bethe-Bloch stopping power relations for inelastic collisions were used to determine the absorption of electron and proton energy in cured neat epoxy resin and the absorption of electron energy in a graphite/epoxy composite. Absorption of electron energy due to bremsstrahlung was determined. Electron energies from 0.2 to 4.0 MeV and proton energies from 0.3 to 1.75 MeV were used. Monoenergetic electron energy absorption profiles for models of pure graphite, cured neat epoxy resin, and graphite/epoxy composites are reported. A relation is determined for depth of uniform energy absorption in a composite as a function of fiber volume fraction and initial electron energy. Monoenergetic proton energy absorption profiles are reported for the neat resin model. A relation for total proton penetration in the epoxy resin as a function of initial proton energy is determined. Electron energy absorption in the composite due to bremsstrahlung is reported. Electron and proton energy absorption profiles in cured neat epoxy resin are reported for environments approximating geosynchronous earth orbit.
Sesion, P D; Henriques, J M; Barboza, C A; Albuquerque, E L; Freire, V N; Caetano, E W S
2010-11-03
CdSnO(3) ilmenite and perovskite crystals were investigated using both the local density and generalized gradient approximations, LDA and GGA, respectively, of the density functional theory (DFT). The electronic band structures, densities of states, dielectric functions, optical absorption and reflectivity spectra related to electronic transitions were obtained, as well as the infrared absorption spectra after computing the vibrational modes of the crystals at q = 0. Dielectric optical permittivities and polarizabilities at ω = 0 and ∞ were also calculated. The results show that GGA-optimized geometries are more accurate than LDA ones, and the Kohn-Sham band structures obtained for the CdSnO(3) polymorphs confirm that ilmenite has an indirect band gap, while perovskite has a direct band gap, both being semiconductors. Effective masses for both crystals are obtained for the first time, being highly isotropic for electrons and anisotropic for holes. The optical properties reveal a very small degree of anisotropy of both crystals with respect to different polarization planes of incident light. The phonon calculation at q = 0 for perovskite CdSnO(3) does not show any imaginary frequencies, in contrast to a previous report suggesting the existence of a more stable crystal of perovskite CdSnO(3) with ferroelectric properties.
NASA Astrophysics Data System (ADS)
Li, Yanli; Dabo, Ismaila
2011-10-01
Plane-wave electronic-structure predictions based upon orbital-dependent density-functional theory (OD-DFT) approximations, such as hybrid density-functional methods and self-interaction density-functional corrections, are severely affected by computational inaccuracies in evaluating electron interactions in the plane-wave representation. These errors arise from divergence singularities in the plane-wave summation of electrostatic and exchange interaction contributions. Auxiliary-function corrections are reciprocal-space countercharge corrections that cancel plane-wave singularities through the addition of an auxiliary function to the point-charge electrostatic kernel that enters into the expression of interaction terms. At variance with real-space countercharge corrections that are employed in the context of density-functional theory (DFT), reciprocal-space corrections are computationally inexpensive, making them suited to more demanding OD-DFT calculations. Nevertheless, there exists much freedom in the choice of auxiliary functions and various definitions result in different levels of performance in eliminating plane-wave inaccuracies. In this work we derive exact point-charge auxiliary functions for the description of molecular structures of arbitrary translational symmetry, including the yet unaddressed one-dimensional case. In addition, we provide a critical assessment of different reciprocal-space countercharge corrections and demonstrate the improved accuracy of point-charge auxiliary functions in predicting the electronic levels and electrical response of conjugated polymers from plane-wave OD-DFT calculations.
NASA Astrophysics Data System (ADS)
Borghi, Giovanni; Fabrizio, Michele; Tosatti, Erio
2014-09-01
The Gutzwiller projector technique has long been known as a method to include correlations in electronic structure calculations. We describe a model implementation for a Gutzwiller +LDA calculation in a localized-orbital restricted basis framework, emphasizing the protocol step by step and illustrating our specific procedure for this and future applications. We demonstrate the method with a classic problem, the ferromagnetism of bulk bcc Fe, whose nature is attracting fresh interest. In the conventional Stoner-Wohlfarth model, and in spin-polarized LDA calculations, the ferromagnetic ordering of iron sets in so that the electrons can reduce their mutual Coulomb repulsion, at the cost of some increase of electron kinetic energy. This balance may, however, be altered by correlations, which are strong for localized d orbitals. The present localized basis Gutzwiller +LDA calculation demonstrates how the ferromagnetic ordering of Fe may, in fact, entrain a decrease of kinetic energy at the cost of some increase of potential energy. This happens because, as foreshadowed long ago by Goodenough and others and more recently supported by LDA-DMFT calculations, correlations cause eg and t2g d orbitals to behave differently, with the weakly propagating eg states fully spin polarized and almost localized, and only t2g states forming a broad partly filled itinerant band. Owing to an intra-atomic Hund's rule exchange that aligns eg and t2g spins, the propagation of itinerant t2g holes is favored when different atomic spins are ferromagnetically aligned. This suggests a strong analogy with double exchange in iron ferromagnetism.
Du, Jincheng; Devanathan, Ram; Corrales, L Rene; Weber, William J
2012-01-01
First principle periodic density functional theory (DFT) calculations have been performed to understand the electronic structure, chemical bonding, phase transition, and physical properties of the zircon (in the chemical composition of ZrSiO4) and its high pressure phase reidite. Temperature effect on phase transition and thermal-mechanical properties such as heat capacity and bulk modulus have been studied by combining the equation of states obtained from DFT calculations with the quasi-harmonic Debye model to take into account the entropy contribution to free energy. Local density approximation (LDA) and generalized gradient approximation (GGA) DFT functionals have been systematically compared in predicting the structure and property of this material. It is found that the LDA functional provides a better description of the equilibrium structure and bulk modulus, while GGA predicts a transition pressure closer to experimental values. Both functionals correctly predict the relative stability of the two phases, with GGA giving slightly larger energy differences. The calculated band structures show that both zircon and reidite have indirect bandgaps and the reidite phase has a narrower bandgap than the zircon phase. The atomic charges determined using the Bader method show that bonding in reidite has a stronger covalent character.
NASA Astrophysics Data System (ADS)
Pimenov, Oleg A.; Belova, Natalya V.; Sliznev, Valery V.
2017-03-01
The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione thulium, or Tm(thd)3, has been studied by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and quantum chemical (DFT) calculations. Both the DFT(PBE0) calculations and the GED data collected at 400(8) K indicate that the molecules have D3 symmetry with a distorted antiprismatic TmO6 coordination geometry. According to GED refinements the twist angle θ, i.e. the angle of rotation of the upper O3 triangles relative to their position in regular prism is θ = 16.9(2.0)0. This value is close to both the equilibrium value obtained from the DFT calculations and to the thermal average value at the temperature of the GED experiment obtained by integration over the DFT potential energy surface. The bond distances (rh1) in the chelate ring are Tmsbnd O = 2.214(5) Å, Csbnd O = 1.278(4) Å, and Csbnd C = 1.404(3) Å. The DFT calculations yielded structure parameters in close agreement with those found experimentally. As an alternative to conventional Lewis model which was realized in NBO the topological analysis of ρ(r) in the frame of Bader's quantum theory of atoms in molecule (QTAIM) was performed.
Kovács, Gábor; Kozlov, Sergey M; Matolínová, Iva; Vorokhta, Mykhailo; Matolín, Vladimír; Neyman, Konstantin M
2015-11-14
The high catalytic activity of Pt-Co nanoalloys in oxygen reduction and other reactions is usually attributed to their Pt-rich surfaces. However, identification of the precise near-surface structure is by no means easily achievable experimentally. In this work we systematically analyzed the chemical ordering and surface composition of PtXCo(79-X) and PtXCo(140-X) bimetallic nanoparticles by means of a recently developed method based on topological energy expressions and electronic structure calculations. Pt is found to segregate on the surface, especially on corner and edge sites, forming a one atomic layer thick skin independent of the size and composition of the nanoparticle. In turn, the subsurface shell of the particle is composed mostly of Co, whereas the core area has a mixed composition, which depends on the overall stoichiometry. The formation of an outer Pt shell is corroborated by thoroughly analyzed data of X-ray photoelectron spectroscopy experiments performed with various photon energies on annealed Pt-Co particles prepared in vacuum by magnetron sputtering. The core-shell structure of Pt-Co particles is calculated to be more stable than the respective L10 structure. The obtained topological energy expressions are shown to depend only very moderately on the nanoparticle size, which allowed us to apply them to determine the ordering in ∼4 nm big PtXCo(1463-X) species. The presented results deepen our understanding of the intrinsic structure of Pt-Co nanoparticles depending on their size and composition.
A Detailed Derivation of Gaussian Orbital-Based Matrix Elements in Electron Structure Calculations
ERIC Educational Resources Information Center
Petersson, T.; Hellsing, B.
2010-01-01
A detailed derivation of analytic solutions is presented for overlap, kinetic, nuclear attraction and electron repulsion integrals involving Cartesian Gaussian-type orbitals. It is demonstrated how s-type orbitals can be used to evaluate integrals with higher angular momentum via the properties of Hermite polynomials and differentiation with…
Tucker, Jon R.; Magyar, Rudolph J.
2012-02-01
High explosives are an important class of energetic materials used in many weapons applications. Even with modern computers, the simulation of the dynamic chemical reactions and energy release is exceedingly challenging. While the scale of the detonation process may be macroscopic, the dynamic bond breaking responsible for the explosive release of energy is fundamentally quantum mechanical. Thus, any method that does not adequately describe bonding is destined to lack predictive capability on some level. Performing quantum mechanics calculations on systems with more than dozens of atoms is a gargantuan task, and severe approximation schemes must be employed in practical calculations. We have developed and tested a divide and conquer (DnC) scheme to obtain total energies, forces, and harmonic frequencies within semi-empirical quantum mechanics. The method is intended as an approximate but faster solution to the full problem and is possible due to the sparsity of the density matrix in many applications. The resulting total energy calculation scales linearly as the number of subsystems, and the method provides a path-forward to quantum mechanical simulations of millions of atoms.
Li, M M; Fan, Xiaofeng; Zheng, W T
2013-10-23
With first-principle calculations, we studied the structural stability and electronic properties of the BxCy compounds based on three kinds of phases including diamond-like, C20-like and B15-like phases. The C20-like structure B8C12 is found to be a new stable structure with relatively low formation energy in middle boron concentration and is expected to be synthesized experimentally. Combined with a microscopic model, the Vickers hardness of the different configurations of BxCy compounds is analyzed with the change of boron concentration. It is found that the hardness of the B-C system has a decreasing trend with the increase of boron concentration. In addition, all the structures have metallic properties, except B12C3 and B14C. With the analysis of Mulliken bond population and charge distribution, the bonds with high electron density and short bond length have an important contribution to the hardness in the B-C system, while the effect of metallicity to hardness can be ignored.
The ab initio calculations of the doping Zr's influence on the electronic structure of AlCo2Ti
NASA Astrophysics Data System (ADS)
Fu, Hongzhi; Peng, Feng; Cheng, Dong; Gao, Tao; Cheng, Xinlu; Yang, Xiangdong
2007-08-01
The electronic structures of the ternary (Hume Rothery) L21-phase compound AlCo2Ti are calculated by first-principles using full potential linearized augmented plane wave (FLAPW) method with the generalized gradient approximation (GGA). The ab initio results are analyzed with a simplified model for Al-based compounds containing transition metal (TM) atoms. The results show that the total DOS depends strongly on the positions of TM atoms, and the TM d DOS plays a crucial role in hybridization with other element valence electrons. However, the Al 3s states are repelled far away from the Fermi energy in studied sample, and the Al 3d states are far more extended-like in the character than the d states. Furthermore, the total DOSs are modulated by Al 3p states and the Al 3p states are more sensitive than d states to change in the electronic interactions. Then, the Al 3p is also important for the ternary stability of the intermetallic compound. The Co Ti interaction becomes stronger by the doping element Zr in the Al4Co8Ti3Zr structure. Especially, the doping Al4Co8Ti3Zr alloy has a larger value DOS at the Fermi level and makes the total DOS gap smaller than the AlCo2Ti.
Muresan, Nicoleta; Lu, Connie C; Ghosh, Meenakshi; Peters, Jonas C; Abe, Megumi; Henling, Lawrence M; Weyhermöller, Thomas; Bill, Eckhard; Wieghardt, Karl
2008-06-02
The electronic structure of a family comprising tetrahedral (alpha-diimine)iron dichloride, and tetrahedral bis(alpha-diimine)iron compounds has been investigated by Mossbauer spectroscopy, magnetic susceptibility measurements, and X-ray crystallography. In addition, broken-symmetry density functional theoretical (B3LYP) calculations have been performed. A detailed understanding of the electronic structure of these complexes has been obtained. A paramagnetic (St=2), tetrahedral complex [FeII(4L)2], where (4L)1- represents the diamagnetic monoanion N-tert-butylquinolinylamide, has been synthesized and characterized to serve as a benchmark for a Werner-type complex containing a tetrahedral FeIIN4 geometry and a single high-spin ferrous ion. In contrast to the most commonly used description of the electronic structure of bis(alpha-diimine)iron(0) complexes as low-valent iron(0) species with two neutral alpha-diimine ligands, it is established here that they are, in fact, complexes containing two (alpha-diiminato)1-* pi radical monoanions and a high-spin ferrous ion (in tetrahedral N4 geometry) (SFe=2). Intramolecular antiferromagnetic coupling between the pi radical ligands (Srad=1/2) and the ferrous ion (SFe=2) yields the observed St=1 ground state. The study confirms that alpha-diimines are redox noninnocent ligands with an energetically low-lying antibonding pi* lowest unoccupied molecular orbital which can accept one or two electrons from a transition metal ion. The (alpha-diimine)FeCl2 complexes (St=2) are shown to contain a neutral alpha-diimine ligand, a high spin ferrous ion, and two chloride ligands.
NASA Astrophysics Data System (ADS)
da Silva, E. Lora; Marinopoulos, A. G.; Vieira, R. B. L.; Vilão, R. C.; Alberto, H. V.; Gil, J. M.; Lichti, R. L.; Mengyan, P. W.; Baker, B. B.
2016-07-01
The electronic structure of hydrogen impurity in Lu2O3 was studied by first-principles calculations and muonium spectroscopy. The computational scheme was based on two methods which are well suited to treat defect calculations in f -electron systems: first, a semilocal functional of conventional density-functional theory (DFT) and secondly a DFT+U approach which accounts for the on-site correlation of the 4 f electrons via an effective Hubbard-type interaction. Three different types of stable configurations were found for hydrogen depending upon its charge state. In its negatively charged and neutral states, hydrogen favors interstitial configurations residing either at the unoccupied sites of the oxygen sublattice or at the empty cube centers surrounded by the lanthanide ions. In contrast, the positively charged state stabilized only as a bond configuration, where hydrogen binds to oxygen ions. Overall, the results between the two methods agree in the ordering of the formation energies of the different impurity configurations, though within DFT+U the charge-transition (electrical) levels are found at Fermi-level positions with higher energies. Both methods predict that hydrogen is an amphoteric defect in Lu2O3 if the lowest-energy configurations are used to obtain the charge-transition, thermodynamic levels. The calculations of hyperfine constants for the neutral interstitial configurations show a predominantly isotropic hyperfine interaction with two distinct values of 926 MHz and 1061 MHz for the Fermi-contact term originating from the two corresponding interstitial positions of hydrogen in the lattice. These high values are consistent with the muonium spectroscopy measurements which also reveal a strongly isotropic hyperfine signature for the neutral muonium fraction with a magnitude slightly larger (1130 MHz) from the ab initio results (after scaling with the magnetic moments of the respective nuclei).
NASA Astrophysics Data System (ADS)
Guillot-Deudon, Catherine; Harel, Sylvie; Mokrani, Arezki; Lafond, Alain; Barreau, Nicolas; Fernandez, Vincent; Kessler, John
2008-12-01
The aim of the present work is to complete a preliminary study concerning the electronic band structure investigations of NaxCu1-xIn5S8 compounds with 0≤x≤1 , which are expected to be formed at the Cu(In,Ga)Se2/In2S3 interface. The band structure calculations demonstrate that for the compounds containing both Na and Cu, as the Cu content increases the band gap tends to decrease, and x-ray photoemission spectroscopy measurements show that this variation is mainly due to valence-band-maximum shift along the solid solution. The band gap strongly depends on the nature of the monovalent cation, and the band structure calculations demonstrate that the d electrons of copper are responsible for the shift of the valence band. In addition, it is worth noting that the Cu-containing compounds have indirect gaps.
Technique Development for Complete Electronic-Structure Calculations in Crystalline Solids
1976-03-01
also have been spending considerable time on a stuHy of the formal structure of the cluster expansion method. We found ways to derive the basic...Harris and Hendrik J. Monkhorst. Solid State Conm. 9, 1449 (1971). 2. "’Exact’ Hartree-Fock Results for Atomic Hydrogen Crystlas ," by Frank E. Harris
Singlet oxygen generation in PUVA therapy studied using electronic structure calculations
NASA Astrophysics Data System (ADS)
Serrano-Pérez, Juan José; Olaso-González, Gloria; Merchán, Manuela; Serrano-Andrés, Luis
2009-06-01
The ability of furocoumarins to participate in the PUVA (Psoralen + UV-A) therapy against skin disorders and some types of cancer, is analyzed on quantum chemical grounds. The efficiency of the process relies on its capability to populate its lowest triplet excited state, and then either form adducts with thymine which interfere DNA replication or transfer its energy, generating singlet molecular oxygen damaging the cell membrane in photoactivated tissues. By determining the spin-orbit couplings, shown to be the key property, in the intersystem crossing yielding the triplet state of the furocoumarin, the electronic couplings in the triplet-triplet energy transfer process producing the singlet oxygen, and the reaction rates and lifetimes, the efficiency in the phototherapeutic action of the furocoumarin family is predicted as: khellin < 5-methoxypsoralen (5-MOP) < 8-methoxypsoralen (8-MOP) < psoralen < 4,5‧,8-trimethylpsoralen (TMP) < 3-carbethoxypsoralen (3-CPS), the latter being the most efficient photosensitizer and singlet oxygen generator.
Fischer, Guntram; Gerber, Iann C
2015-01-14
We have investigated the magnetic properties of low-indexed iron surfaces and the influence of the chemical environment on these properties. We have considered the (1 0 0), (1 1 0), (1 1 1), (2 1 1) and (3 1 0) surfaces, both, bare and with the presence of adsorbates. These were chosen to mimic realistic chemical synthesis environments, being H, Cl, HCl, NH3, NH4Cl, or CH3COOH. We have found an increased magnetization at all bare surfaces. Upon H adsorption the magnetization is generally reduced, but still above the bulk value. All other ligands and their dissociated parts alter the magnetic properties of the surfaces only weakly. Our calculations do not indicate that ligands are responsible for experimental observations of Fe nanoparticles with average magnetizations below the bulk value.
NASA Astrophysics Data System (ADS)
Fischer, Guntram; Gerber, Iann C.
2015-01-01
We have investigated the magnetic properties of low-indexed iron surfaces and the influence of the chemical environment on these properties. We have considered the (1 0 0), (1 1 0), (1 1 1), (2 1 1) and (3 1 0) surfaces, both, bare and with the presence of adsorbates. These were chosen to mimic realistic chemical synthesis environments, being H, Cl, HCl, NH3, NH4Cl, or CH3COOH. We have found an increased magnetization at all bare surfaces. Upon H adsorption the magnetization is generally reduced, but still above the bulk value. All other ligands and their dissociated parts alter the magnetic properties of the surfaces only weakly. Our calculations do not indicate that ligands are responsible for experimental observations of Fe nanoparticles with average magnetizations below the bulk value.
Liebscher, C H; Freysoldt, C; Dennenwaldt, T; Harzer, T P; Dehm, G
2016-07-12
Metastable Cu-Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1-5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for {110}-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu-Cr d-band that lies between the delocalized d-bands of Cu and Cr.
Atomic partial charges on CH3NH3PbI3 from first-principles electronic structure calculations
NASA Astrophysics Data System (ADS)
Madjet, Mohamed E.; El-Mellouhi, Fedwa; Carignano, Marcelo A.; Berdiyorov, Golibjon R.
2016-04-01
We calculated the partial charges in methylammonium (MA) lead-iodide perovskite CH3NH3PbI3 in its different crystalline phases using different first-principles electronic charge partitioning approaches, including the Bader, ChelpG, and density-derived electrostatic and chemical (DDEC) schemes. Among the three charge partitioning methods, the DDEC approach provides chemically intuitive and reliable atomic charges for this material, which consists of a mixture of transition metals, halide ions, and organic molecules. The DDEC charges are also found to be robust against the use of hybrid functionals and/or upon inclusion of spin-orbit coupling or dispersive interactions. We calculated explicitly the atomic charges with a special focus on the dipole moment of the MA molecules within the perovskite structure. The value of the dipole moment of the MA is reduced with respect to the isolated molecule due to charge redistribution involving the inorganic cage. DDEC charges and dipole moment of the organic part remain nearly unchanged upon its rotation within the octahedral cavities. Our findings will be of both fundamental and practical importance, as the accurate and consistent determination of the atomic charges is important in order to understand the average equilibrium distribution of the electrons and to help in the development of force fields for larger scale atomistic simulations to describe static, dynamic, and thermodynamic properties of the material.
Implicit solvent model for linear-scaling first-principles electronic structure calculations
NASA Astrophysics Data System (ADS)
Helal, Hatem H.; Payne, Mike; Mostofi, Arash A.
2009-03-01
Density functional theory (DFT) enables first-principles calculations that exhibit cubic scaling of the computational time required with respect to the number of atoms in the system. This presents an unavoidable difficulty when first-principles accuracy is needed for the study of large-scale biological systems. The ONETEP program reformulates DFT so that the required computational effort scales only linearly with system size, recently demonstrated for up to 32,000 atoms on 64 cores.ootnotetextN. D. M. Hine, P. D. Haynes, A. A. Mostofi, C.-K. Skylaris and M. C. Payne, submitted to J. Chem. Phys. (2008). Further complicating DFT based studies of biomolecular systems is the need for an accurate representation of the electrostatic environment. Rather than introducing explicit solvent molecules into the system, which would be computationally prohibitive, we present our recent efforts to integrate an implicit solvent modelootnotetextD. A. Scherlis et al., J. Chem. Phys. 124, 074103 (2006). with ONETEP in order to study systems in solution consisting of many thousands of atoms. We report preliminary results of our methodology with a study of the DNA nucleosome core particle.
Systematic sparse matrix error control for linear scaling electronic structure calculations.
Rubensson, Emanuel H; Sałek, Paweł
2005-11-30
Efficient truncation criteria used in multiatom blocked sparse matrix operations for ab initio calculations are proposed. As system size increases, so does the need to stay on top of errors and still achieve high performance. A variant of a blocked sparse matrix algebra to achieve strict error control with good performance is proposed. The presented idea is that the condition to drop a certain submatrix should depend not only on the magnitude of that particular submatrix, but also on which other submatrices that are dropped. The decision to remove a certain submatrix is based on the contribution the removal would cause to the error in the chosen norm. We study the effect of an accumulated truncation error in iterative algorithms like trace correcting density matrix purification. One way to reduce the initial exponential growth of this error is presented. The presented error control for a sparse blocked matrix toolbox allows for achieving optimal performance by performing only necessary operations needed to maintain the requested level of accuracy.
Kostko, Oleg; Bravaya, Ksenia; Krylov, Anna; Ahmed, Musahid
2009-12-14
We report a combined theoretical and experimental study of ionization of cytosine monomers and dimers. Gas-phase molecules are generated by thermal vaporization of cytosine followed by expansion of the vapor in a continuous supersonic jet seeded in Ar. The resulting species are investigated by single photon ionization with tunable vacuum-ultraviolet (VUV) synchrotron radiation and mass analyzed using reflectron mass spectrometry. Energy onsets for the measured photoionization efficiency (PIE) spectra are 8.60+-0.05 eV and 7.6+-0.1 eV for the monomer and the dimer, respectively, and provide an estimate for the adiabatic ionization energies (AIE). The first AIE and the ten lowest vertical ionization energies (VIEs) for selected isomers of cytosine dimer computed using equation-of-motion coupled-cluster (EOM-IP-CCSD) method are reported. The comparison of the computed VIEs with the derivative of the PIE spectra, suggests that multiple isomers of the cytosine dimer are present in the molecular beam. The calculations reveal that the large red shift (0.7 eV) of the first IE of the lowest-energy cytosine dimer is due to strong inter-fragment electrostatic interactions, i.e., the hole localized on one of the fragments is stabilized by the dipole moment of the other. A sharp rise in the CH+ signal at 9.20+-0.05 eV is ascribed to the formation of protonated cytosine by dissociation of the ionized dimers. The dominant role of this channel is supported by the computed energy thresholds for the CH+ appearance and the barrierless or nearly barrierless ionization-induced proton transfer observed for five isomers of the dimer.
NASA Astrophysics Data System (ADS)
Cox, Courtney E.; Phifer, Jeremy R.; Ferreira da Silva, Larissa; Gonçalves Nogueira, Gabriel; Ley, Ryan T.; O'Loughlin, Elizabeth J.; Pereira Barbosa, Ana Karolyne; Rygelski, Brett T.; Paluch, Andrew S.
2017-02-01
Solubility parameter based methods have long been a valuable tool for solvent formulation and selection. Of these methods, the MOdified Separation of Cohesive Energy Density (MOSCED) has recently been shown to correlate well the equilibrium solubility of multifunctional non-electrolyte solids. However, before it can be applied to a novel solute, a limited amount of reference solubility data is required to regress the necessary MOSCED parameters. Here we demonstrate for the solutes methylparaben, ethylparaben, propylparaben, butylparaben, lidocaine and ephedrine how conventional molecular simulation free energy calculations or electronic structure calculations in a continuum solvent, here the SMD or SM8 solvation model, can instead be used to generate the necessary reference data, resulting in a predictive flavor of MOSCED. Adopting the melting point temperature and enthalpy of fusion of these compounds from experiment, we are able to predict equilibrium solubilities. We find the method is able to well correlate the (mole fraction) equilibrium solubility in non-aqueous solvents over four orders of magnitude with good quantitative agreement.
NASA Astrophysics Data System (ADS)
Galli, Giulia
2003-03-01
While in the last decade robust experimental results have been obtained for II-VI nanocrystals, group IV elemental nanostructures are much less well characterized. The interplay between quantum confinement effects and surface properties has not been fully understood in these systems, and the effects of preparation conditions on the physical properties of Group IV nanoparticles remain an open issue. In this talk we present results of first principles simulations -using both Density Functional Theory and Quantum Monte Carlo techniques- aimed at understanding the physical and chemical properties of C, Si and Ge nanoparticles with diameters up to 2-3 nm. In particular, we will present investigations of optical gaps[1,2] and surface properties[2,3,4], and simulations of the effect of different preparation conditions [5] on the structure of Si nanoparticles. Work done in collaboration with E.Draeger, J.Grossman, A.Puzder, J-Y Raty and A.Williamson. [1] A. J. Williamson, J.C. Grossman, R.Q. Hood, A. Puzder and G. Galli, Phys. Rev. Lett. 89, 196803, (2002). [2] J-Y. Raty, G. Galli, A.Van Buuren and L. J. Terminello, Phys. Rev. Lett. (accepted). [3] A. Puzder, A. J. Williamson, J. C. Grossman and G. Galli, J. Chem. Phys. 117, 6721 (2002). [4] A. Puzder, A. J. Williamson, J. C. Grossman and G. Galli, Phys. Rev. Lett. 88, 097401 (2002). [5] E. Draeger, J. Grossman, A. Williamson and G. Galli (submitted).
NASA Astrophysics Data System (ADS)
Martin-Samos, Layla; Bussi, Giovanni
2009-08-01
We present here SaX (Self-energies and eXcitations), a plane-waves package aimed at electronic-structure and optical-properties calculations in the GW framework, namely using the GW approximation for quasi-particle properties and the Bethe-Salpeter equation for the excitonic effects. The code is mostly written in FORTRAN90 in a modern style, with extensive use of data abstraction (i.e. objects). SaX employs state of the art techniques and can treat large systems. The package is released with an open source license and can be also download from http://www.sax-project.org/. Program summaryProgram title: SaX (Self-energies and eXcitations) Catalogue identifier: AEDF_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEDF_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU General Public License No. of lines in distributed program, including test data, etc.: 779 771 No. of bytes in distributed program, including test data, etc.: 4 894 755 Distribution format: tar.gz Programming language: FORTRAN, plus some C utilities Computer: Linux PC, Linux clusters, IBM-SP5 Operating system: Linux, Aix Has the code been vectorised or parallelized?: Yes RAM: depending on the system complexity Classification: 7.3 External routines: Message-Passing Interface (MPI) to perform parallel computations. ESPRESSO ( http://www.quantum-espresso.org) Nature of problem: SaX is designed to calculate the electronic band-structure of semiconductors, including quasi-particle effects and optical properties including excitonic effects. Solution method: The electronic band-structure is calculated using the GW approximation for the self-energy operator. The optical properties are calculated solving the Bethe-Salpeter equation in the GW approximation. The wavefunctions are expanded on a plane-waves basis set, using norm-conserving pseudopotentials. Restrictions: Many objects are non-local matrix represented in plane wave basis
NASA Astrophysics Data System (ADS)
Powell, B. J.; Baruah, T.; Bernstein, N.; Brake, K.; McKenzie, Ross H.; Meredith, P.; Pederson, M. R.
2004-05-01
We report first-principles density-functional calculations for hydroquinone (HQ), indolequinone (IQ), and semiquinone (SQ). These molecules are believed to be the basic building blocks of the eumelanins, a class of biomacromolecules with important biological functions (including photoprotection) and with the potential for certain bioengineering applications. We have used the difference of self-consistent fields method to study the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, ΔHL. We show that ΔHL is similar in IQ and SQ, but approximately twice as large in HQ. This may have important implications for our understanding of the observed broadband optical absorption of the eumelanins. The possibility of using this difference in ΔHL to molecularly engineer the electronic properties of eumelanins is discussed. We calculate the infrared and Raman spectra of the three redox forms from first principles. Each of the molecules have significantly different infrared and Raman signatures, and so these spectra could be used in situ to nondestructively identify the monomeric content of macromolecules. It is hoped that this may be a helpful analytical tool in determining the structure of eumelanin macromolecules and hence in helping to determine the structure-property-function relationships that control the behavior of the eumelanins.
Electronic Structure of I-M8Ga16Sn30 (M = Ba, Sr, Yb) by First-Principles Calculation
NASA Astrophysics Data System (ADS)
Wang, Jin-song; Liu, Hong-xia; Deng, Shuping; Li, De-cong; Shen, Lan-xian; Cheng, Feng; Deng, Shu-kang
2016-10-01
Sn-based clathrates possess excellent thermoelectric properties ascribed to their higher Seebeck coefficient and lower thermal conductivity. Guest atoms significantly modulate the thermoelectric properties of Sn-based calculates because of their diverse atomic radius and interactions with framework atoms. Thus, we explored the electronic structure of I-M8Ga16Sn30 (M = Ba, Sr, Yb) by first-principles calculation. Results revealed significant differences between Yb8Ga16Sn30 and M8Ga16Sn30 (M = Ba, Sr,). In particular, the Yb-filled compound substitution possesses lowest formation energy and the off-center distance of the Yb atom is the largest compared with the other structures. I-M8Ga16Sn30 (M = Ba, Sr, Yb) is an indirect band gap semiconductor, and the enhanced hybridization effect between the guest and framework atoms' orbits exists because the Yb f orbit results in a decrease in band gap. Ba- and Sr-filled clathrates have similar valence bands but slightly different conduction bands; however, Yb8Ga16Sn30 possess the spiculate density of states near the Fermi level that reveals excellent thermoelectric properties.
Powell, B J; Baruah, T; Bernstein, N; Brake, K; McKenzie, Ross H; Meredith, P; Pederson, M R
2004-05-08
We report first-principles density-functional calculations for hydroquinone (HQ), indolequinone (IQ), and semiquinone (SQ). These molecules are believed to be the basic building blocks of the eumelanins, a class of biomacromolecules with important biological functions (including photoprotection) and with the potential for certain bioengineering applications. We have used the difference of self-consistent fields method to study the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, Delta(HL). We show that Delta(HL) is similar in IQ and SQ, but approximately twice as large in HQ. This may have important implications for our understanding of the observed broadband optical absorption of the eumelanins. The possibility of using this difference in Delta(HL) to molecularly engineer the electronic properties of eumelanins is discussed. We calculate the infrared and Raman spectra of the three redox forms from first principles. Each of the molecules have significantly different infrared and Raman signatures, and so these spectra could be used in situ to nondestructively identify the monomeric content of macromolecules. It is hoped that this may be a helpful analytical tool in determining the structure of eumelanin macromolecules and hence in helping to determine the structure-property-function relationships that control the behavior of the eumelanins.
NASA Astrophysics Data System (ADS)
Enkovaara, J.; Rostgaard, C.; Mortensen, J. J.; Chen, J.; Dułak, M.; Ferrighi, L.; Gavnholt, J.; Glinsvad, C.; Haikola, V.; Hansen, H. A.; Kristoffersen, H. H.; Kuisma, M.; Larsen, A. H.; Lehtovaara, L.; Ljungberg, M.; Lopez-Acevedo, O.; Moses, P. G.; Ojanen, J.; Olsen, T.; Petzold, V.; Romero, N. A.; Stausholm-Møller, J.; Strange, M.; Tritsaris, G. A.; Vanin, M.; Walter, M.; Hammer, B.; Häkkinen, H.; Madsen, G. K. H.; Nieminen, R. M.; Nørskov, J. K.; Puska, M.; Rantala, T. T.; Schiøtz, J.; Thygesen, K. S.; Jacobsen, K. W.
2010-06-01
Electronic structure calculations have become an indispensable tool in many areas of materials science and quantum chemistry. Even though the Kohn-Sham formulation of the density-functional theory (DFT) simplifies the many-body problem significantly, one is still confronted with several numerical challenges. In this article we present the projector augmented-wave (PAW) method as implemented in the GPAW program package (https://wiki.fysik.dtu.dk/gpaw) using a uniform real-space grid representation of the electronic wavefunctions. Compared to more traditional plane wave or localized basis set approaches, real-space grids offer several advantages, most notably good computational scalability and systematic convergence properties. However, as a unique feature GPAW also facilitates a localized atomic-orbital basis set in addition to the grid. The efficient atomic basis set is complementary to the more accurate grid, and the possibility to seamlessly switch between the two representations provides great flexibility. While DFT allows one to study ground state properties, time-dependent density-functional theory (TDDFT) provides access to the excited states. We have implemented the two common formulations of TDDFT, namely the linear-response and the time propagation schemes. Electron transport calculations under finite-bias conditions can be performed with GPAW using non-equilibrium Green functions and the localized basis set. In addition to the basic features of the real-space PAW method, we also describe the implementation of selected exchange-correlation functionals, parallelization schemes, ΔSCF-method, x-ray absorption spectra, and maximally localized Wannier orbitals.
Electron Affinity Calculations for Thioethers
NASA Technical Reports Server (NTRS)
Sulton, Deley L.; Boothe, Michael; Ball, David W.; Morales, Wilfredo
1997-01-01
Previous work indicated that polyphenyl thioethers possessed chemical properties, related to their electron affinities, which could allow them to function as vapor phase lubricants (VPL). Indeed, preliminary tribological tests revealed that the thioethers could function as vapor phase lubricants but not over a wide temperature and hertzian pressure range. Increasing the electron affinity of the thioethers may improve their VPL properties over this range. Adding a substituent group to the thioether will alter its electron affinity in many cases. Molecular orbital calculations were undertaken to determine the effect of five different substituent groups on the electron affinity of polyphenyl thioethers. It was found that the NO2, F, and I groups increased the thioethers electron affinity by the greatest amount. Future work will involve the addition of these groups to the thioethers followed by tribological testing to assess their VPL properties.
NASA Astrophysics Data System (ADS)
Egawa, Toru; Kachi, Yukari; Takeshima, Tsuguhide; Takeuchi, Hiroshi; Konaka, Shigehiro
2003-10-01
The molecular structure and conformation of carvone, a compound with a minty odor, were investigated by means of gas electron diffraction supported by theoretical calculations. Electron diffraction patterns were recorded by heating the nozzle up to 128 °C to obtain enough scattering intensity. The infrared spectrum was also measured by using an absorption cell with a path length of 10 m. The obtained molecular scattering intensities were analyzed with the aid of theoretical calculations and infrared spectroscopy. It was revealed that the experimental data are well reproduced by assuming that carvone consists of a mixture of three conformers that have the isopropenyl group in the equatorial position and mutually differ in the torsional angle around the single bond connecting the ring and the isopropenyl group. It was also found that the puckering amplitude of the ring of carvone is close to those of menthol and isomenthol, a minty compound and its nonminty isomer. The determined structural parameters ( rg and ∠ α) of the most abundant conformer of carvone are as follows: < r(C-C)>=1.520(3) Å; < r(CC)>=1.360(5) Å; r(CO)=1.225(5) Å; < r(C-H)>=1.104(4)Å; <∠CC-C>=121.1(5)°; <∠C-C-C>=110.4(5)°; ∠C-CO-C=117.1(14)°; <∠C-C-H>=111.1(13)°. Angle brackets denote average values and parenthesized values are the estimated limits of error (3 σ) referring to the last significant digit.
Banerjee, Amartya S; Lin, Lin; Hu, Wei; Yang, Chao; Pask, John E
2016-10-21
The Discontinuous Galerkin (DG) electronic structure method employs an adaptive local basis (ALB) set to solve the Kohn-Sham equations of density functional theory in a discontinuous Galerkin framework. The adaptive local basis is generated on-the-fly to capture the local material physics and can systematically attain chemical accuracy with only a few tens of degrees of freedom per atom. A central issue for large-scale calculations, however, is the computation of the electron density (and subsequently, ground state properties) from the discretized Hamiltonian in an efficient and scalable manner. We show in this work how Chebyshev polynomial filtered subspace iteration (CheFSI) can be used to address this issue and push the envelope in large-scale materials simulations in a discontinuous Galerkin framework. We describe how the subspace filtering steps can be performed in an efficient and scalable manner using a two-dimensional parallelization scheme, thanks to the orthogonality of the DG basis set and block-sparse structure of the DG Hamiltonian matrix. The on-the-fly nature of the ALB functions requires additional care in carrying out the subspace iterations. We demonstrate the parallel scalability of the DG-CheFSI approach in calculations of large-scale two-dimensional graphene sheets and bulk three-dimensional lithium-ion electrolyte systems. Employing 55 296 computational cores, the time per self-consistent field iteration for a sample of the bulk 3D electrolyte containing 8586 atoms is 90 s, and the time for a graphene sheet containing 11 520 atoms is 75 s.
NASA Astrophysics Data System (ADS)
Banerjee, Amartya S.; Lin, Lin; Hu, Wei; Yang, Chao; Pask, John E.
2016-10-01
The Discontinuous Galerkin (DG) electronic structure method employs an adaptive local basis (ALB) set to solve the Kohn-Sham equations of density functional theory in a discontinuous Galerkin framework. The adaptive local basis is generated on-the-fly to capture the local material physics and can systematically attain chemical accuracy with only a few tens of degrees of freedom per atom. A central issue for large-scale calculations, however, is the computation of the electron density (and subsequently, ground state properties) from the discretized Hamiltonian in an efficient and scalable manner. We show in this work how Chebyshev polynomial filtered subspace iteration (CheFSI) can be used to address this issue and push the envelope in large-scale materials simulations in a discontinuous Galerkin framework. We describe how the subspace filtering steps can be performed in an efficient and scalable manner using a two-dimensional parallelization scheme, thanks to the orthogonality of the DG basis set and block-sparse structure of the DG Hamiltonian matrix. The on-the-fly nature of the ALB functions requires additional care in carrying out the subspace iterations. We demonstrate the parallel scalability of the DG-CheFSI approach in calculations of large-scale two-dimensional graphene sheets and bulk three-dimensional lithium-ion electrolyte systems. Employing 55 296 computational cores, the time per self-consistent field iteration for a sample of the bulk 3D electrolyte containing 8586 atoms is 90 s, and the time for a graphene sheet containing 11 520 atoms is 75 s.
NASA Astrophysics Data System (ADS)
Du, Yu-Lei
2009-11-01
We perform a first-principles study on the electronic structure and elastic properties of Ti3AlC with an antiperovskite structure. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of this compound. The elastic constants of Ti3AlC are derived yielding c11 = 356 GPa, c11 = 55 GPa, c44 = 157 GPa. The bulk modulus B, shear modulus G and Young's modulus E are determined to be 156, 151 and 342 GPa, respectively. These properties are compared with those of Ti3AlC2 and Ti2AlC with a layered structure in the Ti-Al-C system and Fe3AlC with the same antiperovskite structure.
NASA Astrophysics Data System (ADS)
Shlykov, Sergey A.; Phien, Tran D.; Gao, Yan; Weber, Peter M.
2017-03-01
Molecular structure and conformational behavior of N-phenylpiperidine (NPhP) were investigated by synchronous gas-phase electron diffraction/mass spectrometry (GED/MS) and quantum chemistry. Due to influence of steric repulsion and hyperconjugation, NPhP may exist in two conformers, equatorial and axial chair forms. Both experiment and theoretical calculations suggest a C1 symmetry of the conformers, with the plane perpendicular to the phenyl group turned by ca. 30-40° (equatorial) and 0-20° (axial) about the plane perpendicular to the piperidine ring symmetry plane. According to the QC calculations, NPhP may exist as two conformers, equatorial and axial, with a ratio of Eq:Ax = 92:8 (B3LYP), 87:13 (B3LYP-GD3), 84:16 (M06-2X), 83:17 (MP2/6-311G**) and 76:24% (MP2/cc-pVTZ). Except for the latter, these values are in good agreement with the experimental GED data of 90(10):10(10)%. A comparative analysis of similar compounds, phenylcyclohexane and 1-phenylheterocyclohexanes, was performed. Conformational properties depend on the CPhsbnd X bond distance and hyperconjugation between the phenyl ring and the lone pair on the heteroatom. The contribution of the axial form of 1-phenylcyclohexane derivatives increases in the series of the heteroatom X in the cyclohexane ring: C → N → Si → P.
NASA Astrophysics Data System (ADS)
Joshi, Bhawani Datt; Srivastava, Anubha; Honorato, Sara Braga; Tandon, Poonam; Pessoa, Otília Deusdênia Loiola; Fechine, Pierre Basílio Almeida; Ayala, Alejandro Pedro
2013-09-01
Oncocalyxone A (C17H18O5) is the major secondary metabolite isolated from ethanol extract from the heartwood of Auxemma oncocalyx Taub popularly known as “pau branco”. Oncocalyxone A (Onco A) has many pharmaceutical uses such as: antitumor, analgesic, antioxidant and causative of inhibition of platelet activation. We have performed the optimized geometry, total energy, conformational study, molecular electrostatic potential mapping, frontier orbital energy gap and vibrational frequencies of Onco A employing ab initio Hartree-Fock (HF) and density functional theory (DFT/B3LYP) method with 6-311++G(d, p) basis set. Stability of the molecule arising from hyperconjugative interactions and/or charge delocalization has been analyzed using natural bond orbital (NBO) analysis. UV-vis spectrum of the compound was recorded in DMSO and MeOH solvent. The TD-DFT calculations have been performed to explore the influence of electronic absorption spectra in the gas phase, as well as in solution environment using IEF-PCM and 6-31G basis set. The 13C NMR chemical shifts have been calculated with the B3LYP/6-311++G(d, p) basis set and compared with the experimental values. These methods have been used as tools for structural characterization of Onco A.
Joshi, Bhawani Datt; Srivastava, Anubha; Honorato, Sara Braga; Tandon, Poonam; Pessoa, Otília Deusdênia Loiola; Fechine, Pierre Basílio Almeida; Ayala, Alejandro Pedro
2013-09-01
Oncocalyxone A (C17H18O5) is the major secondary metabolite isolated from ethanol extract from the heartwood of Auxemma oncocalyx Taub popularly known as "pau branco". Oncocalyxone A (Onco A) has many pharmaceutical uses such as: antitumor, analgesic, antioxidant and causative of inhibition of platelet activation. We have performed the optimized geometry, total energy, conformational study, molecular electrostatic potential mapping, frontier orbital energy gap and vibrational frequencies of Onco A employing ab initio Hartree-Fock (HF) and density functional theory (DFT/B3LYP) method with 6-311++G(d,p) basis set. Stability of the molecule arising from hyperconjugative interactions and/or charge delocalization has been analyzed using natural bond orbital (NBO) analysis. UV-vis spectrum of the compound was recorded in DMSO and MeOH solvent. The TD-DFT calculations have been performed to explore the influence of electronic absorption spectra in the gas phase, as well as in solution environment using IEF-PCM and 6-31G basis set. The (13)C NMR chemical shifts have been calculated with the B3LYP/6-311++G(d,p) basis set and compared with the experimental values. These methods have been used as tools for structural characterization of Onco A.
NASA Astrophysics Data System (ADS)
Luo, Kan; Deng, Qihuang; Zha, Xianhu; Huang, Qing; Francisco, Joseph S.; Yu, Xiaohui; Qiao, Yingjie; He, Jian; Du, Shiyu
2015-07-01
Employing first-principles density functional theory (DFT), the structures and electronic and mechanical properties of Al(111)/ZrB2(0001) heterojunctions are investigated. It is found that both B-terminated ZrB2(0001) and Zr-terminated ZrB2(0001) can form heterojunction interfaces with Al(111) surface. The heterojunction with B-terminated ZrB2(0001) is demonstrated to be most stable by comparing the surface adhesion energies of six different heterojunction models. In the stable configurations, the Al atom is found projecting to the hexagonal hollow site of neighbouring boron layer for the B-terminated ZrB2(001), and locating at the top site of the boron atoms for Zr-terminated ZrB2(001) interface. The mechanisms of interface interaction are investigated by density of states, charge density difference and band structure calculations. It is found that covalent bonds between surface Al atoms and B atoms are formed in the B-terminated heterojunction, whereas the Al atoms and Zr atoms are stabilised by interface metallic bonds for the Zr-terminated case. Mechanical properties of Al/ZrB2 heterojunctions are also predicted in the current work. The values of moduli of Al/ZrB2 heterojunctions are determined to be between those of single crystal Al and ZrB2, which exhibit the transition of mechanical strength between two bulk phases. DFT calculations with the current models provide the mechanical properties for each heterojunction and the corresponding contributions by each type of interface in the composite materials. This work paves the way for industrial applications of Al(111)/ZrB2(0001) heterojunctions.
Ammar, A; Ménétrier, M; Villesuzanne, A; Matar, S; Chevalier, B; Etourneau, J; Villeneuve, G; Rodríguez-Carvajal, J; Koo, H-J; Smirnov, A I; Whangbo, M-H
2004-08-09
The electronic and structural properties of potassium hexaboride, KB(6), were examined by transport, magnetic susceptibility, EPR, and NMR measurements, temperature-dependent crystal structure determination, and electronic band structure calculations. The valence bands of KB(6) are partially empty, but the electrical resistivity of KB(6) reveals that it is not a normal metal. The magnetic susceptibility as well as EPR and NMR measurements show the presence of localized electrons in KB(6). The EPR spectra of KB(6) have two peaks, a broad ( approximately 320 G) and a narrow (less than approximately 27 G) line width, and the temperature-dependence of the magnetic susceptibility of KB(6) exhibits a strong hysteresis below 70 K. The temperature-dependent crystal structure determination of KB(6) shows the occurrence of an unusual variation in the unit cell parameter hence supporting that the hysteresis of the magnetic susceptibility is a bulk phenomenon. The line width DeltaH(pp) of the broad EPR signal is independent of temperature and EPR frequency. This finding indicates that the line broadening results from the dipole-dipole interaction, and the spins responsible for the broad EPR peak has the average distance of approximately 1.0 nm. To explain these apparently puzzling properties, we examined a probable mechanism of electron localization in KB(6) and its implications.
Density Functional Calculation of the Electronic Structures of Some A5B6C7-TYPE Crystals
NASA Astrophysics Data System (ADS)
Akkus, Harun
The electronic band structures of some A5B6C7-type ternary compounds, BiSeI, BiSI, BiSCl, BiSBr, BiSeBr and SbSeBr, are investigated using the density functional theory and pseudopotential theory under the generalized gradient approximation (GGA). The electronic band structures obtained show that these crystals, except for BiSeI, have an indirect band gap.
NASA Astrophysics Data System (ADS)
Ching, Wai-Yim; Rulis, Paul
2009-03-01
Over the last eight years, a large number of x-ray absorption near edge structure (XANES) and/or electron energy loss near edge structure (ELNES) spectroscopic calculations for complex oxides and nitrides have been performed using the supercell-OLCAO (orthogonalized linear combination of atomic orbitals) method, obtaining results in very good agreement with experiments. The method takes into account the core-hole effect and includes the dipole matrix elements calculated from ab initio wavefunctions. In this paper, we describe the method in considerable detail, emphasizing the special advantages of this method for large complex systems. Selected results are reviewed and several hitherto unpublished results are also presented. These include the Y K edge of Y ions segregated to the core of a Σ31 grain boundary in alumina, O K edges of water molecules, C K edges in different types of single walled carbon nanotubes, and the Co K edge in the cyanocobalamin (vitamin B12) molecule. On the basis of these results, it is argued that the interpretation of specific features of the calculated XANES/ELNES edges is not simple for complex material systems because of the delocalized nature of the conduction band states. The long-standing notion of the 'fingerprinting' technique for spectral interpretation of experimental data is not tenable. A better approach is to fully characterize the structure under study, using either crystalline data or accurate ab initio modeling. Comparison between calculated XANES/ELNES spectra and available measurements enables us to ascertain the validity of the modeled structure. For complex crystals or structures, it is necessary to use the weighted sum of the spectra from structurally nonequivalent sites for comparison with the measured data. Future application of the supercell-OLCAO method to complex biomolecular systems is also discussed.
Andrews, Lester; Wang, Xuefeng; Gong, Yu; Kushto, Gary P; Vlaisavljevich, Bess; Gagliardi, Laura
2014-07-17
Reactions of laser-ablated U atoms with N2 molecules upon codeposition in excess argon or neon at 4 K gave intense NUN and weak UN absorptions. Annealing produced progressions of new absorptions for the UN2(N2)1,2,3,4,5 and UN(N2)1,2,3,4,5,6 complexes. The neon-to-argon matrix shift decreases with increasing NN ligation and therefore the number of noble gas atoms left in the primary coordination sphere around the NUN molecule. Small matrix shifts are observed when the secondary coordination layers around the primary UN2(N2)1,2,3,4,5 and UN(N2)1,2,3,4,5,6 complexes are changed from neon-to-argon to nitrogen. Electronic structure, energy, and frequency calculations provide support for the identification of these complexes and the characterization of the N≡U≡N and U≡N core molecules as terminal uranium nitrides. Codeposition of U with pure nitrogen produced the saturated U(NN)7 complex, which UV irradiation converted to the NUN(NN)5 complex with slightly lower frequencies than found in solid argon.
NASA Astrophysics Data System (ADS)
Alaal, Naresh; Loganathan, Vaideesh; Medhekar, Nikhil; Shukla, Alok
2016-03-01
A first principles many-body approach is employed to calculate the band structure and optical response of nanometer-sized ribbons of SiC. Many-body effects are incorporated using the GW approximation, and excitonic effects are included using the Bethe-Salpeter equation. Both unpassivated and hydrogen-passivated armchair SiC nanoribbons are studied. As a consequence of low dimensionality, large quasiparticle corrections are seen to the Kohn-Sham energy gaps. In both cases quasiparticle band gaps are increased by up to 2 eV, as compared to their Kohn-Sham energy values. Inclusion of electron-hole interactions modifies the absorption spectra significantly, giving rise to strongly bound excitonic peaks in these systems. The results suggest that hydrogen passivated armchair SiC nanoribbons have the potential to be used in optoelectronic devices operating in the UV-Vis region of the spectrum. We also compute the formation energies of these nanoribbons as a function of their widths, and conclude that hydrogen-saturated ribbons will be much more stable as compared to bare ones.
NASA Astrophysics Data System (ADS)
Lu, Lai-Yu; Wei, Dong-Qing; Chen, Xiang-Rong; Ji, Guang-Fu
2008-09-01
Structures and electronic properties of the pentaerythritol (PE) crystal under volume compression up to 0.85V0 are studied by E - V fitting method using density functional theory (DFT). The compression dependences of the cell volumes, lattice constants, and molecular geometries of solid PE are presented and discussed. It is found that the solid PE presents anisotropy along a- and c-axes, and the c axis is the most compressible. Decreasing anisotropy ratio (c/a) with elevating compression suggests an enhancement of the vdW interaction with increasing compression. The C-C and C-H bonds are significantly reduced under compression, which may be related to the sensitivity. The solid PE has indirect band gap (X - G) in the range of the researched compression and the band gap is decreased with compression.
NASA Astrophysics Data System (ADS)
Dakkouri, Marwan; Typke, Volker
2010-08-01
The molecular structure of 1,1-dichlorosilacyclopentane (DCSCP) has been investigated by means of gas-phase electron diffraction and quantum mechanical calculation. We applied both a pseudorotation model to account for the dynamic and large amplitude motion in DCSCP, and a one-conformer model of C1 symmetry. Using the computational results we analyzed the dependency of the ring geometrical parameters and vibrational mean amplitudes on the phase angle φ. The joint electron diffraction and ab initio study has led to the following ra structural parameters of DCSCP ( C1 conformer): r(Si-Cl) = 2.047(2) Å, r(Si-C) = 1.867(4) Å, average r(C-C) ring = 1.548(4) Å, average r(C-H) = 1.103(7) Å, <(C-Si-C) = 97.4(6)°, <(Cl-Si-Cl) = 104.8(10)°, and effective phase angle φ = 74.8(58)°. The puckering amplitude for the five-membered ring was determined to be q = 0.480(24) Å. The quantum mechanical calculations were performed by utilizing the UHF, B3LYP, and MP2 methods in combination with basis sets 6-311++G(2df,2pd), 6-311++G(df,pd), 6-311++G(p,d), 6-311+G(d,p), 6-311G(d,p) and Dunning double and triple zeta (with and without augmentation). All these methods have consistently shown that the C2 conformer is more stable than the C s symmetric form. For all calculations we used the MOLPRO and Gaussian03 packages. NBO and AIM analyses were also carried out to explore the bond/anti-bond hyperconjugative interactions and the topological properties of the charge density distribution in DCSCP. NBO scheme including second-order perturbation analysis has shown that the major orbital stabilizing interactions are between the chlorine lone pair ( nπ) Cl and the low-lying σSi-C2∗ and σSi-C5∗ antibonding orbitals. It was found that remote σSi-C → σC-H∗ interactions are stabilized by 4.4 kcal mol -1 and contribute to the stabilization of the C2 conformer in DCSCP. Deletion analysis was performed using various deletion algorithms like NOSTAR, NOVIC, NOGEM (see text). The
NASA Astrophysics Data System (ADS)
Li, Cai-Lin; Wu, Chao-Ling; Chen, Yun-Gui; Zhou, Jing-Jing; Zheng, Xin; Pang, Li-Juan; Deng, Gang
2010-06-01
Molecule geometry structures, frequencies, and energetic stabilities of ammonia borane (AB, NH3BH3) and metal amidoboranes (MAB, MNH2BH3), formed by substituting H atom in AB with one of main group metal atoms, have been investigated by density-functional theory and optimized at the B3LYP levels with 6-311G++ (3df, 3pd) basic set. Their structural parameters and infrared spectrum characteristic peaks have been predicted, which should be the criterion of a successfully synthesized material. Several parameters such as binding energies, vibrational frequencies, and the energy gaps between the HOMO and the LUMO have been adopted to characterize and evaluate their structure stabilities. It is also found that the binding energies and HOMO-LUMO energy gaps of the MAB obviously change with the substitution of the atoms. MgAB has the lowest binding energy and is easier to decompose than any other substitutional structures under same conditions, while CaAB has the highest chemical activity.
NASA Astrophysics Data System (ADS)
Kong, Yi; Liu, Baixin
2007-02-01
We investigate, in the present study, the structural properties, magnetic moments and charge distribution of the solid solution in an immiscible Co-Cu system at equilibrium by first-principles calculation using special quasirandom structures (SQS). In order to mimic the pair and multisite correlation functions of the randomly substitutional fcc solid solutions, the original SQS is developed to include five 16-atom SQS unit cells, i.e., 1/16, 2/16, 3/16, 4/16, and 8/16, enabling to mimic at nine specific alloys compositions. Correspondingly, a new error analysis method is proposed for comparing the situations of various alloy compositions within the SQS unit cells having a same number of atoms. The developed SQS are then applied in the first-principles calculation to study the CoxCu1-x solid solutions (x refers to the Co concentration). It turns out that the calculated results of the lattice constants and magnetic moments versus the Co concentration are in good agreement with the experimental data, and especially, the sharp drop in the magnetic moment near the composition x=0.1 is well reproduced. The heats of formation are also calculated and in good agreement with those obtained from Mediema’s thermodynamic theory and available experimental data. At the alloy compositions x=0.25 and 0.75, some hypothetical crystalline structures of the Co-Cu compounds are respectively calculated and their heats of formation are found to be higher than the solid solution counterparts. Finally, the electron distribution among the atoms in the CoxCu1-x solid solutions is studied and the obtained charge densities show that in the CoxCu1-x solid solutions, the charge distributes mostly between the Co-Co atoms, thus forming attractive covalent bonding.
Sasaki, Takashi; Ono, Tomoya; Hirose, Kikuji
2006-11-01
We present an efficient and highly accurate first-principles calculation method with linear system-size scaling to determine the self-consistent ground-state electron-charge densities of nanostructures suspended between semi-infinite bulks by directly minimizing the energy functional. By making efficient use of the advantages of the real-space finite-difference method, we can impose arbitrary boundary conditions on models and employ spatially localized orbitals. These advantages enable us to calculate the ground-state electron-charge densities in semi-infinite systems. Examples of electronic structure calculations for a one-dimensional case and a conductance calculation for sodium nanowires are presented. The calculated electronic structure of the one-dimensional system agrees well with the exact analytical solution, and the conduction properties of the sodium nanowires are consistent with experimental and other theoretical results. These results imply that our procedure enables us to accurately compute self-consistent electronic structures of semi-infinite systems.
NASA Astrophysics Data System (ADS)
Uzdin, V. M.; Vega, A.; Khrenov, A.; Keune, W.; Kuncser, V. E.; Jiang, J. S.; Bader, S. D.
2012-01-01
Magnetization reversal in nanoscale (Sm-Co)/Fe (hard/soft) bilayer exchange-spring magnets with in-plane uniaxial magnetic anisotropy was investigated by magnetometry, conversion-electron Mössbauer spectroscopy (CEMS) and atomistic Fe spin-structure calculations. Magnetization loops along the easy direction exhibit signatures typical of exchange-spring magnets. In-field CEMS at inclined γ-ray incidence onto thin (2 nm) 57Fe probe layers embedded at various depths in the 20-nm-thick natural (soft) Fe layer provides depth-dependent information (via the line-intensity ratio R23 as a function of the applied field H) about the in-plane rotation of Fe spins. A minimum in the R23-vs-H dependence at (Hmin, Rmin) determines the field where Fe magnetic moments roughly adopt an average perpendicular orientation during their reversal from positive to negative easy-axis orientation. A monotonic decrease of Hmin with distance from the hard/soft interface is observed. Rotation of Fe spins takes place even in the interface region in applied fields far below the field of irreversible switching, Hirr, of the hard phase. Formation of an Fe-Co alloy is detected in the interface region. For comparison, the noncollinear Fe spin structure during reversal and the resulting R23 ratio were obtained by electronic-structure calculations based on a quantum-mechanical Hamiltonian for itinerant electrons. The coupling at the hard/soft interface is described by the uniaxial exchange-anisotropy field, hint, as a parameter. Our calculated R23 ratios as a function of the (reduced) applied field h exhibit similar features as observed in the experiment, in particular a minimum at (hmin, Rmin). Rmin is found to increase with hint, thus providing a measure of the interface coupling. Evidence is provided for the existence of fluctuations of the interface coupling. The calculations also show that the Fe spin spiral formed during reversal is highly inhomogeneous. In general, our simulation of the Fe spin
Uzdin, V. M.; Vega, A.; Khrenov, A.; Keune, W.; Kuncser, V. E.; Jiang, J. S.; Bader, S. D.
2012-01-01
Magnetization reversal in nanoscale (Sm-Co)/Fe (hard/soft) bilayer exchange-spring magnets with in-plane uniaxial magnetic anisotropy was investigated by magnetometry, conversion-electron Moessbauer spectroscopy (CEMS) and atomistic Fe spin-structure calculations. Magnetization loops along the easy direction exhibit signatures typical of exchange-spring magnets. In-field CEMS at inclined {gamma}-ray incidence onto thin (2 nm) {sup 57}Fe probe layers embedded at various depths in the 20-nm-thick natural (soft) Fe layer provides depth-dependent information (via the line-intensity ratio R{sub 23} as a function of the applied field H) about the in-plane rotation of Fe spins. A minimum in the R{sub 23}-vs-H dependence at (H{sub min}, R{sub min}) determines the field where Fe magnetic moments roughly adopt an average perpendicular orientation during their reversal from positive to negative easy-axis orientation. A monotonic decrease of H{sub min} with distance from the hard/soft interface is observed. Rotation of Fe spins takes place even in the interface region in applied fields far below the field of irreversible switching, H{sub irr}, of the hard phase. Formation of an Fe-Co alloy is detected in the interface region. For comparison, the noncollinear Fe spin structure during reversal and the resulting R{sub 23} ratio were obtained by electronic-structure calculations based on a quantum-mechanical Hamiltonian for itinerant electrons. The coupling at the hard/soft interface is described by the uniaxial exchange-anisotropy field, hint, as a parameter. Our calculated R{sub 23} ratios as a function of the (reduced) applied field h exhibit similar features as observed in the experiment, in particular a minimum at (h{sub min}, R{sub min}). R{sub min} is found to increase with hint, thus providing a measure of the interface coupling. Evidence is provided for the existence of fluctuations of the interface coupling. The calculations also show that the Fe spin spiral formed
NASA Astrophysics Data System (ADS)
Ünal, Hatice; Gunceler, Deniz; Gülseren, Oğuz; Ellialtıoğlu, Şinasi; Mete, Ersen
2015-11-01
The electronic and optical properties of thin anatase TiO2 (1 0 1) and (0 0 1) nanowires have been investigated using the screened Coulomb hybrid density functional calculations. For the bare nanowires with sub-nanometer diameters, the calculated band gaps are larger relative to the bulk values due to size effects. The role of organic light harvesting sensitizers on the absorption characteristics of the anatase nanowires has been examined using the hybrid density functional method incorporating partial exact exchange with range separation. For the lowest lying excitations, directional charge redistribution of tetrahydroquinoline (C2-1) dye shows a remarkably different profile in comparison to a simple molecule which is chosen as the coumarin skeleton. The binding modes and the adsorption energies of C2-1 dye and coumarin core on the anatase nanowires have been studied including non-linear solvation effetcs. The calculated optical and electronic properties of the nanowires with these two different types of sensitizers have been interpreted in terms of their electron-hole generation, charge carrier injection and recombination characteristics.
NASA Astrophysics Data System (ADS)
rehman Hashmi, Muhammad Raza ur; Zafar, Muhammad; Shakil, M.; Sattar, Atif; Ahmed, Shabbir; Ahmad, S. A.
2016-11-01
First-principles calculations by means of the full-potential linearized augmented plane wave method using the generalized gradient approximation with correlation effect correction (GGA+U) within the framework of spin polarized density functional theory (DFT+U) are used to study the structural, electronic, and magnetic properties of cubic perovskite compounds RbXF3 (X = Mn, V, Co, and Fe). It is found that the calculated structural parameters, i.e., lattice constant, bulk modulus, and its pressure derivative are in good agreement with the previous results. Our results reveal that the strong spin polarization of the 3d states of the X atoms is the origin of ferromagnetism in RbXF3. Cohesive energies and the magnetic moments of RbXF3 have also been calculated. The calculated electronic properties show the half-metallic nature of RbCoF3 and RbFeF3, making these materials suitable for spintronic applications.
Electron Structure of Francium
NASA Astrophysics Data System (ADS)
Koufos, Alexander
2012-02-01
This talk presents the first calculations of the electronic structure of francium for the bcc, fcc and hcp structures, using the Augmented Plane Wave (APW) method in its muffin-tin and linearized general potential forms. Both the Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA), were used to calculate the electronic structure and total energy of francium (Fr). The GGA and LDA both found the total energy of the hcp structure slightly below that of the fcc and bcc structure, respectively. This is in agreement with similar results for the other alkali metals using the same methodology. The equilibrium lattice constant, bulk modulus and superconductivity parameters were calculated. We found that under pressures, in the range of 1-5 GPa, Fr could be a superconductor at a critical temperature of about 4K.
Using a Hand-Held Electronic Calculator
ERIC Educational Resources Information Center
North, Roger
1975-01-01
The arithmetic needed for complex calculation using an electronic calculator is explained and exemplified. Problems involving square roots, number theory, Fibonacci numbers, and electrical resistances are solved. (SD)
NASA Astrophysics Data System (ADS)
Ksenafontov, Denis N.; Moiseeva, Natalia F.; Khristenko, Lyudmila V.; Karasev, Nikolai M.; Shishkov, Igor F.; Vilkov, Lev V.
2010-12-01
The geometric structure of piracetam was studied by quantum chemical calculations (DFT and ab initio), gas electron diffraction (GED), and FTIR spectroscopy. Two stable mirror symmetric isomers of piracetam were found. The conformation of pyrrolidine ring is an envelope in which the C4 atom deviates from the ring plane, the angle between the planes (C3 sbnd C4 sbnd C5) and (C2 sbnd C3 sbnd C5) is 154.1°. The direction of the deviation is the same as that of the side acetamide group. The piracetam molecule is stabilized in the gas phase by an intramolecular hydrogen bond between the N9H 2 group and the oxygen O6, bonded to C2. The principal structural parameters ( re, Å and ∠e, degrees; uncertainties are 3 σLS values) were found to be: r(С3 sbnd С4) = 1.533(1), r(C4 sbnd C5) = 1.540(1), r(N1 sbnd C5) = 1.456(1), r(C2 sbnd C3) = 1.520(1), r(N1 sbnd C7) = 1.452(1), r(C7 sbnd C8) = 1.537(1), r(N1 sbnd C2) = 1.365(2), r(C8 sbnd N9) = 1.360(2), r(C2 dbnd O6) = 1.229(1), r(C8 dbnd O10) = 1.221(1), ∠C2 sbnd N1 sbnd C5 = 113.4(6), ∠N1 sbnd C2 sbnd C3 = 106.9(6), ∠N1 sbnd C7 sbnd C8 = 111.9(6), ∠C7 sbnd C8 sbnd N9 = 112.5(6), ∠N1 sbnd C2 sbnd O6 = 123.0(4), ∠C3 sbnd N1 sbnd C7 = 120.4(4), ∠C7 sbnd C8 sbnd O10 = 120.2(4), ∠C5 sbnd N1 sbnd C2 sbnd O6 = 170(6), ∠C3 sbnd C2 sbnd N1 sbnd C7 = 178(6), ∠C2 sbnd N1 sbnd C7 sbnd C8 = 84.2, ∠N1 sbnd C7 sbnd C8 sbnd O10 = 111.9.
ERIC Educational Resources Information Center
Wetsel, Grover C., Jr.
1978-01-01
Calculates the energy-band structure of noninteracting electrons in a one-dimensional crystal using exact and approximate methods for a rectangular-well atomic potential. A comparison of the two solutions as a function of potential-well depth and ratio of lattice spacing to well width is presented. (Author/GA)
NASA Astrophysics Data System (ADS)
Wen, Jing; Zhang, Xitian; Gao, Hong
2015-02-01
Many conflicting electron microscopy data for In2O3(ZnO)m indicate that it may have the polymorphous and polytypoid structures. We investigate their stabilities based on four controversial models. The calculated results confirm that the models with the zigzag feature are more stable than the others and it is possible to form different zigzag configurations in the samples as observed in the experiments. The dynamic process of eliminating the dangling bonds and the requirements of maximizing the symmetry and the distances between the In atoms in the slabs can be regarded as the dominant rules to stabilize the system, but the statistical equilibrium processes have the chances to transform it from the ground state structures to the other model structures. The study of the electronic structures based on the plane and zigzag models reveals that their band gaps and effective masses increase monotonically with m. The predicted band gaps are consistent with the experimental results. The anisotropic feature of electron effective mass tensor exhibited in the plane model differs from that of the zigzag one, which is so notable that can be employed to determine which model is more close to the actual structure of a given sample. The calculated results confirm the possibilities of the separation of conduction electrons and defects and the existence of the natural optimized transport channels in the layered structures, which demonstrate its advantage over ZnO to transport electrons and benefit its applications in the optoelectronic devices.
Bengu, Erman; Marks, Laurence D; Ovali, Rasim V; Gulseren, Oguz
2008-10-01
Cubic boron nitride (c-BN) nucleation takes place on hexagonal boron nitride (h-BN) layers growing perpendicular to the substrate surface during thin film synthesis. Studies focused on the nucleation of the cubic phase suggest the possibility that transient phases and/or defects on these h-BN structures have a role in sp3-bonded cubic phase nucleation. In this study, we have investigated the nature, energetics, and structure of several possible defects on BN basal planes, including point defects, 4-, and 5-fold BN rings, that may possibly match the experimentally observed transient phase fine structure. TEM image observations are used to build approximate atomic models for the proposed structures, and DFT calculations are used to relax these structures while minimizing their respective total energies. These optimized atomic geometries are then used to simulate TEM images, which are compared to the experimentally observed structures. Data from DFT calculations and analysis of simulated images from the proposed atomic structures suggest that 4-fold BN rings are more likely to exist on the transient phase possibly leading to c-BN nucleation.
NASA Astrophysics Data System (ADS)
Kandasamy, M.; Velraj, G.
2012-08-01
Fourier transform infrared (FTIR) and FT-Raman spectra have been recorded and extensive spectroscopic investigations have been carried out on 5-bromo-2-pyridinecarbonitrile (5B2PC). The optimized geometries, wavenumber and intensity of the vibrational bands of (5B2PC) have been calculated using density functional level of theory (DFT/B3LYP) employing 6-311G(d,p) basis set. On the basis of the comparison between calculated and experimental results, assignments of the fundamental vibrational modes are examined. The molecular stability and bond strength were investigated by applying the natural bond orbital (NBO) analysis. The electronic properties like HOMO-LUMO analysis of (5B2PC) have been reported.
Singh, Swapnil; Singh, Harshita; Srivastava, Anubha; Tandon, Poonam; Sinha, Kirti; Bharti, Purnima; Kumar, Sudhir; Kumar, Padam; Maurya, Rakesh
2014-11-11
In the present work, a detailed conformational study of cladrin (3-(3,4-dimethoxy phenyl)-7-hydroxychromen-4-one) has been done by using spectroscopic techniques (FT-IR/FT-Raman/UV-Vis/NMR) and quantum chemical calculations. The optimized geometry, wavenumber and intensity of the vibrational bands of the cladrin in ground state were calculated by density functional theory (DFT) employing 6-311++G(d,p) basis sets. The study has been focused on the two most stable conformers that are selected after the full geometry optimization of the molecule. A detailed assignment of the FT-IR and FT-Raman spectra has been done for both the conformers along with potential energy distribution for each vibrational mode. The observed and scaled wavenumber of most of the bands has been found to be in good agreement. The UV-Vis spectrum has been recorded and compared with calculated spectrum. In addition, 1H and 13C nuclear magnetic resonance spectra have been also recorded and compared with the calculated data that shows the inter or intramolecular hydrogen bonding. The electronic properties such as HOMO-LUMO energies were calculated by using time-dependent density functional theory. Molecular electrostatic potential has been plotted to elucidate the reactive part of the molecule. Natural bond orbital analysis was performed to investigate the molecular stability. Non linear optical property of the molecule have been studied by calculating the electric dipole moment (μ) and the first hyperpolarizability (β) that results in the nonlinearity of the molecule.
Barboza, C.A.; Henriques, J.M.; Albuquerque, E.L.; Caetano, E.W.S.; Freire, V.N.; Costa, J.A.P. da
2010-02-15
Orthorhombic perovskite CdGeO{sub 3} was studied using the density-functional theory (DFT) formalism. The electronic band structure, density of states, effective masses, dielectric function and optical absorption were obtained. Comparing with orthorhombic CaGeO{sub 3}, which is an indirect S->GAMMA gap material, the substitution of calcium by cadmium changes the valence band maximum from the S point to the GAMMA point in reciprocal space, and decreases the Kohn-Sham band gap energy. Our results suggest that orthorhombic CdGeO{sub 3} has features of a semiconductor and is potentially useful for optoelectronic applications. - Abstract: Graphical Abstract Legend (TOC Figure): Different views of the unit cell of orthorhombic CdGeO{sub 3} (left, top). The electronic band structure near the main gap and the partial density of states (PDOS) are shown also (right), as well as the optical absorption for different polarizations of incident light (left, bottom).
NASA Astrophysics Data System (ADS)
Tatemizo, N.; Imada, S.; Miura, Y.; Yamane, H.; Tanaka, K.
2017-03-01
The valence band (VB) structures of wurtzite AlCrN (Cr concentration: 0-17.1%), which show optical absorption in the ultraviolet-visible-infrared light region, were investigated via photoelectron yield spectroscopy (PYS), x-ray/ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio density of states (DOS) calculations. An obvious photoelectron emission threshold was observed ~5.3 eV from the vacuum level for AlCrN, whereas no emission was observed for AlN in the PYS spectra. Comparisons of XPS and UPS VB spectra and the calculated DOS imply that Cr 3d states are formed both at the top of the VB and in the AlN gap. These data suggest that Cr doping could be a viable option to produce new materials with relevant energy band structures for solar photoelectric conversion.
NASA Technical Reports Server (NTRS)
Bates, Kevin R.; Daniels, Andrew D.; Scuseria, Gustavo E.
1998-01-01
We report a comparison of two linear-scaling methods which avoid the diagonalization bottleneck of traditional electronic structure algorithms. The Chebyshev expansion method (CEM) is implemented for carbon tight-binding calculations of large systems and its memory and timing requirements compared to those of our previously implemented conjugate gradient density matrix search (CG-DMS). Benchmark calculations are carried out on icosahedral fullerenes from C60 to C8640 and the linear scaling memory and CPU requirements of the CEM demonstrated. We show that the CPU requisites of the CEM and CG-DMS are similar for calculations with comparable accuracy.
Reshak, Ali H; Shalaginov, Mikhail Y; Saeed, Yasir; Kityk, I V; Auluck, S
2011-03-31
We report a first-principles study of structural and phase stability in three different structures of perovskite-types KMgH(3) according to H position. While electronic and optical properties were measured only for stable perovskite-type KMgH(3), our calculated structural parameters are found in good agreement with experiment and other theoretical results. We also study the electronic charge density space distribution contours in the (200), (101), and (100) crystallographic planes, which gives better insight picture of chemical bonding between K-H, K-Mg-H, and Mg-H. Moreover, we have calculated the electronic band structure dispersion, total, and partial density of electron states to study the band gap origin and the contribution of s-band of H, s and p-band of Mg in the valence band, and d-band of K in the conduction band. Furthermore, optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients, optical conductivities, and loss functions of stable KMgH(3) were calculated for photon energies up to 40 eV.
NASA Astrophysics Data System (ADS)
Flandorfer, Hans; Richter, Klaus W.; Giester, Gerald; Ipser, Herbert
2002-02-01
The new ternary compound Pd13In5.25Sb3.75 was found. Its crystal structure was determined using a CCD diffractometer at room temperature. Evaluations and refinements finally yielded a C-centered monoclinic structure (space group, C2/c; Pearson symbol, mC88, Z=4) with a=15.189(2) Å, b=8.799(1) Å, c=13.602(2) Å, and β=123.83(1)°. For the entire data set of 3706 independent reflections residual values are R=0.0461 and Rw=0.0789. The structure was found to be isotypic to Pd13Pb9 with In and Sb on the Pb sites. The existence of a further ternary compound, which was already described as Pd3In4Sb2, could be confirmed. Its composition range was determined by EPMA to be PdIn1.2-1.3Sb0.8-0.7. It does not melt congruently and we were not able to find suitable single crystals. However, we were able to prepare the pure ternary compound in order to perform X-ray powder diffraction using a Guinier image plate technique. The entire diffraction spectrum was refined by full profile Rietveld method using the program Fullprof. The α-PdSn2 structure type (space group, I41/acd; Pearson symbol, t148, Z=16), proposed for this compound, was confirmed and the lattice parameters are a=6.4350(1) Å and c=24.3638(3) Å. The residual values were Rp=5.34 and Rwp=6.70. The tetragonal PdSn2 structure type is a mixed variant of the CaF2 type and the CuAl2 type structure. Also in this ternary compound we assumed a random contribution of In and Sb over the 16e and 16f positions. The electronic structures of both compounds were investigated by extended Hückel calculations. Crystal orbital overlap populations show extended bonding interactions between the main group elements. The bonding interactions of the main group elements are almost optimized at the experimentally observed In/Sb ratio of the ternary compound. The In/Sb ratio in Pd13In5.25Sb3.75 can thus be rationalized on the basis of the electronic structure.
NASA Astrophysics Data System (ADS)
Hoyer, Chad E.; Manni, Giovanni Li; Truhlar, Donald G.; Gagliardi, Laura
2014-11-01
The diatomic molecule Fe2 was investigated using restricted active space second-order perturbation theory (RASPT2). This molecule is very challenging to study computationally because predictions about the ground state and excited states depend sensitively on the choice of the quantum chemical method. For Fe2 we show that one needs to go beyond a full-valence active space in order to achieve even qualitative agreement with experiment for the dissociation energy, and we also obtain a smooth ground-state potential curve. In addition we report the first multireference study of Fe_2^ +, for which we predict an 8Σ _u^ - ground state, which was not predicted by previous computational studies. By using an active space large enough to remove the most serious deficiencies of previous theoretical work and by explicitly investigating the interpretations of previous experimental results, this study elucidates previous difficulties and provides - for the first time - a qualitatively correct treatment of Fe2, Fe_2^ +, and Fe_2^ -. Moreover, this study represents a record in terms of the number or active electrons and active orbitals in the active space, namely 16 electrons in 28 orbitals. Conventional CASPT2 calculations can be performed with at most 16 electrons in 16 orbitals. We were able to overcome this limit by using the RASPT2 formalism.
NASA Astrophysics Data System (ADS)
Liu, Qi-Jun; Zhang, Ning-Chao; Sun, Yan-Yun; Liu, Fu-Sheng; Liu, Zheng-Tang
2014-05-01
The structural, elastic, mechanical, electronic, optical properties and effective masses of CuMIIIBO2 (MIIIB = Sc, Y, La) compounds have been investigated by the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory under local density approximation. The equilibrium structural parameters are in good agreement with previous experimental and theoretical data. To our knowledge, there are no available data of elastic constants for comparison. The bulk, shear and Young's modulus, ratio of B/G, Poisson's ratio and Lamé's constants of CuMIIIBO2 have been studied. The electronic structures of CuMIIIBO2 are consistent with other calculations. The population analysis, charge densities and effective masses have been shown and analyzed. The imaginary and real parts of the dielectric function, refractive index and extinction coefficient of CuMIIIBO2 are calculated. The interband transitions to absorption of CuMIIIBO2 have been analyzed.
Nuzhdin, Kirill B; Nesterov, Sergej V; Tyurin, Daniil A; Feldman, Vladimir I; Wei, Liu; Lund, Anders
2005-07-21
The radical cations of piperazine, morpholine, thiomorpholine, and thioxane were investigated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy in a solid Freon matrix. Optimized geometry and magnetic parameters of the radical cations were calculated using a density functional theory (DFT)/Perdew-Burke-Ernzerhof (PBE) method. Both experimental and theoretical results suggest that all the studied species adopt chair (or distorted chair) conformations. No evidence for the boat conformers with intramolecular sigma-bonding between heteroatoms were obtained. In the cases of morpholine and thioxane, the oxygen atoms are characterized by relatively small spin populations, whereas a major part of spin density is located at N and S atoms, respectively. The thiomorpholine radical cation exhibits nearly equal spin population of N and S atoms. In most cases (except for thioxane), the calculated magnetic parameters agree with the experimental data reasonably well.
NASA Astrophysics Data System (ADS)
Uǧurlu, Güventürk
2017-02-01
The molecular structure and conformational analysis of isonicotinic acid (3-methoxy-4-hydroxy-benzylidene)- hydrazide were investigated by Ab initio and density functional theory DFT/B3LYP levels of theory with complete relaxation in the potential energy surface using varied basis set. The four stable conformers of the studied molecule (C1, C2, C3 and C4) were computed. The computational results diagnose the most stable conformer of (3-methoxy-4-hydroxy-benzylidene)-hydrazide as the C1 form. Molecular structure, dipole moment, polarizability and first static hyperpolarizability of the four stable conformers have been calculated by using 6-311++G (d, p) basis set for both models. Besides, EHOMO (the highest occupied molecular orbital energy), ELUMO (the lowest unoccupied molecular orbital energy) and HOMO-LUMO energy gap (ΔEg) are investigated. The dipole moment for C1, C2, C3 and C4 conformers are calculated at 2.44, 7.74, 7.75 and 6.58 with DFT/B3LYP level of the theory 6-311++G (d, p) basis set and at the HF/6-311++ G (d, p) 2.60, 7.42, 7.41 and 6.36 Debye, respectively. The structural parameters of the studied molecule compared with data in the literature.
Nigam, Sandeep; Gupta, Sanjeev; Banyai, Douglas; Pandey, Ravindra; Majumder, Chiranjib
2015-03-14
Two dimensional nanostructures of group IV elements have attracted a great deal of attention because of their fundamental and technological applications. A graphene-like single layer of tin atoms, commonly called stanene, has recently been predicted to behave like a quantum spin Hall insulator. Here we report the atomic structure, stability and electron transport properties of stanene stabilized on a gold substrate. The optimization of geometry and electronic structure was carried out using a plane-wave based pseudo-potential approach. This work is divided into three parts: (i) the nature of chemical interaction between tin atoms and the gold support, (ii) the geometrical shape and electronic structure of the tin layer on the gold support and (iii) the electron transport behavior of the gold supported tin layer. The results show that tin atoms bind to the gold support through strong chemical bonds and significant electronic charge transfer occurs from tin to the gold support. Remarkably, for a layer of tin atoms, while a buckled structure is preferred in the free state, a planar graphene-like atomic arrangement is stabilized on the gold support. This structural change corroborates the metal-like band structure of the planar stanene in comparison to the semi-metallic buckled configuration. The tunneling current of the supported tin layer shows Ohmic-like behavior and the calculated STM pattern of the supported tin layer shows distinct images of 'holes', characteristic of the hexagonal lattice.
NASA Astrophysics Data System (ADS)
Middleton, Kirsten; Zhang, Guoping; George, Thomas F.
2012-02-01
Memantine is currently used as a treatment for mild to severe Alzheimer's disease, although its functionality is complicated. Using various density functional theory calculations and basis sets, we first examine memantine alone and then add ions which are present in the human body. This provides clues as to how the compound may react in the calcium ion channel, where it is believed to treat the disease. In order to understand the difference between calcium and magnesium ions interacting with memantine, we compute the electron affinity of each complex. We find that memantine is more strongly attracted to magnesium ions than calcium ions within the channel. By observing the HOMO-LUMO gap within memantine in comparison to adamantane, we find that memantine is more excitable than the anti-flu drug. We believe these factors to affect the efficiency of memantine as a treatment of Alzheimer's disease.
Choi, Sunghwan; Hong, Kwangwoo; Kim, Jaewook; Kim, Woo Youn
2015-03-07
We developed a self-consistent field program based on Kohn-Sham density functional theory using Lagrange-sinc functions as a basis set and examined its numerical accuracy for atoms and molecules through comparison with the results of Gaussian basis sets. The result of the Kohn-Sham inversion formula from the Lagrange-sinc basis set manifests that the pseudopotential method is essential for cost-effective calculations. The Lagrange-sinc basis set shows faster convergence of the kinetic and correlation energies of benzene as its size increases than the finite difference method does, though both share the same uniform grid. Using a scaling factor smaller than or equal to 0.226 bohr and pseudopotentials with nonlinear core correction, its accuracy for the atomization energies of the G2-1 set is comparable to all-electron complete basis set limits (mean absolute deviation ≤1 kcal/mol). The same basis set also shows small mean absolute deviations in the ionization energies, electron affinities, and static polarizabilities of atoms in the G2-1 set. In particular, the Lagrange-sinc basis set shows high accuracy with rapid convergence in describing density or orbital changes by an external electric field. Moreover, the Lagrange-sinc basis set can readily improve its accuracy toward a complete basis set limit by simply decreasing the scaling factor regardless of systems.
NASA Astrophysics Data System (ADS)
Behtash, Maziar; Joo, Paul H.; Nazir, Safdar; Yang, Kesong
2015-05-01
We studied the electronic properties and relative thermodynamic stability of several pentavalent-ion (Ta, Nb, P, Sb, and I) doped SnO2 systems using first-principles hybrid density functional theory calculations, in order to evaluate their potential as transparent conducting oxides (TCOs). I-doped SnO2, though conductive, shows a narrowed optical band gap with respect to the undoped system due to the formation of gap states above the valence band. Nb-doped SnO2 forms localized impurity states below the conduction band bottom, suggesting that the Nb dopant exists as an Nb4+-like cation, which is consistent with the recent experimental finding of the formation of the impurity level below the conduction band bottom [Appl. Phys. Express 5, 061201 (2012)]. Ta- and Sb-doped SnO2 display n-type conductivity, high charge carrier density, and widened optical band gap. P-doped SnO2 shows similar n-type electronic properties with that of Sb- and Ta-doped systems, and thus P-doped SnO2 is proposed as a promising candidate TCO for further experimental validation.
NASA Astrophysics Data System (ADS)
Bounab, S.; Bentabet, A.; Bouhadda, Y.; Belgoumri, Gh.; Fenineche, N.
2017-03-01
We have investigated the structural and electronic properties of the BAs x Sb 1-x , AlAs x Sb 1-x , GaAs x Sb 1-x and InAs x Sb 1-x semiconductor alloys using first-principles calculations under the virtual crystal approximation within both the density functional perturbation theory and the pseudopotential approach. In addition the optical properties have been calculated by using empirical methods. The ground state properties such as lattice constants, both bulk modulus and derivative of bulk modulus, energy gap, refractive index and optical dielectric constant have been calculated and discussed. The obtained results are in reasonable agreement with numerous experimental and theoretical data. The compositional dependence of the lattice constant, bulk modulus, energy gap and effective mass of electrons for ternary alloys show deviations from Vegard's law where our results are in agreement with the available data in the literature.
Fakhim Lamrani, A.; Belaiche, M.; Benyoussef, A.; and others
2014-01-07
The electronic and magnetic properties of double-impurities-doped SnO{sub 2} (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed for double impurities (M1 and M2) from M1 = Cr, and M2 = Mn, and Re. The origins of ferromagnetism are shown to be different in the two cases. For Sn{sub 1-2x}Cr{sub x}Mn{sub x}O2, the hybridization between Cr-3d and O-2p results in Cr becoming ferromagnetic with a magnetic moment of about 5.0 μ{sub B} per supercell. The Cr-and Mn-doped SnO{sub 2} system exhibits half-metallic ferromagnetism. The strong ferromagnetic couplings between local magnetic moments can be attributed to p-d hybridization. In contrast, in (Cr, Re) codoped TiO{sub 2}, the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Re, which would lead to the ferromagnetic through the double-exchange mechanism in transition metal oxides. Since there are two possible couplings between the impurities, we studied both configurations (ferromagnetic and antiferromagnetic (AF)) for double-impurities-doped SnO{sub 2}. Our calculations show that a ferromagnetic alignment of the spins is energetically always more stable than simple AF arrangements, which makes these materials possible candidates for spin injection in spintronic devices.
Structural and Electronic properties of β- In2 X 3 (X = O, S, Se, Te) using ab initio calculations
NASA Astrophysics Data System (ADS)
Khare, S. V.; Marsillac, S.; Mangale, N. S.; Gade, V.
2011-03-01
Several III-VI body-centered tetragonal layered compounds belonging to space group I 4 1 /amd have been a subject of interest recently because of their potential applications in high efficiency and environmentally friendly copper-indium-gallium-selenide (CIGS) solar cells and molecules. Here we have studied the structural, energetic, and electronic properties of four compounds β - In 2 X3 (X = O, S, Se, Te), in this space group. Using first principles computations, we have fully determined the lattice constants a and c, as well as 10 internal parameters that define this unique structure of primitive unit cells of 40 atoms. For β - In 2 S3 our computed values are found to be consistent with experimental measurements. The bulk modulus B, local electronic density of states (LDOS), total density of states (DOS), and band gap Ef of these phases have been investigated. Supported by Ohio Supercomputing Center, National Center for Supercomputing Applications, Wright Center for PVIC, National Science Foundation, DARPA.
NASA Astrophysics Data System (ADS)
Li, Zongbao; Wang, Xia; Xing, Xiaobo; Wang, Ying
2017-02-01
Using density functional theory, we calculated the geometries, band structures and densities of states of W-doped, N-doped, and W/N-codoped anatase TiO4 (001) and (101) surfaces, as well as while the formation energies, based on the overall reaction energy diagram. The calculated results reveal that, on the two surfaces, the absorption of W atoms are more stable than that of N atoms while a larger energy barrier blocks the transfer of W atoms from the surfaces to the body. For TiO2(001), the W-doping and the N/W-codoping lead to a visible lattice distortion while the recombination of photo-generated electron-holes pairs is reduced. A comprehensive analysis of the electronic structures show that the band-gap narrows and a new W-N bond appears, which obviously enhance the photocatalytic activity.
Structural, mechanical, and electronic properties of Rh2B and RhB2: first-principles calculations
Chu, Binhua; Li, Da; Tian, Fubo; Duan, Defang; Sha, Xiaojing; Lv, Yunzhou; Zhang, Huadi; Liu, Bingbing; Cui, Tian
2015-01-01
The crystal structures of Rh2B and RhB2 at ambient pressure were explored by using the evolutionary methodology. A monoclinic P21/m structure of Rh2B was predicted and donated as Rh2B-I, which is energetically much superior to the previously experimentally proposed Pnma structure. At the pressure of about 39 GPa, the P21/m phase of Rh2B transforms to the C2/m phases. For RhB2, a new monoclinic P21/m phase was predicted, named as RhB2-II, it has the same structure type with Rh2B. Rh2B-I and RhB2-II are both mechanically and dynamically stable. They are potential low compressible materials. The analysis of electronic density of states and chemical bonding indicates that the formation of strong and directional covalent B-B and Rh-B bonds in these compounds contribute greatly to their stabilities and high incompressibility. PMID:26123399
NASA Astrophysics Data System (ADS)
Mokadem, A.; Bouslama, M.; Benhelal, O.; Assali, A.; Ghaffour, M.; Chelahi Chikr, Z.; Boulenouar, K.; Boubaia, A.
2014-03-01
The semiconductor ZnO of large gap of 3,4 eV is of great interest for the technological applications as chemical sensors, UV light emission, optical memories, laser emission, solar cells, etc. These applications depend on the electron structure of material. We adopt the density functional theory (DFT) calculation by using the program Wien2K, within the Generalized Gradient Approximation (GGA) and modified Becke-Johnson (mBJ) for studying the electron behavior of ZnO. The features of the valence band derived from the hybridization of Zn-3d and O-2p states. The electron charge density calculated by these simulation methods indicates a charge transfer between zinc and oxygen inducing a difference in electronegativity between both species (Zn and O), responsible to ionic character of bonding in ZnO. The predictions based on the GGA and mBJ calculations are confirmed by the results of the experimental spectroscopic analysis Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS).
NASA Astrophysics Data System (ADS)
Sharma, Sheetal; Verma, A. S.; Sarkar, B. K.; Jindal, V. K.
2012-06-01
The structural, electronic, optical and elastic properties of zinc-blende compounds (ZnX, X = S, Se and Te) have been investigated using the full-potential augmented plane wave plus local orbitals method within density functional theory. The generalized gradient approximation (GGA) formalism is used for the exchange correlation energy to calculate the bulk properties including lattice constant, bulk modulus and its pressure derivative. Electronic band gap, complex refractive index and elastic constants (C11, C12 and C44) are also evaluated. Our results are in reasonable agreement with the available theoretical and experimental data.
NASA Astrophysics Data System (ADS)
Abid, O. Miloud; Menouer, S.; Yakoubi, A.; Khachai, H.; Omran, S. Bin; Murtaza, G.; Prakash, Deo; Khenata, R.; Verma, K. D.
2016-05-01
The structural, electronic, elastic, thermoelectric and thermodynamic properties of NbMSb (M = Fe, Ru, Os) half heusler compounds are reported. The full-potential linearized augmented plane wave (FP-LAPW) plus local orbital (lo) method, based on the density functional theory (DFT) was employed for the present study. The equilibrium lattice parameter results are in good compliance with the available experimental measurements. The electronic band structure and Boltzmann transport calculations indicated a narrow indirect energy band gap for the compound having electronic structure favorable for thermoelectric performance as well as with substantial thermopowers at temperature ranges from 300 K to 800 K. Furthermore, good potential for thermoelectric performance (thermopower S ≥ 500 μeV) was found at higher temperature. In addition, the analysis of the charge density, partial and total densities of states (DOS) of three compounds demonstrate their semiconducting, ionic and covalent characters. Conversely, the calculated values of the Poisson's ratio and the B/G ratio indicate their ductile makeup. The thermal properties of the compounds were calculated by quasi-harmonic Debye model as implemented in the GIBBS code.
Sheu, Hong-Li; Kim, Sunghwan; Laane, Jaan
2013-12-19
The infrared and Raman spectra of 2,6-difluoropyridine (26DFPy) along with ab initio and DFT computations have been used to assign the vibrations of the molecule in its S0 electronic ground state and to calculate its structure. The ultraviolet absorption spectrum showed the electronic transition to the S1(π,π*) state to be at 37,820.2 cm(-1). With the aid of ab initio computations the vibrational frequencies for this excited state were also determined. TD-B3LYP and CASSCF computations for the excited states were carried out to calculate the structures for the S1(π,π*) and S2(n,π*) excited states. The CASSCF results predict that the S1(π,π*) state is planar and that the S2(n,π*) state has a barrier to planarity of 256 cm(-1). The TD-B3LYP computations predict a barrier of 124 cm(-1) for the S1(π,π*) state, but the experimental results support the planar structure. Hypothetical models for the ring-puckering potential energy function were calculated for both electronic excited states to show the predicted quantum states. The changes in the vibrational frequencies in the two excited states reflect the weaker π bonding within the pyridine ring.
Kasireddy, Chandana; Bann, James G; Mitchell-Koch, Katie R
2015-11-11
Fluorine NMR spectroscopy is a powerful tool for studying biomolecular structure, dynamics, and ligand binding, yet the origins of (19)F chemical shifts are not well understood. Herein, we use electronic structure calculations to describe the changes in (19)F chemical shifts of 2F- and 4F-histidine/(5-methyl)-imidazole upon acid titration. While the protonation of the 2F species results in a deshielded chemical shift, protonation of the 4F isomer results in an opposite, shielded chemical shift. The deshielding of 2F-histidine/(5-methyl)-imidazole upon protonation can be rationalized by concomitant decreases in charge density on fluorine and a reduced dipole moment. These correlations do not hold for 4F-histidine/(5-methyl)-imidazole, however. Molecular orbital calculations reveal that for the 4F species, there are no lone pair electrons on the fluorine until protonation. Analysis of a series of 4F-imidazole analogues, all with delocalized fluorine electron density, indicates that the deshielding of (19)F chemical shifts through substituent effects correlates with increased C-F bond polarity. In summary, the delocalization of fluorine electrons in the neutral 4F species, with gain of a lone pair upon protonation may help explain the difficulty in developing a predictive framework for fluorine chemical shifts. Ideas debated by chemists over 40 years ago, regarding fluorine's complex electronic effects, are shown to have relevance for understanding and predicting fluorine NMR spectra.
NASA Astrophysics Data System (ADS)
Chan, Kevin T.; Lee, Hoonkyung; Cohen, Marvin L.
2011-10-01
Graphene provides many advantages for controlling the electronic structure of adatoms and other adsorbates via gating. Using the projected density of states and charge density obtained from first-principles density-functional periodic supercell calculations, we investigate the possibility of performing “alchemy” of adatoms on graphene, i.e., transforming the electronic structure of one species of adatom into that of another species by application of a gate voltage. Gating is modeled as a change in the number of electrons in the unit cell, with the inclusion of a compensating uniform background charge. Within this model and the generalized gradient approximation to the exchange-correlation functional, we find that such transformations are possible for K, Ca, and several transition-metal adatoms. Gate control of the occupation of the p states of In on graphene is also investigated. The validity of the supercell approximation with uniform compensating charge and the model for exchange and correlation is also discussed.
Walsh, Aron; Wei, S.-H.; Yan Yanfa; Al-Jassim, M. M.; Turner, John A.; Woodhouse, Michael; Parkinson, B. A.
2007-10-15
A systematic study of nine binary and ternary spinel oxides formed from Co, Al, and Fe is presented by means of density functional theory. Analysis of the structural, magnetic, and electronic properties through the series of materials is carried out. Preference for the octahedral spinel sites are found in the order Fe
NASA Astrophysics Data System (ADS)
Quinet, Pascal
2014-09-01
A detailed investigation of the atomic structure and radiative parameters involving the lowest states within the 6p4, 6p36d, 6p37s, 6p37p and 6p37d configurations of neutral polonium is reported in the present paper. Using different physical models based on the pseudo-relativistic Hartree-Fock approach, the influence of intravalence, core-valence and core-core electron correlation on the atomic parameters is discussed in detail. This work allowed us to fix the spectroscopic designation of some experimental level energy values and to provide for the first time a set of reliable oscillator strengths corresponding to 31 Po I spectral lines in the wavelength region from 175 to 987 nm.
NASA Astrophysics Data System (ADS)
Berdiyorov, G. R.; Madjet, M. E.
2016-12-01
Using the first-principles density functional theory, we study the effect of surface functionalization on the structural and optoelectronic properties of recently proposed quasi-two-dimensional material TiC2 [T. Zhao, S. Zhang, Y. Guo, Q. Wang, Nanoscale 8 (2016) 233]. Hydrogenated, fluorinated, oxidized and hydroxylated surfaces are considered. Significant changes in the lattice parameters and partial charge distributions are found due to the surface termination. Direct contribution of the adatoms to the system density of states near the Fermi level is obtained, which has a major impact on the optoelectronic properties of the material. For example, surface termination results in larger absorption in the visible range of the spectrum. The electronic transport is also affected by the surface functionalization: the current in the system can be reduced by an order of magnitude. These findings indicate the importance of the effects of surface passivation on optoelectronic properties of this quasi-2D material.
NASA Astrophysics Data System (ADS)
Slassi, A.; Hammi, M.; El Rhazouani, O.
2017-02-01
The surface relaxations, surface energies and electronic structures of BaO- and SnO2-terminated BaSnO3 (001) surfaces have been studied by employing the first-principles density functional theory. For both terminations, we find that the upper-layer Ba and Sn atoms move inward, whereas upper-layer O atoms move outward from the surface. Moreover, the largest relaxations are occurred on the first-layer atoms of both terminations. The surface rumpling of BaO-terminated BaSnO3 (001) is slightly less than that of the SnO2-terminated BaSnO3 (001) surface. The surface energies show that both terminated surfaces are energetically stable and favorable. Finally, the surface band gap is slightly decreased for the BaO termination, while it is dramatically decreased for the SnO2 termination.
NASA Astrophysics Data System (ADS)
Zhou, Zhaobo; Zhou, Xiaolong; Zhang, Kunhua
2016-12-01
First-principle calculations were performed to investigate the structural, phase stability, electronic, elastic properties and hardness of monoclinic structure IrN2 (m-IrN2), orthorhombic structure IrN2 (o-IrN2) and zinc blende structure IrN (ZB IrN). The results show us that only m-IrN2 is both thermodynamic and dynamic stability. The calculated band structure and density of states (DOS) curves indicate that o-IrN2 and ZB Ir-N compounds we calculated have metallic behavior while m-IrN2 has a small band gap of 0.3 eV, and exist a common hybridization between Ir-5d and N-2p states, which forming covalent bonding between Ir and N atoms. The difference charge density reveals the electron transfer from Ir atom to N atom for three Ir-N compounds, which forming strong directional covalent bonds. Notable, a strong N-N bond appeared in m-IrN2 and o-IrN2. The ratio of bulk to shear modulus (B/G) indicate that three Ir-N compounds we calculated are ductile, and ZB IrN possesses a better ductility than two types IrN2. m-IrN2 has highest Debye temperature (736 K), illustrating it possesses strongest covalent bonding. The hardness of three Ir-N compounds were also calculated, and the results reveal that m-IrN2 (18.23 GPa) and o-IrN2 (18.02 GPa) are ultraincompressible while ZB IrN has a negative value, which may be attributed to phase transition at ca. 1.98 GPa.
NASA Astrophysics Data System (ADS)
Mizutani, U.; Asahi, R.; Sato, H.; Noritake, T.; Takeuchi, T.
2008-07-01
The first-principles FLAPW (full potential linearized augmented plane wave) electronic structure calculations were performed for the Ag5Li8 gamma-brass, which contains 52 atoms in a unit cell and has been known for many years as one of the most structurally complex alloy phases. The calculations were also made for its neighboring phase AgLi B2 compound. The main objective in the present work is to examine if the Ag5Li8 gamma-brass is stabilized at the particular electrons per atom ratio e/a = 21/13 in the same way as some other gamma-brasses like Cu5Zn8 and Cu9Al4, obeying the Hume-Rothery electron concentration rule. For this purpose, the e/a value for the Ag5Li8 gamma-brass as well as the AgLi B2 compound was first determined by means of the FLAPW-Fourier method we have developed. It proved that both the gamma-brass and the B2 compound possess an e/a value equal to unity instead of 21/13. Moreover, we could demonstrate why the Hume-Rothery stabilization mechanism fails for the Ag5Li8 gamma-brass and proposed a new stability mechanism, in which the unique gamma-brass structure can effectively lower the band-structure energy by forming heavily populated bonding states near the bottom of the Ag-4d band.
Martineau, Charlotte; Allix, Mathieu; Suchomel, Matthew R; Porcher, Florence; Vivet, François; Legein, Christophe; Body, Monique; Massiot, Dominique; Taulelle, Francis; Fayon, Franck
2016-10-04
The room temperature structure of Ba5AlF13 has been investigated by coupling electron, synchrotron and neutron powder diffraction, solid-state high-resolution NMR ((19)F and (27)Al) and first principles calculations. An initial structural model has been obtained from electron and synchrotron powder diffraction data, and its main features have been confirmed by one- and two-dimensional NMR measurements. However, DFT GIPAW calculations of the (19)F isotropic shieldings revealed an inaccurate location of one fluorine site (F3, site 8a), which exhibited unusual long F-Ba distances. The atomic arrangement was reinvestigated using neutron powder diffraction data. Subsequent Fourier maps showed that this fluorine atom occupies a crystallographic site of lower symmetry (32e) with partial occupancy (25%). GIPAW computations of the NMR parameters validate the refined structural model, ruling out the presence of local static disorder and indicating that the partial occupancy of this F site reflects a local motional process. Visualisation of the dynamic process was then obtained from the Rietveld refinement of neutron diffraction data using an anharmonic description of the displacement parameters to account for the thermal motion of the mobile fluorine. The whole ensemble of powder diffraction and NMR data, coupled with first principles calculations, allowed drawing an accurate structural model of Ba5AlF13, including site-specific dynamical disorder in the fluorine sub-network.
NASA Astrophysics Data System (ADS)
Errandonea, D.; Segura, A.; Manjón, F. J.; Chevy, A.; Machado, E.; Tobias, G.; Ordejón, P.; Canadell, E.
2005-03-01
This paper reports on Hall effect and resistivity measurements under high pressure up to 3-4 GPa in p -type γ -indium selenide (InSe) (doped with As, Cd, or Zn) and ɛ -gallium selenide (GaSe) (doped with N or Sn). The pressure behavior of the hole concentration and mobility exhibits dramatic differences between the two layered compounds. While the hole concentration and mobility increase moderately and monotonously in ɛ -GaSe, a large increase of the hole concentration near 0.8 GPa and a large continuous increase of the hole mobility, which doubled its ambient pressure value by 3.2 GPa, is observed in γ -InSe. Electronic structure calculations show that the different pressure behavior of hole transport parameters can be accounted for by the evolution of the valence-band maximum in each material under compression. While the shape of the valence band maximum is virtually pressure-insensitive in ɛ -GaSe, it changes dramatically in γ -InSe, with the emergence of a ring-shaped subsidiary maximum that becomes the absolute valence-band maximum as pressure increases. These differences are shown to be a consequence of the presence or absence of a symmetry element (mirror plane perpendicular to the anisotropy axis) in the point group of each polytype ( D3h for the ɛ -polytype and C3v for the γ -polytype), resulting in different selection rules that affect the k⃗•p⃗ interaction between valence bands.
Electronic structure and optical properties of F-doped β-Ga2O3 from first principles calculations
NASA Astrophysics Data System (ADS)
Jinliang, Yan; Chong, Qu
2016-04-01
The effects of F-doping concentration on geometric structure, electronic structure and optical property of β-Ga2O3 were investigated. All F-doped β-Ga2O3 with different concentrations are easy to be formed under Ga-rich conditions, the stability and lattice parameters increase with the F-doping concentration. F-doped β-Ga2O3 materials display characteristics of the n-type semiconductor, occupied states contributed from Ga 4s, Ga 4p and O 2p states in the conduction band increase with an increase in F-doping concentration. The increase of F concentration leads to the narrowing of the band gap and the broadening of the occupied states. F-doped β-Ga2O3 exhibits the sharp band edge absorption and a broad absorption band. Absorption edges are blue-shifted, and the intensity of broad band absorption has been enhanced with respect to the fluorine content. The broad band absorption is ascribed to the intra-band transitions from occupied states to empty states in the conduction band. Project supported by the Innovation Project of Shandong Graduate Education, China (No. SDYY13093) and the National Natural Science Foundation of China (No. 10974077).
NASA Astrophysics Data System (ADS)
Shein, I. R.; Ivanovskii, A. L.
2011-10-01
The hexagonal phase SrPtAs (s.g. P6/ mmm; #194) with a honeycomb lattice structure was recently declared as a new low-temperature ( T C ∼ 4.2 K) superconductor. Here, by means of first-principles calculations the optimized structural parameters, electronic bands, Fermi surface, total and partial densities of states, inter-atomic bonding picture, independent elastic constants, bulk and shear moduli for SrPtAs were obtained for the first time and analyzed in comparison with the related layered superconductor SrPt 2As 2.
NASA Astrophysics Data System (ADS)
Çiftci, Yasemin Ö.; Çoban, Cansu
2016-02-01
The structural, mechanical, electronic, dynamic, and optical properties of the ZrPdSn compound crystallising into the MgAgAs structure are investigated by the ab initio calculations based on the density functional theory. The lattice constant, bulk modulus, and first derivative of bulk modulus were obtained by fitting the calculated total energy-atomic volume results to the Murnaghan equation of state. These results were compared to the previous data. The band structure and corresponding density of states (DOS) were also calculated and discussed. The elastic properties were calculated by using the stress-strain method, which shows that the MgAgAs phase of this compound is mechanically stable. The presented phonon dispersion curves and one-phonon DOS confirms that this compound is dynamically stable. In addition, the heat capacity, entropy, and free energy of ZrPdSn were calculated by using the phonon frequencies. Finally, the optical properties, such as dielectric function, reflectivity function, extinction coefficient, refractive index, and energy loss spectrum, were obtained under different pressures.
Multidimensional Deterministic Electron Transport Calculations
1992-05-01
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Long, Run
2013-04-18
The electronic structure of the TiO2(110) surface interfaced with both a semiconducting and metallic carbon nanotube (CNT) was investigated by density functional theory. Our simulations rationalized visible light photocatalytic activity of CNT/TiO2 hybrid materials higher than that under ultraviolent irradiation and showed that the photoactivity of a semiconducting CNT decorating TiO2 is better than that of the metallic CNT/TiO2 system due to efficient charge separation across the interface. This suggests that semiconducting CNT/TiO2 could be a potential photovoltaic material. In contrast, strong interaction between a metallic CNT and TiO2 leads to large charge transfer. Such charge transfer reduces the built-in potential, in turn resulting in inefficient charge separation. Functionalizing the metallic CNT with a small platinum cluster can increase the built-in potential and drive charge separation. These observations indicate that the CNT/TiO2 interface can be a potential photovoltaic material by a metal cluster decorating a CNT despite a real tube being composed of the mixture of metallic and semiconducting CNTs.
NASA Astrophysics Data System (ADS)
Fan, Shuai-wei; Wang, Ri-gao; Xu, Pemg
2016-09-01
The electronic structures and magnetism for carbon-doped group III-nitrides are investigated by utilizing the first principle method with the modified Becke-Johnson potential. Calculations show that carbon substituting cations (anions) would induce the group III-nitrides to be paramagnetic metals (half-metallic ferromagnets). Single carbon substituting nitrogen could produce 1.00μB magnetic moment. Electronic structures indicate that the carriers-mediated double-exchange interaction plays a crucial role in forming the ferromagnetism. Based on the mean-field theory, the Curie temperature for carbon-doped group III-nitrides would be above the room temperature. Negative chemical pair interactions imply that carbon dopants tend to form clustering distribution in group III-nitrides. The nitrogen vacancy would make the carbon-doped group III-nitrides lose the half-metallic ferromagnetism.
NASA Astrophysics Data System (ADS)
Huang, X. N.; Fan, S. W.; Pan, L. Q.
2017-01-01
The electronic structures and ferromagnetism for bulk and surface CdS with CS defects are investigated by the full potential linearized augmented plane wave method together with the modified Becke-Johnson potential. Calculations show bulk and surface CdS with CS defects are half-metallic ferromagnet. Each CS defect could produce the total magnetic moment of 2.00 μB. Electronic structures indicate the stable ferromagnetism could be attributed to the p-d exchange-like p-p coupling mechanism. Sulfur vacancies would give rise to the magnetism vanishing. For the nonpolar (10 1 bar 0) surfaces, the CS defects prefer to occupy the surface layer sites.
NASA Astrophysics Data System (ADS)
Liu, Zhen; Wei, Xinyuan; Wang, Jiajia; Pan, Hong; Ji, Fuhao; Ye, Mao; Yang, Zhongqin; Qiao, Shan
2015-09-01
The local atomic and electronic structures around the dopants in Cr-doped (BixSb1 -x )2Te3 are studied by x-ray absorption fine structure (XAFS) measurements and first-principles calculations. Both Cr and Bi are confirmed substituting Sb sites (CrSb and BiSb). The six nearest Te atoms around Cr move towards Cr and shorten the Cr-Te bond lengths to 2.76 Å and 2.77 Å for x =0.1 and x =0.2 , respectively. Importantly, we reveal the hybridization between the Sb/Te p states and Cr d states by the presence of a pre-edge peak at Cr K -absorption edge, which is also supported by our ab initio calculations. These findings provide important clues to understand the mechanism of ferromagnetic order in this system with quantum anomalous Hall effect.
Lambrecht, Daniel S.; McCaslin, Laura; Xantheas, Sotiris S.; Epifanovsky, Evgeny; Head-Gordon, Martin
2012-09-06
This work reports refinements of the energetic ordering of the known low-energy structures of sulfate-water clusters SO_{4}^{2-} (H2O)n (n = 3-6) using high-level electronic structure methods. Coupled cluster singles and doubles with perturbative triples (CCSD(T)) is used in combination with an estimate of basis set effects up to the complete basis set limit using second order Møller-Plesset theory. Harmonic zero point energy (ZPE), included at the B3LYP/6-311++G** level, was found to have a significant effect on the energetic ordering. Limitations of the ZPE calculations, both due to electronic structure errors, and use of the harmonic approximation, probably constitute the largest remaining errors. Due to the often small energy differences between cluster isomers, and the significant role of ZPE, deuteration can alter the relative energies of low-lying structures, and, when it is applied in conjunction with calculated harmonic ZPE’s, even alters the global minimum for n = 4.
NASA Astrophysics Data System (ADS)
Lazar, Petr; Martincová, Jana; Otyepka, Michal
2015-12-01
TaS2 is a transition-metal dichalcogenide having an exceptionally rich phase diagram, which includes exotic phenomena such as a charge density wave. We analyzed the structure, bonding, ground state, and dynamical stability of 1 T , 2 H , and 3 R phases of TaS2, and a commensurate charge density wave phase from the first principles. Van der Waals interaction among layers and strong electron-electron interactions were included by using the exact exchange plus random phase approximation, a high-level quantum mechanical approach. The calculated structural parameters agree well with the available experimental data. The individual sheets of TaS2 are bound by dispersive forces, which are stronger than dispersive forces in graphite and fluorographite. 1 T -TaS2 is dynamically unstable at low temperature, which leads to the formation of charge density wave and opening of the in-plane band gap. Anharmonic phonon-phonon interactions stabilize the 1 T structure at elevated temperatures. The calculated phase diagram of TaS2 reveals that the 1 T phase is the ground state at temperatures above 1300 K, 2 H below this point, and the charge density wave phase becomes more stable than the perfect 1 T structure below 480 K.
NASA Astrophysics Data System (ADS)
Mousa, Ahmad A.; Khalifeh, Jamil M.
2015-10-01
Structural, electronic, elastic and mechanical properties of ScM (M =Au, Hg and Tl) intermetallic compounds are studied using the full potential-linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT), within the generalized gradient approximation (GGA) and the local density approximation (LDA) to the exchange-correlation approximation energy as implemented in the Wien2k code. The ground state properties including lattice parameters, bulk modulus and elastic constants were all computed and compared with the available previous theoretical and experimental results. The lattice constant was found to increase in contrast to the bulk modulus which was found to decrease with every substitution of the cation (M) starting from Au till Tl in ScM. Both the electronic band structure and density-of-states (DOS) calculations show that these compounds possess metallic properties. The calculated elastic constants (C11, C12 and C44) confirmed the elastic stability of the ScM compounds in the B2-phase. The mechanical properties and ductile behaviors of these compounds are also predicted based on the calculated elastic constants.
NASA Astrophysics Data System (ADS)
Huang, Chang-Bao; Wu, Hai-Xin; Ni, You-Bao; Wang, Zhen-You; Qi, Ming; Zhang, Chun-Li
2016-08-01
The structural, electronic, mechanical properties, and frequency-dependent refractive indexes of GaSe1-x S x (x = 0, 0.25, and 1) are studied by using the first-principles pseudopotential method within density functional theory. The calculated results demonstrate the relationships between intralayer structure and elastic modulus in GaSe1-x S x (x = 0, 0.25, and 1). Doping of ɛ-GaSe with S strengthens the Ga-X bonds and increases its elastic moduli of C 11 and C 66. Born effective charge analysis provides an explanation for the modification of cleavage properties about the doping of ɛ-GaSe with S. The calculated results of band gaps suggest that the distance between intralayer atom and substitution of SSe, rather than interlayer force, is a key factor influencing the electronic exciton energy of the layer semiconductor. The calculated refractive indexes indicate that the doping of ɛ-GaSe with S reduces its refractive index and increases its birefringence. Project supported by the National Natural Science Foundation of China (Grant No. 51202250).
NASA Astrophysics Data System (ADS)
Kabita, Kh; Maibam, Jameson; Indrajit Sharma, B.; Brojen Singh, R. K.; Thapa, R. K.
2016-01-01
We report first principles phase transition, elastic properties and electronic structure for cadmium telluride (CdTe) under induced pressure in the light of density functional theory using the local density approximation (LDA), generalised gradient approximation (GGA) and modified Becke-Johnson (mBJ) potential. The structural phase transition of CdTe from a zinc blende (ZB) to a rock salt (RS) structure within the LDA calculation is 2.2 GPa while that within GGA is found to be at 4 GPa pressure with a volume collapse of 20.9%. The elastic constants and parameters (Zener anisotropy factor, Shear modulus, Poisson’s ratio, Young’s modulus, Kleinmann parameter and Debye’s temperature) of CdTe at different pressures of both the phases have been calculated. The band diagram of the CdTe ZB structure shows a direct band gap of 1.46 eV as predicted by mBJ calculation which gives better results in close agreement with experimental results as compared to LDA and GGA. An increase in the band gap of the CdTe ZB phase is predicted under induced pressure while the metallic nature is retained in the CdTe RS phase.
NASA Astrophysics Data System (ADS)
Yuan, H. K.; Chen, H.; Kuang, A. L.; Ahmed, A. S.; Xiong, Z. H.
2007-05-01
The all-electron spin-polarized generalized gradient approximation to the density-functional theory is used to determine the binding energies, ground-state structures, electronic structures, and magnetic properties of the Yn clusters (n⩽17) . The structural evolution of yttrium clusters, which favors a compact and icosahedral structural growth pattern, is elucidated and compared with the other group-III elemental clusters. The results show that clusters with n=7,13 are more stable than their respective neighbors. Furthermore, the maxima of magnetism at n=8 and n=13 observed experimentally are well described and the magnetic moments for most yttrium clusters are quite small except for Y6 , Y8 , and Y12-Y14 . Particularly, the regular icosahedron structure with a giant moment of 19μB is favored for the Y13 cluster. The similar magnetic features of the scandium and yttrium clusters shown in experiments can be attributed to a common structural motif for these two series of clusters. A change of magnetic ordering from ferromagnetic to antiferromagnetic is observed at n=7 , the exception being the systems Yn with n=8,13,14 which are found to be ferromagnetic. In addition, the calculated ionization potentials are in good agreement with the experimental results, which imply that the predictions of the ground-state geometries of those clusters are accurate.
Wurmehl, Sabine; Fecher, Gerhard H.; Kandpal, Hem C.; Ksenofontov, Vadim; Felser, Claudia; Lin Hongji; Morais, Jonder
2005-11-01
In this work a simple concept was used for a systematic search for materials with high spin polarization. It is based on two semiempirical models. First, the Slater-Pauling rule was used for estimation of the magnetic moment. This model is well supported by electronic structure calculations. The second model was found particularly for Co{sub 2} based Heusler compounds when comparing their magnetic properties. It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment. Stimulated by these models, Co{sub 2}FeSi was revisited. The compound was investigated in detail concerning its geometrical and magnetic structure by means of x-ray diffraction, x-ray absorption, and Moessbauer spectroscopies as well as high and low temperature magnetometry. The measurements revealed that it is, currently, the material with the highest magnetic moment (6{mu}{sub B}) and Curie temperature (1100 K) in the classes of Heusler compounds as well as half-metallic ferromagnets. The experimental findings are supported by detailed electronic structure calculations.
Li, C. Zhao, Y. F.; Fu, C. X.; Gong, Y. Y.; Chi, B. Q.; Sun, C. Q.
2014-10-15
The effects of vertical compressive stress on Sb{sub 2}Te{sub 3} nano-films have been investigated by the first principles calculation, including stability, electronic structure, crystal structure, and bond order. It is found that the band gap of nano-film is sensitive to the stress in Sb{sub 2}Te{sub 3} nano-film and the critical thickness increases under compressive stress. The band gap and band order of Sb{sub 2}Te{sub 3} film has been affected collectively by the surface and internal crystal structures, the contraction ratio between surface bond length of nano-film and the corresponding bond length of bulk decides the band order of Sb{sub 2}Te{sub 3} film.
NASA Astrophysics Data System (ADS)
Das, Pratik Kr.; Mandal, Nibir; Arya, A.
2017-02-01
Naturally occurring zinc sulfide (ZnS) contains a substantial amount of iron (Fe) in its crystal structure. This study explores the possible effects of such Fe impurity on the physical properties of its two phases: B3 and B1, crystallizing in a cubic system with zinc blend (ZB, space group: F-43m) and rock salt (RS, space group: Fm-3m) structures. We have performed ab-initio calculations within density functional theory (DFT) to determine the equilibrium volumes of B3- and B1-ZnS phases, doped with Fe in varying concentrations (0% to 25%), and their corresponding lattice structures. Using the enthalpy cross-over, we determine the pressure-dependent B3 to B1 transition as a function of Fe concentration. Our DFT calculations suggest an inverse relation of the transition pressure with Fe content. For pure ZnS, the transition occurs at 17 GPa, which drops to ˜12 GPa for 25% Fe. This study also provides a first-hand analysis of the elastic constants (C11, C12, and C44) to show the effects of Fe impurity on the mechanical properties of ZnS phases. Their values generally drop due to Fe and the differences widen with increasing pressure. Fe causes large softening of C44, especially for the B1 phase. We have also performed phonon calculations to characterize the vibrational properties and explain the pressure dependent structural instability of the B3- ZnS. Finally, our calculations of the electronic structures show a transition of semi-conductor to conductor behavior of ZnS with incorporation of Fe impurity.
Structural, electronic and magnetic properties of Cd1-xTMxS (TM=Co and V) by ab-initio calculations
NASA Astrophysics Data System (ADS)
Yahi, Hakima; Meddour, Athmane
2016-03-01
The structural, electronic and ferromagnetic properties of Cd1-xTMxS (TM=Co and V) compounds at x=0.25, 0.50 and 0.75 in zinc blende (B3) phase, have been investigated using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the frame work of the density functional theory and the generalized gradient approximation. The electronic properties exhibit half-metallic behavior at x=0.25, 0.50, and 0.75 for Cd1-xVxS and x=0.25 and 0.50 for Cd1-xCoxS, while Cd1-xCoxS with x=0.75 is nearly half-metallic. The calculated magnetic moment per substituted transition metal (TM) atom for half-metallic compounds is found to be 3 μB, whereas that of a nearly half-metallic compound is 2.29 μB. The analysis of band structure and density of states shows that the TM-3d states play a key role in generating spin-polarization and magnetic moment in these compounds. Furthermore, we establish that the p-d hybridization reduces the local magnetic moment of Co and enhances that of V from their free space charge value of 3 μB and creates small local magnetic moments on nonmagnetic Cd and S sites. The exchange constant N0α and N0β have been calculated to validate the effects resulting from exchange splitting process.
NASA Astrophysics Data System (ADS)
Liu, Qi-Jun; Liu, Zheng-Tang; Che, Xing-Sen; Feng, Li-Ping; Tian, Hao
2011-12-01
Structural parameters, elastic, electronic, bonding and optical properties of zinc-blende and rocksalt GeC have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The ground-state properties obtained by minimizing the total energy are in favorable agreement with the previous work. Two phases of GeC are found to be elastically stable and we have obtained the bulk, shear and Young's modulus, Poisson's coefficient and Lamé's constants for zinc-blende and rocksalt GeC. We estimated the Debye temperature of zinc-blende and rocksalt GeC from the acoustic velocity. Electronic and chemical bonding properties have been studied. Moreover, the complex dielectric function, refractive index, extinction coefficient, optical reflectivity, absorption coefficient, energy-loss spectrum and the complex conductivity function are calculated.
Usman, Muhammad; Tan, Yui-Hong Matthias; Ryu, Hoon; Ahmed, Shaikh S; Krenner, Hubert J; Boykin, Timothy B; Klimeck, Gerhard
2011-08-05
Atomistic electronic structure calculations are performed to study the coherent inter-dot couplings of the electronic states in a single InGaAs quantum dot molecule. The experimentally observed excitonic spectrum by Krenner et al (2005) Phys. Rev. Lett. 94 057402 is quantitatively reproduced, and the correct energy states are identified based on a previously validated atomistic tight binding model. The extended devices are represented explicitly in space with 15-million-atom structures. An excited state spectroscopy technique is applied where the externally applied electric field is swept to probe the ladder of the electronic energy levels (electron or hole) of one quantum dot through anti-crossings with the energy levels of the other quantum dot in a two-quantum-dot molecule. This technique can be used to estimate the spatial electron-hole spacing inside the quantum dot molecule as well as to reverse engineer quantum dot geometry parameters such as the quantum dot separation. Crystal-deformation-induced piezoelectric effects have been discussed in the literature as minor perturbations lifting degeneracies of the electron excited (P and D) states, thus affecting polarization alignment of wavefunction lobes for III-V heterostructures such as single InAs/GaAs quantum dots. In contrast, this work demonstrates the crucial importance of piezoelectricity to resolve the symmetries and energies of the excited states through matching the experimentally measured spectrum in an InGaAs quantum dot molecule under the influence of an electric field. Both linear and quadratic piezoelectric effects are studied for the first time for a quantum dot molecule and demonstrated to be indeed important. The net piezoelectric contribution is found to be critical in determining the correct energy spectrum, which is in contrast to recent studies reporting vanishing net piezoelectric contributions.
NASA Astrophysics Data System (ADS)
Yang, Kun; He, Yanqing; Cheng, Yi; Che, Li; Yao, Li
2017-03-01
First-principles density functional theory (DFT) calculations have been used to investigate the structural and electronic properties of the cubic KCaF3 and NaCaF3 (001) surfaces with MF (M = K or Na) and CaF2 terminations. For both KCaF3 and NaCaF3 (001) surfaces, the MF termination has stronger surface rumpling than the CaF2 termination. All the computed band gaps for the KCaF3 and NaCaF3 (001) surfaces are smaller than those of the bulks. Furthermore, separated bands that originate from surface layer F p states are introduced at the top of the valance band of MF-terminated surfaces, indicating the emergence of the surface states. The calculated surface energies show that the MF-terminated surface is energetically more favorable than the CaF2-terminated surface.
Zapata-Rivera, Jhon; Caballol, Rosa; Calzado, Carmen J
2011-04-30
A computational strategy to analyze Cu-O(2) adducts based on the use of difference-dedicated configuration interaction (DDCI) calculations is presented. The electronic structure, vertical gaps and nature of the metal-O(2) interaction, and the extension of the charge transfer between both fragments have been investigated. Relative stabilities between isomers are determined from triplet states CCSD(T) calculations. The key point of the here proposed strategy rests on the use of a rationally designed active space, containing only those orbitals, which optimize the interaction pathways between LCu and O(2) fragments. The procedure has been tested on a broad set of model and synthetic biomimetic systems, the results compared with previous theoretical evaluations and/or available experimental data. Our study indicates that this strategy can be considered as an alternative approach to multireference second-order perturbation theory methods to deal with this type of systems with remarkable biradical nature.
Thanthiriwatte, K Sahan; Wang, Xuefeng; Andrews, Lester; Dixon, David A; Metzger, Jens; Vent-Schmidt, Thomas; Riedel, Sebastian
2014-03-20
Laser-ablated Th atoms react with F2 in condensing noble gases to give ThF4 as the major product. Weaker higher frequency infrared absorptions at 567.2, 564.8 (576.1, 573.8) cm(-1), 575.1 (582.7) cm(-1) and 531.0, (537.4) cm(-1) in solid argon (neon) are assigned to the ThF, ThF2 and ThF3 molecules based on annealing and photolysis behavior and agreement with CCSD(T)/aug-cc-pVTZ vibrational frequency calculations. Bands at 528.4 cm(-1) and 460 cm(-1) with higher fluorine concentrations are assigned to the penta-coordinated species (ThF3)(F2) and ThF5(-). These bands shift to 544.2 and 464 cm(-1) in solid neon. The ThF5 molecule has the (ThF3)(F2) Cs structure and is essentially the unique [ThF3(+)][F2(-)] ion pair based on charge and spin density calculations. Electron capture by (ThF3)(F2) forms the trigonal bipyramidal ThF5(-) anion in a highly exothermic process. Extensive structure and frequency calculations were also done for thorium oxyfluorides and Th2F4,6,8 dimer species. The calculations provide the ionization potentials, electron affinities, fluoride affinities, Th-F bond dissociation energies, and the energies to bind F2 and F2(-) to a cluster as well as dimerization energies.
NASA Astrophysics Data System (ADS)
Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Parasyuk, O. V.; Fedorchuk, A. O.; Khyzhun, O. Y.
2016-10-01
A high-quality single crystal of cesium mercury tetrabromide, Cs2HgCl4, was synthesized by using the vertical Bridgman-Stockbarger method and its electronic structure was studied from both experimental and theoretical viewpoints. In particular, X-ray photoelectron spectra were measured for both pristine and Ar+ ion-bombarded Cs2HgCl4 single crystal surfaces. The present XPS measurements indicate that the Cs2HgCl4 single crystal surface is sensitive with respect to Ar+ ion-bombardment: such a treatment changes substantially its elemental stoichiometry. With the aim of exploring total and partial densities of states within the valence band and conduction band regions of the Cs2HgCl4 compound, band-structure calculations based on density functional theory (DFT) using the augmented plane wave + local orbitals (APW + lo) method as incorporated within the WIEN2k package are performed. The calculations indicate that the Cl 3p states are the principal contributors in the upper portion of the valence band, while the Hg 5d and Cs 5p states dominate in its lower portion. In addition, the calculations allow for concluding that the unoccupied Cl p and Hg s states are the main contributors to the bottom of the conduction band. Furthermore, main optical characteristics of Cs2HgCl4, namely dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity, are elucidated based on the DFT calculations.
Thompson, Damien; Coleman, Simon; Diamond, Dermot; Byrne, Robert
2011-04-07
Liquid ion association in ionic liquids (ILs) has been examined using a comprehensive series of electronic structure calculations that measure the relative extents of ion association and probe stabilisation for the photochromic dye nitrobenzospiropyran (BSP) in a range of ILs featuring both long-tailed phosphonium cations and short-tailed imidazolium cations, paired with both chloride and NTf(2) anions. New physicochemical experiments measured the photochromic properties of BSP in the phosphonium-based room temperature ILs. Taken together, the computed complexation energies and measured spectroscopic properties support recent Walden plots of unusual conductivity-viscosity behaviour obtained for the same ILs and reveal some new features in the atom-scale structure and energetics of local, ion-ion and ion-molecule interactions. Calculations show inter-ion interactions strengthened by between 0.4 and 0.7 eV as stronger constituent ions are used, which contributes to the longer range rigidity of the Cl-based IL structure as reflected in the doubled |zwitterion → closed| probe relaxation time measured for Cl(-)vs. NTf(2)(-) in phosphonium-based ILs. Calculations further reveal a similar, approximately 0.6-0.7 eV maximum "residual" IL headgroup-mediated probe stabilisation potentially available for the anion-probe-cation complexes via the stabilising interaction that remains following the "quenching" interaction between the IL anion and cation. This potential stabilisation, however, is offset by both longer-range charge networks, beyond the scope of the current purely quantum mechanical simulations, and also energetic penalties for disruption of the highly-interdigitated alkyl tail networks in the phosphonium-based ILs which may be estimated from known diffusion data. Overall the electronic calculations of local, individual ion-ion and ion-molecule interactions serve to clarify some of the measured physicochemical properties and provide new data for the development of
Ikuhara, Yuichi
2011-01-01
Grain boundaries and interfaces of crystals have peculiar electronic structures, caused by the disorder in periodicity, providing the functional properties, which cannot be observed in a perfect crystal. In the vicinity of the grain boundaries and interfaces, dopants or impurities are often segregated, and they play a crucial role in deciding the properties of a material. Spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM), allowing the formation of sub-angstrom-sized electron probes, can directly observe grain boundary-segregated dopants. On the other hand, ceramic materials are composed of light elements, and these light elements also play an important role in the properties of ceramic materials. Recently, annular bright-field (ABF)-STEM imaging has been proposed, which is now known to be a very powerful technique in producing images showing both light- and heavy-element columns simultaneously. In this review, the atomic structure determination of ceramic grain boundaries and direct observation of grain boundary-segregated dopants and light elements in ceramics were shown to combine with the theoretical calculations. Examples are demonstrated for well-defined grain boundaries in rare earth-doped Al(2)O(3) and ZnO ceramics, CeO(2) and SrTiO(3) grain boundary, lithium battery materials and metal hydride, which were characterized by Cs-corrected high-angle annular dark-field and ABF-STEM. It is concluded that the combination of STEM characterization and first-principles calculation is very useful in interpreting the structural information and in understanding the origin of the properties in various ceramics.
Rustad, James R.; Dixon, David A.; Rosso, Kevin M.; Felmy, Andrew R.
1999-04-07
Metal ion hydrolysis is fundamental in aqueous chemistry because of the influence of coordinating hydroxide ions on reaction rates; examples include enhanced labilization of coordinating water molecules in hydrolyzed complexes1 and stabilization of oxidized products in electron-transfer reactions involving hydrolyzed reductants.2 Moreover, the role of metal hydrolysis reactions in defining a baseline for establishing trends in metal ligand binding has motivated efforts toward comprehensive integration of Mz+ xOHy stability constants.3-5
Rustad, J.R.; Dixon, D.A.; Rosso, K.M.; Felmy, A.R.
1999-04-07
Metal ion hydrolysis is fundamental in aqueous chemistry because of the influence of coordinating hydroxide ions on reaction rates; examples include enhanced labilization of coordinating water molecules in hydrolyzed complexes and stabilization of oxidized products in electron-transfer reactions involving hydrolyzed reductants. Moreover, the role of metal hydrolysis reactions in defining a baseline for establishing trends in metal-ligand binding has motivated efforts toward comprehensive integration of M{sup z+}{sub x}OH{sub y} stability constants.
Saravanan, Chandra; Shao, Yihan; Baer, Roi; Ross, Philip N; Head-Gordon, Martin
2003-04-15
A sparse matrix multiplication scheme with multiatom blocks is reported, a tool that can be very useful for developing linear-scaling methods with atom-centered basis functions. Compared to conventional element-by-element sparse matrix multiplication schemes, efficiency is gained by the use of the highly optimized basic linear algebra subroutines (BLAS). However, some sparsity is lost in the multiatom blocking scheme because these matrix blocks will in general contain negligible elements. As a result, an optimal block size that minimizes the CPU time by balancing these two effects is recovered. In calculations on linear alkanes, polyglycines, estane polymers, and water clusters the optimal block size is found to be between 40 and 100 basis functions, where about 55-75% of the machine peak performance was achieved on an IBM RS6000 workstation. In these calculations, the blocked sparse matrix multiplications can be 10 times faster than a standard element-by-element sparse matrix package.
NASA Astrophysics Data System (ADS)
Mazin, I. I.
2007-03-01
LiCrO2 is a two-dimensional triangular antiferromagnet, isostructural with the common battery material LiCoO2 and a well-known Jahn-Teller antiferromagnet NaNiO2 . As opposed to the latter, LiCrO2 exibits antiferromagnetic exchange in the Cr planes, which has been ascribed to direct Cr-Cr d-d overlap. Using local density approximation (LDA) and LDA+U first-principles calculations, I confirm this conjecture and show that (a) direct d-d overlap is indeed enhanced compared to isostructural Ni and Co compounds, (b) the p-d charge-transfer gap is also enhanced, thus suppressing the ferromagnetic superexchange, (c) the calculated magnetic Hamiltonian maps well onto the nearest-neighbor Heisenberg exchange model, and (d) the interplanar inteaction is antiferromagnetic.
NASA Astrophysics Data System (ADS)
Zhang, Weibin; Cho, Hoon Young; Zhang, Zhijun; Yang, Woochul; Kim, Ki Kang; Zhang, Fuchun
2016-11-01
The electronic structure and the optical properties of Mn-decorated graphitic carbon nitride (g-C3N4) were investigated using the density functional method. The large absorption energy of the Mn atoms on the g-C3N4 surface was found to suppress the clustering of the Mn atoms, which led to a conservation of the photocatalytic activity. The electronic structures of the Mn-decorated g-C3N4 showed that impurity energy levels emerged in the forbidden band of g-C3N4 and that the band edge of g-C3N4 shifted upward to 0.40 eV. In addition, the calculated optical constants showed that the novel photon absorption in the range of visible light originated from electronic transitions from the N 2 p states in the upper valence band to impurity Mn 3 d states. Moreover, the photon absorption reached a maximum when all sites of triangular N holes were decorated with Mn atoms. Our results provide evidence that the Mn-decorated C3N4 system could be a highly-efficient photocatalyst for solar light due to the extension of the range of photon absorption to include almost all visible light.
NASA Astrophysics Data System (ADS)
Boumaza, A.; Ghemid, S.; Chouahda, Z.; Meradji, H.; El Haj Hassan, F.
2012-09-01
The structural, electronic, elastic, thermal and thermodynamic properties of Zn1-xBexTe semiconductor alloys have been investigated using the full-potential linearized augmented plane wave method within density functional theory. We use both the Wu-Cohen and the Engel-Vosko generalized gradient approximations of the exchange-correlation energy that are based on the optimization of the total energy and the corresponding potential, respectively. The ground state properties such as lattice constants, bulk modulus and elastic constants are in good agreement with numerous experimental and theoretical data. The calculated band structures show that the band gap undergoes a direct to indirect transition at a given concentration. A regular-solution model is used to investigate the thermodynamic stability of the alloy that mainly indicates a phase miscibility gap. In addition, the quasi-harmonic Debye model is applied to determine the thermal properties of the alloy.
Bilić, Ante; Reimers, Jeffrey R; Hush, Noel S
2005-03-01
The adsorption of phenylthiol on the Au(111) surface is modeled using Perdew and Wang density-functional calculations. Both direct molecular physisorption and dissociative chemisorption via S-H bond cleavage are considered as well as dimerization to form disulfides. For the major observed product, the chemisorbed thiol, an extensive potential-energy surface is produced as a function of both the azimuthal orientation of the adsorbate and the linear translation of the adsorbate through the key fcc, hcp, bridge, and top binding sites. Key structures are characterized, the lowest-energy one being a broad minimum of tilted orientation ranging from the bridge structure halfway towards the fcc one. The vertically oriented threefold binding sites, often assumed to dominate molecular electronics measurements, are identified as transition states at low coverage but become favored in dense monolayers. A similar surface is also produced for chemisorption of phenylthiol on Ag(111); this displays significant qualitative differences, consistent with the qualitatively different observed structures for thiol chemisorption on Ag and Au. Full contours of the minimum potential energy as a function of sulfur translation over the crystal face are described, from which the barrier to diffusion is deduced to be 5.8 kcal mol(-1), indicating that the potential-energy surface has low corrugation. The calculated bond lengths, adsorbate charge and spin density, and the density of electronic states all indicate that, at all sulfur locations, the adsorbate can be regarded as a thiyl species that forms a net single covalent bond to the surface of strength 31 kcal mol(-1). No detectable thiolate character is predicted, however, contrary to experimental results for alkyl thiols that indicate up to 20%-30% thiolate involvement. This effect is attributed to the asymptotic-potential error of all modern density functionals that becomes manifest through a 3-4 eV error in the lineup of the adsorbate and
Huang, Meng; Kline, Neal; Miller, Terry A; Dawes, Richard
2017-01-12
Near-infrared cavity ringdown spectra were recorded following the photolysis of dihalomethanes in O2/N2 mixtures. In particular, photolysis of CH2I2 under conditions previously reported to produce the simplest Criegee intermediate, CH2O2, gave a complex, structured spectrum between 6800 and 9000 cm(-1), where the lowest triplet-singlet transition (ã-X̃) of CH2O2 might be expected. To help identify the carrier of the spectrum, extensive electronic structure calculations were performed on the ã and X̃ states of CH2O2 and the lowest two doublet states of the iodomethylperoxy radical, CH2IO2, which also could be produced by the chemistry and whose Ã-X̃ transition likely lies in this spectral region. The conclusion of these calculations is that the ã-X̃ transition of CH2O2 clearly falls outside the observed spectral range and would be extremely weak both because it is spin-forbidden and because of a large geometric change between the ã and X̃ states. Moreover, only a shallow well (with a barrier to dissociation of less than 1900 cm(-1)) is predicted on the ã state, which likely precludes the existence of long-lived states. Calculations for the Ã-X̃ transition of CH2IO2 are generally consistent with the observed spectrum in terms of both the electronic origin and vibrational frequencies in the Ã state. To confirm the carrier assignment to CH2IO2, calculations beyond the Franck-Condon approximation were carried out to explain the hot band structure of the large-amplitude, low-frequency O-O-C-I torsion mode, ν12. Photolysis of other dihalomethanes produced similar spectra which were analyzed and assigned to CH2ClO2 and CH2BrO2. Experimental values for the electronic energies and frequencies for several Ã state vibrations and the ν12 vibration of the X̃ state of each are reported. In addition, the observed spectra were used to follow the self-reaction of the CH2IO2 species and its reaction with SO2. The rates of these reactions are dramatically faster than
NASA Astrophysics Data System (ADS)
Guan, Jiwen; Hu, Yongjun; Zou, Hao; Cao, Lanlan; Liu, Fuyi; Shan, Xiaobin; Sheng, Liusi
2012-09-01
In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH3COOH)n.H+, the feature related to the fragment ions (CH3COOH)H+.COO (105 amu) via β-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH3COOH).H+ and (CH3COOH)H+.COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH3COOH)H+.COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH3COOH)+ becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH3COOH).CH3CO+. Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.
NASA Technical Reports Server (NTRS)
Smith, Grant D.; Jaffe, R. L.; Yoon, D. Y.; Arnold, James O. (Technical Monitor)
1994-01-01
Conformational energy contours of perfluoroalkanes, determined from ab initio calculations, confirm the well-known spitting of trans states into two minima at plus or minus 17 degrees but also show that the gauche states split as well, with minima at plus or minus 124 degrees and plus or minus 84 in order to relieve steric crowding. The directions of such split distortions from the perfectly staggered states are strongly coupled for adjacent pairs of bonds in a manner identical to the intradyad pair for poly (isobutylene) chains. These conformational characteristics are fully represented by a six-state rotational isomeric state (RIS) model for PTFE comprised of t(+), t(-), g(sup +)+, g(sup +)-, g(sup -) + and g(sup -)-states, located at the split energy minima. The resultant 6 x 6 statistical weight matrix is described by first-order interaction parameters for the g+(+) (ca. 0.6 kcal/mol) and g+- (ca. 2.0 kcal/mol) states, and second order parameters for the g(sup +)+g(sup +)+ (ca 0.6 kcal/mol) and g(sup +)+g(sup -)+ (ca. 1.0 kcal/mol) states. This six-state RIS model, without adjustment of the geometric or energy parameters as determined from the ab initio calculations, predicts the unperturbed chain dimensions and the fraction of gauche bonds as a function of temperature for PTFE in good agreement with available experimental values.
NASA Astrophysics Data System (ADS)
Rustad, James R.; Dixon, David A.; Felmy, Andrew R.
2000-05-01
Density functional calculations are performed on M 3(OH) 7(H 2O) 62+ and M 3O(OH) 6(H 2O) 6+ clusters for MAl, Cr(III), and Fe(III), allowing determination of the relative acidities of the μ 3-hydroxo and aquo functional groups. Contrary to previous predictions and rationalizations, Fe 3OH and Al 3OH groups have nearly the same intrinsic acidity, while Cr 3OH groups are significantly more acidic. The gas-phase acidity of the Fe 3OH site is in good agreement with the value predicted by the molecular mechanics model previously used to estimate the relative acidities of surface sites on iron oxides. [ J. R. Rustad et al. (1996)Geochim. Cosmochim. Acta 60, 1563]. Acidities of aquo functional groups were also computed for Al and Cr. The AlOH 2 site is more acidic than the Al 3OH site, whereas the Cr 3OH site is more acidic than the CrOH 2 site. These findings predict that the surface charging behavior of chromium oxides/oxyhydroxides should be distinguishable from their Fe, Al counterparts. The calculations also provide insight into why the lepidocrocite/boehmite polymorph is not observed for CrOOH.
NASA Astrophysics Data System (ADS)
Tsuzuki, Seiji; Honda, Kazumasa; Uchimaru, Tadafumi; Mikami, Masuhiro
2005-04-01
The intermolecular interaction energy of the toluene dimer has been calculated with the ARS-F model (a model chemistry for the evaluation of intermolecular interaction energy between ARomatic Systems using Feller's method), which was formerly called as the AIMI model III. The CCSD(T) (coupled cluster calculations with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the second-order Møller-Plesset perturbation interaction energy at the basis set limit obtained by Feller's method and the CCSD(T) correction term obtained using a medium-size basis set. The cross (C2) dimer has the largest (most negative) interaction energy (-4.08kcal/mol). The antiparallel (C2h) and parallel (CS) dimers (-3.77 and -3.41kcal/mol, respectively) are slightly less stable. The dispersion interaction is found to be the major source of attraction in the toluene dimer. The dispersion interaction mainly determines the relative stability of the stacked three dimers. The electrostatic interaction of the stacked three dimers is repulsive. Although the T-shaped and slipped-parallel benzene dimers are nearly isoenergetic, the stacked toluene dimers are substantially more stable than the T-shaped toluene dimer (-2.62kcal/mol). The large dispersion interaction in the stacked toluene dimers is the cause of their enhanced stability.
Uğur, Şule; İyigör, Ahmet
2014-10-06
The electronic, elastic and dynamical properties of the quaternary alloy FeNiMnAl have been investigated using a pseudopotential plane wave method within the generalized gradient approximation (GGA). We determined the lattice parameters and the bulk modulus B. In addition, the elastic properties such as elastic constans (C{sub 11}, C{sub 12} and C{sub 44}), the shear modulus G, the young modulus E, the poisson's ratio σ and the B/G ratio are also given. The FeNiMnAl Heusler alloy exhibit a ferromagnetic half-metallic behavior with the total magnetic moment of 4.02 μ{sub B}. The phonon dispersion of FeNiMnAl has been performed using the density functional theory and the direct method with 2×2×2 supercell.
Guan, Jiwen; Hu, Yongjun; Zou, Hao; Cao, Lanlan; Liu, Fuyi; Shan, Xiaobin; Sheng, Liusi
2012-09-28
In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH(3)COOH)(n)·H(+), the feature related to the fragment ions (CH(3)COOH)H(+)·COO (105 amu) via β-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH(3)COOH)·H(+) and (CH(3)COOH)H(+)·COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH(3)COOH)H(+)·COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH(3)COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH(3)COOH)·CH(3)CO(+). Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.
Guan Jiwen; Hu Yongjun; Zou Hao; Cao Lanlan; Liu Fuyi; Shan Xiaobin; Sheng Liusi
2012-09-28
In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH{sub 3}COOH){sub n}{center_dot}H{sup +}, the feature related to the fragment ions (CH{sub 3}COOH)H{sup +}{center_dot}COO (105 amu) via {beta}-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH{sub 3}COOH){center_dot}H{sup +} and (CH{sub 3}COOH)H{sup +}{center_dot}COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH{sub 3}COOH)H{sup +}{center_dot}COO. After surmounting the methyl hydrogen-transfer barrier 10.84 {+-} 0.05 eV, the opening of dissociative channel to produce ions (CH{sub 3}COOH){sup +} becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH{sub 3}COOH){center_dot}CH{sub 3}CO{sup +}. Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.
NASA Astrophysics Data System (ADS)
Dai, Xian-Qi; Zhao, Jian-Hua; Sun, Yong-Can; Wei, Shu-Yi; Wei, Guo-Hong
2010-09-01
The atomic and electronic structures of Tl and In on Si(111) surfaces are investigated using the first-principles total energy calculations. Total energy optimizations show that the energetically favored structure is 1/3 ML Tl adsorbed at the T4 sites on Si(111) surfaces. The adsorption energy difference of one Tl adatom between (√3 × √3) and (1 × 1) is less than that of each In adatom. The DOS indicates that Tl 6p and Si 3p electrons play a very important role in the formation of the surface states. It is concluded that the bonding of Tl adatoms on Si(111) surfaces is mainly polar covalent, which is weaker than that of In on Si(111). So Tl atom is more easy to be migrated than In atom in the same external electric field and the structures of Tl on Si(111) is prone to switch between (√3 × √3) and (1 × 1).
NASA Astrophysics Data System (ADS)
Hamad, B. A.
2009-07-01
First principle calculations using density functional theory (DFT) and full-potential linearized augmented plane waves (FP-LAPW) method are performed to investigate the structural and electronic properties of rutile phase titanium, vanadium, ruthenium, iridium and tin dioxides, TiO2, VO2, RuO2, IrO2, and SnO2, respectively. The exchange correlation function is described using the local density approximation (LDA) and the generalized gradient approximation (GGA). The structural parameters of the dioxides are found to be in a fair agreement with experimental values and previous calculations. TiO2 exhibits the maximum cohesive energy and RuO2 exhibits the minimum, which is opposite to the trend of pure bulk metals. Titanium dioxide in the left of the periodic table exhibits an insulating behavior with an underestimated bandgap of 2 eV. As the d-band filling increases in VO2, the energy bands shift by 3 eV from those of TiO2 to cross the Fermi level and exhibit a metallic behavior with a pseudo gap to the right of the Fermi level. The energy bands coalescence in RuO2 and IrO2 exhibiting metallic behaviors. However, for a complete filled d-band SnO2, the insulating behavior is retrieved. The distortion of the octahedrons in the rutile structure lifts the degeneracy of the eg orbitals causing further splittings.
Interaction between LiH molecule and Li atom from state-of-the-art electronic structure calculations
NASA Astrophysics Data System (ADS)
Skomorowski, Wojciech; Pawłowski, Filip; Korona, Tatiana; Moszynski, Robert; Żuchowski, Piotr S.; Hutson, Jeremy M.
2011-03-01
State-of-the-art ab initio techniques have been applied to compute the potential energy surface for the lithium atom interacting with the lithium hydride molecule in the Born-Oppenheimer approximation. The interaction potential was obtained using a combination of the explicitly correlated unrestricted coupled-cluster method with single, double, and noniterative triple excitations [UCCSD(T)-F12] for the core-core and core-valence correlation and full configuration interaction for the valence-valence correlation. The potential energy surface has a global minimum 8743 cm-1 deep if the Li-H bond length is held fixed at the monomer equilibrium distance or 8825 cm-1 deep if it is allowed to vary. In order to evaluate the performance of the conventional CCSD(T) approach, calculations were carried out using correlation-consistent polarized valence X-tuple-zeta basis sets, with X ranging from 2 to 5, and a very large set of bond functions. Using simple two-point extrapolations based on the single-power laws X-2 and X-3 for the orbital basis sets, we were able to reproduce the CCSD(T)-F12 results for the characteristic points of the potential with an error of 0.49% at worst. The contribution beyond the CCSD(T)-F12 model, obtained from full configuration interaction calculations for the valence-valence correlation, was shown to be very small, and the error bars on the potential were estimated. At linear LiH-Li geometries, the ground-state potential shows an avoided crossing with an ion-pair potential. The energy difference between the ground-state and excited-state potentials at the avoided crossing is only 94 cm-1. Using both adiabatic and diabatic pictures, we analyze the interaction between the two potential energy surfaces and its possible impact on the collisional dynamics. When the Li-H bond is allowed to vary, a seam of conical intersections appears at C2v geometries. At the linear LiH-Li geometry, the conical intersection is at a Li-H distance which is only slightly
Skomorowski, Wojciech; Pawłowski, Filip; Korona, Tatiana; Moszynski, Robert; Żuchowski, Piotr S; Hutson, Jeremy M
2011-03-21
State-of-the-art ab initio techniques have been applied to compute the potential energy surface for the lithium atom interacting with the lithium hydride molecule in the Born-Oppenheimer approximation. The interaction potential was obtained using a combination of the explicitly correlated unrestricted coupled-cluster method with single, double, and noniterative triple excitations [UCCSD(T)-F12] for the core-core and core-valence correlation and full configuration interaction for the valence-valence correlation. The potential energy surface has a global minimum 8743 cm(-1) deep if the Li-H bond length is held fixed at the monomer equilibrium distance or 8825 cm(-1) deep if it is allowed to vary. In order to evaluate the performance of the conventional CCSD(T) approach, calculations were carried out using correlation-consistent polarized valence X-tuple-zeta basis sets, with X ranging from 2 to 5, and a very large set of bond functions. Using simple two-point extrapolations based on the single-power laws X(-2) and X(-3) for the orbital basis sets, we were able to reproduce the CCSD(T)-F12 results for the characteristic points of the potential with an error of 0.49% at worst. The contribution beyond the CCSD(T)-F12 model, obtained from full configuration interaction calculations for the valence-valence correlation, was shown to be very small, and the error bars on the potential were estimated. At linear LiH-Li geometries, the ground-state potential shows an avoided crossing with an ion-pair potential. The energy difference between the ground-state and excited-state potentials at the avoided crossing is only 94 cm(-1). Using both adiabatic and diabatic pictures, we analyze the interaction between the two potential energy surfaces and its possible impact on the collisional dynamics. When the Li-H bond is allowed to vary, a seam of conical intersections appears at C(2v) geometries. At the linear LiH-Li geometry, the conical intersection is at a Li-H distance which is
Kafader, Jared O.; Ray, Manisha; Jarrold, Caroline Chick
2015-07-21
The anion photoelectron (PE) spectra of EuH{sup −} and the PE spectrum of overlapping EuOH{sup −} and EuO{sup −} anions are presented and analyzed with supporting results from density functional theory calculations on the various anions and neutrals. Results point to ionically bound, high-spin species. EuH and EuOH anions and neutrals exhibit analogous electronic structures: Transitions from {sup 8}Σ{sup −} anion ground states arising from the 4f{sup 7}σ{sub 6s}{sup 2} superconfiguration to the close-lying neutral {sup 9}Σ{sup −} and {sup 7}Σ{sup −} states arising from the 4f{sup 7}σ{sub 6s} superconfiguration are observed spaced by an energy interval similar to the free Eu{sup +} [4f{sup 7}6s] {sup 9}S - {sup 7}S splitting. The electron affinities (EAs) of EuH and EuOH are determined to be 0.771 ± 0.009 eV and 0.700 ± 0.011 eV, respectively. Analysis of spectroscopic features attributed to EuO{sup −} photodetachment is complicated by the likely presence of two energetically competitive electronic states of EuO{sup −} populating the ion beam. However, based on the calculated relative energies of the close-lying anion states arising from the 4f{sup 7}σ{sub 6s} and 4f{sup 6}σ{sub 6s}{sup 2} configurations and the relative energies of the one-electron accessible 4f{sup 7} and 4f{sup 6}σ{sub 6s} neutral states based on ligand-field theory [M. Dulick, E. Murad, and R. F. Barrow, J. Chem. Phys. 85, 385 (1986)], the remaining features are consistent with the 4f{sup 6}σ{sub 6s}{sup 2} {sup 7}Σ{sup −} and 4f{sup 7}σ{sub 6s}{sup 7}Σ{sup −} anion states lying very close in energy (the former was calculated to be 0.15 eV lower in energy than the latter), though the true anion ground state and neutral EA could not be established unambiguously. Calculations on the various EuO anion and neutral states suggest 4f-orbital overlap with 2p orbitals in species with 4f{sup 6} occupancy.
NASA Astrophysics Data System (ADS)
Lahiji, Mohammadreza Askaripour; Ziabari, Ali Abdolahzadeh
2016-11-01
The structural, elastic, electronic, and optical properties of undoped and Cu-doped ZnS nanostructured layers have been studied in the zincblende (ZB) phase, by first-principle approach. Density functional theory (DFT) has been employed to calculate the fundamental properties of the layers using full-potential linearized augmented plane-wave (FPLAPW) method. Mechanical analysis revealed that the bulk modulus increases with the increase of Cu content. Cu doping was found to reduce the band gap value of the material. In addition, DOS effective mass of the electrons and heavy holes was evaluated. Adding Cu caused the decrement/increment of transmission/reflectance of nanolayers in the UV-vis region. The substitution by Cu increased the intensity of the peaks, and a slight red shift was observed in the absorption peak. Moreover, the static dielectric constant, and static refractive index increased with Cu content. The optical conductivity also followed a similar trend to that of the dielectric constants. Energy loss function of the modeled compounds was also evaluated. All calculated parameters were compared with the available experimental and other theoretical results.
Gonis, A.; Turchi, P.E.A.; Schulthess, T.C.; Ek, J. van
1997-08-01
The works referred to above indicate the usefulness of viewing an N-particle system from a higher-dimensional perspective. In doing so, one should attempt to strike a balance between conceptual clarity and computational efficiency, which mitigates against considering calculations in 3n-dimensional space except for rather small values of n. It appears that such a procedure may be profitably employed if a system of N particles were to be considered as consisting of a collection of units or sets, (I{sub k}), each containing n{sub k} particles so that {Sigma}{sub k} n{sub k} = N. The resulting problem associated with these sets of particles that interact with one another is obviously formally identical to the original one. However, it possesses the formal advantage of allowing, in principle, the systematic approach to an exact solution by treating the entire system as a single unit. The operative words here are in principle, as practical applications do not seem to be possible but for the smallest number of particles in a unit, say n = 2 or n = 3. However, in such an implementation, the interparticle correlation is treated directly and explicitly within a unit, resulting in a more accurate treatment of the system the larger the number of particle in a unit.
Electron Correlation in 4-Component Relativistic Calculations
NASA Technical Reports Server (NTRS)
Visscher, Luuk; Arnold, James O. (Technical Monitor)
1994-01-01
The full 4-component Dirac-Coulomb equation can nowadays be used in molecular calculations, The first step in solving this relativistic many-electron equation usually consists of solving the closed or open-shell Diarc-Fock equations. Like in non-relativistic calculations the outcome does not account for the effects of electron correlation. This can in principle be remedied by developing relativistic variants of electron correlation methods like Configuration Interaction or Coupled Cluster. In this talk the differences and similarities of such relativistic approaches as compared to non-relativistic methods will be reviewed. Results of Configuration Interaction calculations on the PtH molecule and on the MeF(sub 6, sup 2-) (Me= Co, Rh, Ir) complexes will be presented to give an impression of the kind of results that currently can be obtained.
Zhang, Junying; Dang, Wenqiang; Yan, Xingchen; Li, Min; Gao, Hong; Ao, Zhimin
2014-11-14
β-Bi2O3 is an efficient visible-light photocatalyst, however, it is unable to split water to produce hydrogen because of the positive conduction band minimum (CBM). In this paper, using hybrid density functional theory (DFT) calculations, we demonstrated that by doping indium in β-Bi2O3, the CBM shifts upward because of the orbital hybridization of Bi, In and O. In-doped β-Bi2O3 photocatalysts synthesized using a precipitation method can photocatalytically split water to produce hydrogen in experiments. In-doping also causes the morphological change of β-Bi2O3 from the hierarchical bulk assembled by nano-sheets to a spongy-like brick. Furthermore, In-doping induces the formation of electric dipoles along the tunnel in the crystal and decreases the effective mass of the electrons, favouring the separation of electron-hole pairs and electron mobility. Therefore, In-doped β-Bi2O3 has much better performance than that of the pristine β-Bi2O3 for photocatalytically decomposing methyl orange (MO) solution. This idea of simply incorporating an isovalent single element into photocatalysts to elevate the CBM and tune the local crystal structure is anticipated to be very useful for designing efficient photocatalysts.
NASA Astrophysics Data System (ADS)
Jeeva Jasmine, N.; Arunagiri, C.; Subashini, A.; Stanley, N.; Thomas Muthiah, P.
2017-02-01
Theoretical Spectrograms, namely, FT-Raman (3500-50 cm-1) and FT-Infrared (4000-400 cm-1) spectra have been studied for 4-acetamido benzaldehyde (4ABA) and are assigned to different normal modes of the molecule. Vibrational spectral analysis was compared with the experimental and theoretical, FT-IR and FT-Raman spectra. The effect of polarity on the Harmonic vibrational frequencies, intensities, optimized geometrical parameters and several thermodynamic parameters in the ground state have been computed by the B3LYP method using 6-311 + G(d,p) basis set. The results of the optimized molecular structure is presented and compared with the XRD values. The global chemical reactivity relate to some parameters, such as HOMO, LUMO, gap energy (ΔE) and other parameters, including electronegativity (χ) and global hardness (η). The values of the reactivity descriptors indicated that the interaction between 4ABA molecules reduced its reactivity in comparison with the exhibited in gas phase. In addition, the local reactivity has been analyzed through the Fukui function and condensed softness indices.
Approximate Bruechner orbitals in electron propagator calculations
Ortiz, J.V.
1999-12-01
Orbitals and ground-state correlation amplitudes from the so-called Brueckner doubles approximation of coupled-cluster theory provide a useful reference state for electron propagator calculations. An operator manifold with hold, particle, two-hole-one-particle and two-particle-one-hole components is chosen. The resulting approximation, third-order algebraic diagrammatic construction [2ph-TDA, ADC (3)] and 3+ methods. The enhanced versatility of this approximation is demonstrated through calculations on valence ionization energies, core ionization energies, electron detachment energies of anions, and on a molecule with partial biradical character, ozone.
Local orbitals in electron scattering calculations*
NASA Astrophysics Data System (ADS)
Winstead, Carl L.; McKoy, Vincent
2016-05-01
We examine the use of local orbitals to improve the scaling of calculations that incorporate target polarization in a description of low-energy electron-molecule scattering. After discussing the improved scaling that results, we consider the results of a test calculation that treats scattering from a two-molecule system using both local and delocalized orbitals. Initial results are promising. Contribution to the Topical Issue "Advances in Positron and Electron Scattering", edited by Paulo Limao-Vieira, Gustavo Garcia, E. Krishnakumar, James Sullivan, Hajime Tanuma and Zoran Petrovic.
NASA Astrophysics Data System (ADS)
Varley, J. B.; Lordi, V.; Miglio, A.; Hautier, G.
2014-07-01
B6O is a member of icosahedral boron-rich solids known for their physical hardness and stability under irradiation bombardment, but it has also recently emerged as a promising high mobility p -type transparent conducting oxide. Using a combination of hybrid functional and many-body perturbation theory calculations, we report on the electronic structure and defect properties of this material. Our calculations identify B6O has a direct band gap in excess of 3.0 eV and possesses largely isotropic and low effective masses for both holes and electrons. Of the native defects, we identify no intrinsic origin to the reported p -type conductivity and confirm that p-type doping is not prevented by intrinsic defects such as oxygen vacancies, which we find act exclusively as neutral defects rather than hole-killing donors. We also investigate a number of common impurities and plausible dopants, finding that isolated acceptor candidates tend to yield deep states within the band gap or act instead as donors, and cannot account for p-type conductivity. Our calculations identify the only shallow acceptor candidate to be a complex consisting of interstitial H bonded to C substituting on the O site (CH)O. We therefore attribute the origins of p-type conductivity to these complexes formed during growth or more likely via isolated CO which later binds with H within the crystal. Lastly, we identify Si as a plausible n -type dopant, as it favorably acts as a shallow donor and does not suffer from self-compensation as may the C-related defects. Thus, in addition to the observed p-type conductivity, B6O exhibits promise of n -type dopability if the stoichiometry and both native and extrinsic sources of compensation can be sufficiently controlled.
NASA Astrophysics Data System (ADS)
Krosley, Kevin; Hagen, Kolbjørn; Hedberg, Kenneth
1995-06-01
Gas-phase electron diffraction data at 23°C together with molecular mechanics (MM3) and ab initio (HF/6-31G∗, gaussian 86) calculations have been used to determine the structure and conformations of 1,4-difluorobutane. The object was to ascertain whether effects similar to the gauche effect in 1,2-difluoroethane, which serves to stabilize the gauche form with the fluorine atoms in close proximity, could also operate in 1,4-difluorobutane. It was found both theoretically and experimentally that the proportion of those conformers having close fluorine atoms was small, implying the absence of effects similar to the gauche effect. The conformational composition estimated from the theoretical calculations is in good agreement with the experimental data. The experimental electron diffraction results constrained by assumptions drawn from the theoretical calculations, ED/MM3 [ED/ab initio], for the principal distances ( {r g}/{Å}) and angles ( {∠ α}/{deg}) with estimated 2σ uncertainties are as follows: r(CH) = 1.105(3) [1.106(3)], r(CF) = 1.398(2) [1.398(2)], r(C 1C 2) = 1.513(2) [1.516(2)], r(C 2C 3) = 1.537(2) [1.532(2)], ∠FCC = 110.9(3) [111.1(3)], ∠CCC = 112.9(4) [112.9(4)], and ∠HCH = 100(3) [100(3)].
NASA Astrophysics Data System (ADS)
Pan, Feng-chun; Chen, Zhi-peng; Lin, Xue-ling; Zheng, Fu; Wang, Xu-ming; Chen, Huan-ming
2016-09-01
The electronic structures and magnetic properties of the Cu and N codoped 3C-SiC system have been investigated by the first-principles calculation. The results show that the Cu doped SiC system prefers the anti-ferromagnetic (AFM) state. Compared to the Cu doped system, the ionicities of C-Cu and C-Si in Cu and N codoped SiC are respectively enhanced and weakened. Especially, the Cu and N codoped SiC systems favor the ferromagnetic (FM) coupling. The FM interactions can be explained by virtual hopping. However, higher N concentration will weaken the ferromagnetism. In order to keep the FM interaction, the N concentration should be restricted within 9.3% according to our analysis. Project supported by the Higher School Science Research Outstanding Youth Fund Project of Ningxia, China (Grant No. NGY2015049).
NASA Astrophysics Data System (ADS)
Enyashin, A. N.; Ivanovskii, A. L.
2013-11-01
The structural, electronic properties and stability of the new MXene compounds—two-dimensional pristine carbonitrides Ti3C2-xNx and their hydroxylated derivatives Ti3C2-xNx(OH)2 are studied by means of DFTB calculations. The genesis of the properties is discussed in the sequence: binary MXenes Ti3C2 (Ti3N2)→hydroxylated forms Ti3C2(OH)2 (Ti3N2(OH)2)→pristine MXene Ti3C2-xNx→hydroxylated Ti3C2-xNx(OH)2. All examined materials are metallic-like. The most favorable type of OH-covering is presented by the occupation of the hollow sites between three neighboring carbon (nitrogen) atoms. Two-dimensional MXene carbonitrides with random distribution of C and N atoms are found to be thermodynamically more favorable.
NASA Astrophysics Data System (ADS)
Bronisz, K.; Ostafin, M.; Poleshchuk, O. Kh.; Mielcarek, J.; Nogaj, B.
2006-11-01
Selected derivatives of 1,4-benzodiazepine: lorazepam, lormetazepam, oxazepam and temazepam, used as active substances in anxiolytic drugs, have been studied by 35Cl NQR method in order to find the correlation between electronic structure and biological activity. The 35Cl NQR resonance frequencies ( νQ) measured at 77 K have been correlated with the following parameters characterising their biological activity: biological half-life period ( t0.5), affinity to benzodiazepine receptor (IC 50) and mean dose equivalent. The results of experimental study of some benzodiazepine derivatives by nuclear quadrupole resonance of 35Cl nuclei are compared with theoretical results based on DFT calculations which were carried out by means of Gaussian'98 W software.
NASA Astrophysics Data System (ADS)
Shen, Ke-Sheng; Jiao, Zhao-Yong; Zhang, Xian-Zhou; Huang, Xiao-Fen
2013-11-01
The structural, electronic and optical properties of the CuGa (Se x S1- x )2 alloy system have been performed systematic within generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) implemented in the Cambridge serial total energy package (CASTEP) code. We calculate the lattice parameters and axial ratio, which agree with the experimental values quite well. The anion position parameters u are also predicted using the model of Abrahams and Bernstein and the results seem to be trustworthy as compared to the experimental and theoretical values. The total and part density of states are discussed which follow the common rule of the conventional semiconductors. The static dielectric tenser and refractive index are summarized compared with available experimental and theoretical values. Also the spectra of the dielectric functions, refractive index, reflectance, absorption coefficient and real parts of photoconductivity are discussed in details.
Greene-Diniz, Gabriel; Greer, J. C.; Fischetti, M. V.
2016-02-07
Several theoretical electronic structure methods are applied to study the relative energies of the minima of the X- and L-conduction-band satellite valleys of In{sub x}Ga{sub 1−x}As with x = 0.53. This III-V semiconductor is a contender as a replacement for silicon in high-performance n-type metal-oxide-semiconductor transistors. The energy of the low-lying valleys relative to the conduction-band edge governs the population of channel carriers as the transistor is brought into inversion, hence determining current drive and switching properties at gate voltages above threshold. The calculations indicate that the position of the L- and X-valley minima are ∼1 eV and ∼1.2 eV, respectively, higher in energy with respect to the conduction-band minimum at the Γ-point.
Guillot-Deudon, Catherine; Harel, Sylvie; Mokrani, Arezki; Lafond, Alain; Barreau, Nicolas; Fernandez, Vincent; Kessler, John
2008-12-15
The aim of the present work is to complete a preliminary study concerning the electronic band structure investigations of Na{sub x}Cu{sub 1-x}In{sub 5}S{sub 8} compounds with 0{<=}x{<=}1, which are expected to be formed at the Cu(In,Ga)Se{sub 2}/In{sub 2}S{sub 3} interface. The band structure calculations demonstrate that for the compounds containing both Na and Cu, as the Cu content increases the band gap tends to decrease, and x-ray photoemission spectroscopy measurements show that this variation is mainly due to valence-band-maximum shift along the solid solution. The band gap strongly depends on the nature of the monovalent cation, and the band structure calculations demonstrate that the d electrons of copper are responsible for the shift of the valence band. In addition, it is worth noting that the Cu-containing compounds have indirect gaps.
Moreira, E.; Henriques, J.M.; Azevedo, D.L.; Caetano, E.W.S.; Freire, V.N.; Albuquerque, E.L.
2012-03-15
Neutron diffraction data for Sr{sub x}Ba{sub 1-x}SnO{sub 3} (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) solid solutions were used as inputs to obtain optimized geometries and electronic properties using the density functional theory (DFT) formalism considering both the local density and generalized gradient approximations, LDA and GGA, respectively. The crystal structures and SnO{sub 6} octahedra tilting angles found after total energy minimization agree well with experiment, specially for the GGA data. Elastic constants were also obtained and compared with theoretical and experimental results for cubic BaSnO{sub 3}. While the alloys with cubic unit cell have an indirect band gap, tetragonal and orthorhombic alloys exhibit direct band gaps (exception made to x=1.0). The Kohn-Sham minimum electronic band gap oscillates from 1.52 eV (cubic x=0.0, LDA) to 2.61 eV (orthorhombic x=1.0, LDA), and from 0.74 eV (cubic BaSnO{sub 3}, GGA) to 1.97 eV (orthorhombic SrSnO{sub 3}, GGA). Parabolic interpolation of bands has allowed us to estimate the effective masses for charge carriers, which are shown to be anisotropic and larger for holes. - Graphical Abstract: Highlights: Black-Right-Pointing-Pointer DFT calculations were performed on Sr{sub x}Ba{sub 1-x}SnO{sub 3} solid solutions. Black-Right-Pointing-Pointer Calculated crystal structures agree well with experiment. Black-Right-Pointing-Pointer Alloys have direct or indirect gaps depending on the Sr molar fraction. Black-Right-Pointing-Pointer The Kohn-Sham gap variation from x=0.0 to x=1.0 is close to the experimental value. Black-Right-Pointing-Pointer Carrier effective masses are very anisotropic, specially for holes.
Strenalyuk, Tatyana; Samdal, Svein; Volden, Hans Vidar
2008-05-29
The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are
Ab initio MCDHF calculations of electron-nucleus interactions
NASA Astrophysics Data System (ADS)
Bieroń, Jacek; Froese Fischer, Charlotte; Fritzsche, Stephan; Gaigalas, Gediminas; Grant, Ian P.; Indelicato, Paul; Jönsson, Per; Pyykkö, Pekka
2015-05-01
We present recent advances in the development of atomic ab initio multiconfiguration Dirac-Hartree-Fock theory, implemented in the GRASP relativistic atomic structure code. For neutral atoms, the deviations of properties calculated within the Dirac-Hartree-Fock (DHF) method (based on independent particle model of an atomic cloud) are usually dominated by electron correlation effects, i.e. the non-central interactions of individual electrons. We present the recent advances in accurate calculations of electron correlation effects in small, medium, and heavy neutral atoms. We describe methods of systematic development of multiconfiguration expansions leading to systematic, controlled improvement of the accuracy of the ab initio calculations. These methods originate from the concept of the complete active space (CAS) model within the DHF theory, which, at least in principle, permits fully relativistic calculations with full account of electron correlation effects. The calculations within the CAS model on currently available computer systems are feasible only for very light systems. For heavier atoms or ions with more than a few electrons, restrictions have to be imposed on the multiconfiguration expansions. We present methods and tools, which are designed to extend the numerical calculations in a controlled manner, where multiconfiguration expansions account for all leading electron correlation effects. We show examples of applications of the GRASP code to calculations of hyperfine structure constants, but the code may be used for calculations of arbitrary bound-state atomic properties. In recent years it has been applied to calculations of atomic and ionic spectra (transition energies and rates), to determinations of nuclear electromagnetic moments, as well as to calculations related to interactions of bound electrons with nuclear electromagnetic moments leading to violations of discrete symmetries.
NASA Astrophysics Data System (ADS)
Bai, J.; Raulot, J. M.; Zhang, Y. D.; Esling, C.; Zhao, X.; Zuo, L.
2011-01-01
The crystallographic, magnetic and electronic structures of the ferromagnetic shape memory alloys Ni2XGa (X=Mn, Fe, and Co), are systematically investigated by means of the first-principles calculations within the framework of density functional theory using the VIENNA AB INITIO SOFTWARE PACKAGE. The lattice parameters of both austenitic and martensitic phases in Ni2MnGa have been calculated. The formation energies of the cubic phase of Ni2XGa are estimated, and show a destabilization tendency if Mn atom is substituted by Fe or Co. From Ni2MnGa to Ni2CoGa, the down spin total density of states (DOS) at Fermi level is gradually increasing, whereas that of the up spin part remains almost unchanged. This is the main origin of the difference of the magnetic moment in these alloys. The partial DOS is dominated by the Ni and Mn 3d states in the bonding region below EF. There are two bond types existing in Ni2XGa: one is between neighboring Ni atoms in Ni2MnGa; the other is between Ni and X atoms in Ni2FeGa and Ni2CoGa alloys.
Cao, Jun
2015-06-28
In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore photocleavages of 2-formyl-2H-azirine and isoxazole in the gas phase and the subsequent rearrangement reactions. The carbonyl n → π{sup *} transition induces a cleavage of the C—N single bond of 2-formyl-2H-azirine to yield β-formylvinylnitrene in open-shell singlet state. However, the n → π{sup *} excitation of the imine chromophore results in a cleavage of the C—C single bond, producing a nitrile ylide intermediate through an internal conversion to the ground state. β-formylvinylnitrene and nitrile ylide with the carbonyl group are easily transformed into 2-formyl-2H-azirine and oxazole, respectively. The N—O bond cleavages on both S{sub 1}({sup 1}ππ{sup *}) and S{sub 2}({sup 1}n{sub N}π{sup *}) of isoxazole are ultrafast processes, and they give products of 2-formyl-2H-azirine, 3-formylketenimine, HCN + CHCHO, and HCO + CHCHN. Both 2H-azirines and ketenimines were suggested to be formed from the triplet vinylnitrenes by intersystem crossing in the previous studies. However, our calculations show that the singlet β-formylvinylnitrene is responsible for the formation of 2-formyl-2H-azirine and 3-formylketenimine, and the singlet vinylnitrenes can play a key role in the photoinduced reactions of both 2H-azirines and isoxazoles.
NASA Astrophysics Data System (ADS)
Cao, Jun
2015-06-01
In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore photocleavages of 2-formyl-2H-azirine and isoxazole in the gas phase and the subsequent rearrangement reactions. The carbonyl n → π* transition induces a cleavage of the C—N single bond of 2-formyl-2H-azirine to yield β-formylvinylnitrene in open-shell singlet state. However, the n → π* excitation of the imine chromophore results in a cleavage of the C—C single bond, producing a nitrile ylide intermediate through an internal conversion to the ground state. β-formylvinylnitrene and nitrile ylide with the carbonyl group are easily transformed into 2-formyl-2H-azirine and oxazole, respectively. The N—O bond cleavages on both S1(1ππ*) and S2(1nNπ*) of isoxazole are ultrafast processes, and they give products of 2-formyl-2H-azirine, 3-formylketenimine, HCN + CHCHO, and HCO + CHCHN. Both 2H-azirines and ketenimines were suggested to be formed from the triplet vinylnitrenes by intersystem crossing in the previous studies. However, our calculations show that the singlet β-formylvinylnitrene is responsible for the formation of 2-formyl-2H-azirine and 3-formylketenimine, and the singlet vinylnitrenes can play a key role in the photoinduced reactions of both 2H-azirines and isoxazoles.
NASA Astrophysics Data System (ADS)
Uba, S.; Bonda, A.; Uba, L.; Bekenov, L. V.; Antonov, V. N.; Ernst, A.
2016-08-01
In this joint experimental and ab initio study, we focused on the influence of the chemical composition and martensite phase transition on the electronic, magnetic, optical, and magneto-optical properties of the ferromagnetic shape-memory Ni-Mn-Ga alloys. The polar magneto-optical Kerr effect (MOKE) spectra for the polycrystalline sample of the Ni-Mn-Ga alloy of Ni60Mn13Ga27 composition were measured by means of the polarization modulation method over the photon energy range 0.8 ≤h ν ≤5.8 eV in magnetic field up to 1.5 T. The optical properties (refractive index n and extinction coefficient k ) were measured directly by spectroscopic ellipsometry using the rotating analyzer method. To complement experiments, extensive first-principles calculations were made with two different first-principles approaches combining the advantages of a multiple scattering Green function method and a spin-polarized fully relativistic linear-muffin-tin-orbital method. The electronic, magnetic, and MO properties of Ni-Mn-Ga Heusler alloys were investigated for the cubic austenitic and modulated 7M-like incommensurate martensitic phases in the stoichiometric and off-stoichiometric compositions. The optical and MOKE properties of Ni-Mn-Ga systems are very sensitive to the deviation from the stoichiometry. It was shown that the ab initio calculations reproduce well experimental spectra and allow us to explain the microscopic origin of the Ni2MnGa optical and magneto-optical response in terms of interband transitions. The band-by-band decomposition of the Ni2MnGa MOKE spectra is presented and the interband transitions responsible for the prominent structures in the spectra are identified.
Woywod, Clemens; Vallet, Valerie; Li, Jingrui; Goerling, Andreas
2008-12-08
Understanding the molecular mechanism for the photoinduced transmembrane proton pump in the bacteriorhodopsin system is of fundamental importance. This study attempts to investigate the energetics of the initial step of the proton transport cycle, the photoisomerization of the retinal chromophore. The exact reaction pathway and the question of how many excited electronic states are involved in the internal conversion process are still unresolved. The problem is approached by constructing a reaction coordinate suggested by crystallographic studies for a simplified chromophore model system. The CASSCF and CASPT2 electronic structure methods are employed to calculate the energies of the four lowest lying singlet states as a function of the reaction coordinate. The effect of negatively charged protein residues on the reaction is simulated by inclusion of a negative point charge in the model. The results indicate that trans{yields}cis isomerization around the C{sub {beta}} = C{sub {gamma}} bond may be accompanied by twisting around the C{sub {alpha}}-C{sub {beta}} bond in order to drive the proton pump. The presence of a counterion does not seem to reduce the barrier for isomerization or the S{sub 0}-S{sub 1} energy difference but clearly stabilizes the cis--product. At first sight the results appear to support the idea of a participation of no other electronic states beyond S{sub 0} and first singly {pi}{pi}* excited state in the photoreaction. However, the relevance of this prediction is rather limited because of the small size of the model system. Other states of retinal, corresponding in particular to the partly doubly {pi}{pi}* excited S{sub 2} state of the model, are likely to have a vertical excitation energy similar to the first singly {pi}{pi}* excited state or even below.
Dey, Abhishek; Chow, Marina; Taniguchi, Kayoko; Lugo-Mas, Priscilla; Davin, Steven; Maeda, Mizuo; Kovacs, Julie A.; Odaka, Masafumi; Hodgson, Keith O.; Hedman, Britt; Solomon, Edward I.; /SLAC, SSRL
2006-09-28
The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS{sup -})-, sulfenate (RSO{sup -})-, and sulfinate (RSO{sub 2}{sup -})-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO- species changes upon protonation as the S-O bond is elongated (by {approx}0.1 {angstrom}). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe{sup III} in the active site of NHase as CysS{sup -}, CysSOH, and CysSO{sub 2}{sup -} both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The pre-edge region of the X-ray absorption spectrum is sensitive to the Z{sub eff} of the Fe and reveals that the Fe in [FeNO]{sup 6} NHase species has a Z{sub eff} very similar to that of its photolyzed Fe{sup III} counterpart. DFT calculations reveal that this results from the strong {pi} back-bonding into the {pi}* antibonding orbital of NO, which shifts significant charge from the formally t{sub 2}{sup 6} low-spin metal to the coordinated NO.
NASA Astrophysics Data System (ADS)
Lu, Huansheng; Xu, Bo; Shi, Jing; Wu, Musheng; Hu, Yinquan; Ouyang, Chuying
2016-11-01
Sodium-ion batteries (NIBs) as an alternative to lithium-ion batteries (LIBs) have recently received great attentions because of the relatively high abundance of sodium. Searching for suitable anode materials has always been a hot topic in the field of NIB study. Recent reports show that phosphorus-based materials are potential as the anode materials for NIBs. Using first-principles calculations, herein, we study the atomic and electronic structures, diffusion dynamics and intrinsic elastic properties of various Na-P alloy compounds (NaP5, Na3P11, NaP and Na3P) as the intermediate phases during Na extraction/insertion in phosphorus-based anode materials. It is found that all the crystalline phases of Na-P alloy phases considered in our study are semiconductors with band gaps larger than that of black phosphorus (BP). The calculations of Na diffusion dynamics indicate a relatively fast Na diffusion in these materials, which is important for good rate performance. In addition, the diffusion channels of sodium ions are one-dimensional in NaP5 phase and three-dimensional in other three phases (Na3P11, NaP and Na3P). Elastic constant calculations indicate that all four phases are mechanically stable. Among them, however, NaP5, Na3P11 and NaP alloy phases are ductile, while the fully sodiated phase Na3P is brittle. In order to improve the electrochemical performance of Na-P alloy anodes for NIBs, thus, promoting ductility of Na-P phase with high sodium concentration may be an effective way.
Program Calculates Power Demands Of Electronic Designs
NASA Technical Reports Server (NTRS)
Cox, Brian
1995-01-01
CURRENT computer program calculates power requirements of electronic designs. For given design, CURRENT reads in applicable parts-list file and file containing current required for each part. Program also calculates power required for circuit at supply potentials of 5.5, 5.0, and 4.5 volts. Written by use of AWK utility for Sun4-series computers running SunOS 4.x and IBM PC-series and compatible computers running MS-DOS. Sun version of program (NPO-19590). PC version of program (NPO-19111).
Urech, Tracy H.; Woodard, LeChauncy D.; Virani, Salim S.; Dudley, R. Adams; Lutschg, Meghan Z.; Petersen, Laura A.
2015-01-01
Background Hospital report cards and financial incentives linked to performance require clinical data that are reliable, appropriate, timely, and cost-effective to process. Pay-for-performance plans are transitioning to automated electronic health record (EHR) data as an efficient method to generate data needed for these programs. Objective To determine how well data from automated processing of structured EHR fields (AP-EHR) reflect data from manual chart review and the impact of these data on performance rewards. Research Design Cross-sectional analysis of performance measures used in a cluster randomized trial assessing the impact of financial incentives on guideline-recommended care for hypertension. Subjects A total of 2,840 patients with hypertension assigned to participating physicians at 12 Veterans Affairs hospital-based outpatient clinics. Fifty-two physicians and 33 primary care personnel received incentive payments. Measures Overall, positive and negative agreement indices and Cohen's kappa were calculated for assessments of guideline-recommended antihypertensive medication use, blood pressure (BP) control, and appropriate response to uncontrolled BP. Pearson's correlation coefficient was used to assess how similar participants’ calculated earnings were between the data sources. Results By manual chart review data, 72.3% of patients were considered to have received guideline-recommended antihypertensive medications compared to 65.0% by AP-EHR review (k=0.51). Manual review indicated 69.5% of patients had controlled BP compared to 66.8% by AP-EHR review (k=0.87). Compared to 52.2% of patients per the manual review, 39.8% received an appropriate response by AP-EHR review (k=0.28). Participants’ incentive payments calculated using the two methods were highly correlated (r≥0.98). Using the AP-EHR data to calculate earnings, participants’ payment changes ranged from a decrease of $91.00 (−30.3%) to an increase of $18.20 (+7.4%) for medication use
Guo, Yongliang; Wang, Changying; Qiu, Wujie; Ke, Xuezhi; Huai, Ping; Cheng, Cheng; Zhu, Zhiyuan; Chen, Changfeng
2016-10-07
Performed a systematic study using first-principles methods of the pressure-induced structural and electronic phase transitions in ThS_{2}, which may play an important role in the next generation nuclear energy fuel technology.
Li, Jia; Zhang, Shengli; Huang, Shiping; Wang, Peng; Tian, Huiping
2013-02-15
R{sub 3}ZnH{sub 5} (R=K, Rb, Cs) series have been investigated with respect to the crystal structure, electronic and thermodynamic properties using first-principle methods based on density functional theory with generalized gradient approximation. The optimized structures and atomic coordinates are in good agreement with the experimental data. The strong covalent interactions are obtained between Zn and H atoms in the 18-electron [ZnH{sub 4}]{sup 2-} complex, while an ionic interaction is found between [ZnH{sub 4}]{sup 2-} and R atom. The formation enthalpies show that the formations of R{sub 3}ZnH{sub 5} hydrides are all exothermic at 298 K. The vibration free energies of R{sub 3}ZnH{sub 5} show that the thermodynamic stabilities of R{sub 3}ZnH{sub 5} hydrides decrease with the increasing diameter of R atom. Two possible decomposition reactions of R{sub 3}ZnH{sub 5} series have been suggested in our work. One (reaction one) is that R{sub 3}ZnH{sub 5} hydrides decomposes to elements directly, and the other (reaction two) is that R{sub 3}ZnH{sub 5} hydrides decomposes to RH hydride. The results show that the first decomposition reaction is more favorable one. The spontaneous decomposition reaction of K{sub 3}ZnH{sub 5} hydrides occur upon 465 K via reaction one, and 564 K via reaction two, respectively. - Graphical abstract: Total charge density of K{sub 3}ZnH{sub 5}. Highlights: Black-Right-Pointing-Pointer Electronic and thermodynamic properties of R{sub 3}ZnH{sub 5} (R=K, Rb, Cs) were calculated. Black-Right-Pointing-Pointer The formations of R{sub 3}ZnH{sub 5} hydrides are all exothermic at 298 K. Black-Right-Pointing-Pointer The thermodynamic stabilities decrease with the increasing diameter of R atom. Black-Right-Pointing-Pointer Two possible decomposition pathways of R{sub 3}ZnH{sub 5} were investigated.
NASA Astrophysics Data System (ADS)
Lan, Hai-Ping; Zhang, Shuang
2009-11-01
Recently, a new switching characteristic of double-walled carbon nanotubes (DWNTs) transistors is found in during experiments. We carry out a series of ab intio calculations on DWNTs' electronic properities, together with verification on the electronic response under the electric field. Our results reveal that the peculiar energy states relation in DWNTs and related contact modes should account for the distinct switching behavior of DWNT transistors. We believe these results have important implications in the fabrication and understanding of electronic devices with DWNTs.
NASA Astrophysics Data System (ADS)
Kong, Yuanyuan; Duan, Yonghua; Ma, Lishi; Li, Runyue
2016-10-01
By performing first-principles calculations within the generalized gradient approximation, the phase stability, elastic constant and anisotropy, and density of states of cubic C15-type MAl2 (M = Mg, Ca, Sr and Ba) Laves phases have been investigated. Optimized equilibrium lattice parameters and formation enthalpies agree well with the available experimental data. Elastic constants C ij have been evaluated, and these C15-type MAl2 Laves phases are mechanically stable due to the meeting of C ij to the mechanical stability criteria. Polycrystalline elastic moduli have been deduced from elastic constants by Voigt-Reuss-Hill approximation. Plastic properties were characterized via values of B/G, Poisson’s ratio ν and Cauchy pressure (C 12-C 44). The elastic anisotropy has been considered by several anisotropy indexes (A U , A Z , A shear and A comp), anisotropy of shear modulus, and 3D surface constructions of bulk and Young’s moduli. Additionally, the sound velocity anisotropy and Debye temperature were predicted. Finally, electronic structures were carried out to reveal the underlying phase stability mechanism of these Laves phases.
NASA Astrophysics Data System (ADS)
Demkov, Alexander A.; Navrotsky, Alexandra
2001-03-01
The International Technology Roadmap for Semiconductors (ITRS) predicts that the strategy of scaling complementary metal-oxide-semiconductor (CMOS) devices will come to an abrupt end around the year 2012. The main reason for this will be the unacceptably high leakage current through the silicon dioxide gate with a thickness below 20 ÅFinding a gate insulator alternative to SiO2 has proven to be far from trivial. Hafnium and zirconium dioxides and silicates have been recently considered as gate dielectrics with intermediate dielectric constants. Hafnia and ziconia are important ceramic materials as well, and their phase relations are rather well studied. There is also interest in hafnia as a constituent of ceramic waste forms for plutonium, based on its refractory nature and high neutron absorption cross section. We use a combination of the ab-initio calculations and calorimetry to investigate thermodynamic and electronic properties of hafnia and zirconia. We describe the cubic to tetragonal phase transition in the fluorite structure by computing the total energy surface for zone-edge distortions correct to fourth order in the soft-mode displacement with the strain coupling renormalization included. We compare the two materials using some simple chemical concepts.
NASA Astrophysics Data System (ADS)
Azadegan, B.
2013-03-01
The presented Mathematica code is an efficient tool for simulation of planar channeling radiation spectra of relativistic electrons channeled along major crystallographic planes of a diamond-structure single crystal. The program is based on the quantum theory of channeling radiation which has been successfully applied to study planar channeling at electron energies between 10 and 100 MeV. Continuum potentials for different planes of diamond, silicon and germanium single crystals are calculated using the Doyle-Turner approximation to the atomic scattering factor and taking thermal vibrations of the crystal atoms into account. Numerical methods are applied to solve the one-dimensional Schrödinger equation. The code is designed to calculate the electron wave functions, transverse electron states in the planar continuum potential, transition energies, line widths of channeling radiation and depth dependencies of the population of quantum states. Finally the spectral distribution of spontaneously emitted channeling radiation is obtained. The simulation of radiation spectra considerably facilitates the interpretation of experimental data. Catalog identifier: AEOH_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOH_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 446 No. of bytes in distributed program, including test data, etc.: 209805 Distribution format: tar.gz Programming language: Mathematica. Computer: Platforms on which Mathematica is available. Operating system: Operating systems on which Mathematica is available. RAM: 1 MB Classification: 7.10. Nature of problem: Planar channeling radiation is emitted by relativistic charged particles during traversing a single crystal in direction parallel to a crystallographic plane. Channeling is modeled as the motion
The calculation of vibrational intensities in forbidden electronic transitions
NASA Astrophysics Data System (ADS)
Johnson, Philip M.; Xu, Haifeng; Sears, Trevor J.
2006-10-01
A method is described for the use of electronic structure and Franck-Condon factor programs in the calculation of the vibrational intensities in forbidden electronic transitions. Using the B˜B22-X˜B12 electronic transition of benzonitrile cation as a test case, transition moments were calculated using the symmetry adapted cluster/configuration interaction method at various points along the normal mode displacements of the molecule, from which transition moment derivatives were obtained. The transition moments were found to vary almost linearly with respect to the normal mode displacements. Using these, along with Franck-Condon factors, an expansion of the transition moment with respect to the normal coordinates provides a measure of vibrational intensities, including the effects of geometry change and Duschinsky rotation [Acta Physicochim. URSS 7, 551 (1937)]. Second order terms in the moment expansion are calculated, and it is determined that they must be included if the intensity of combination bands is to be properly obtained.
The calculation of vibrational intensities in forbidden electronic transitions.
Johnson, Philip M; Xu, Haifeng; Sears, Trevor J
2006-10-28
A method is described for the use of electronic structure and Franck-Condon factor programs in the calculation of the vibrational intensities in forbidden electronic transitions. Using the B 2B2-X 2B1 electronic transition of benzonitrile cation as a test case, transition moments were calculated using the symmetry adapted cluster/configuration interaction method at various points along the normal mode displacements of the molecule, from which transition moment derivatives were obtained. The transition moments were found to vary almost linearly with respect to the normal mode displacements. Using these, along with Franck-Condon factors, an expansion of the transition moment with respect to the normal coordinates provides a measure of vibrational intensities, including the effects of geometry change and Duschinsky rotation [Acta Physicochim. URSS 7, 551 (1937)]. Second order terms in the moment expansion are calculated, and it is determined that they must be included if the intensity of combination bands is to be properly obtained.
The electronic structure of Lu
NASA Astrophysics Data System (ADS)
Tibbetts, T. A.; Harmon, B. N.
1982-12-01
The electronic structure of hcp Lu has been calculated using a linearized augmented plane wave (LAPW) method and the Hedin-Lundqvist local density approximation for exchange and correlation. Although complete self-consistency was hindered by the proximity of the 4f levels to the Fermi energy, the valence bands were converged and the calculation yielded a Fermi surface remarkably similar to that calculated by Keeton and Loucks. Comparison is made with recent de Haas-van Alphen and neutron magnetic form factor experiments.
Yu, Jen-Shiang K; Hwang, Jenn-Kang; Tang, Chuan Yi; Yu, Chin-Hui
2004-01-01
A number of recently released numerical libraries including Automatically Tuned Linear Algebra Subroutines (ATLAS) library, Intel Math Kernel Library (MKL), GOTO numerical library, and AMD Core Math Library (ACML) for AMD Opteron processors, are linked against the executables of the Gaussian 98 electronic structure calculation package, which is compiled by updated versions of Fortran compilers such as Intel Fortran compiler (ifc/efc) 7.1 and PGI Fortran compiler (pgf77/pgf90) 5.0. The ifc 7.1 delivers about 3% of improvement on 32-bit machines compared to the former version 6.0. Performance improved from pgf77 3.3 to 5.0 is also around 3% when utilizing the original unmodified optimization options of the compiler enclosed in the software. Nevertheless, if extensive compiler tuning options are used, the speed can be further accelerated to about 25%. The performances of these fully optimized numerical libraries are similar. The double-precision floating-point (FP) instruction sets (SSE2) are also functional on AMD Opteron processors operated in 32-bit compilation, and Intel Fortran compiler has performed better optimization. Hardware-level tuning is able to improve memory bandwidth by adjusting the DRAM timing, and the efficiency in the CL2 mode is further accelerated by 2.6% compared to that of the CL2.5 mode. The FP throughput is measured by simultaneous execution of two identical copies of each of the test jobs. Resultant performance impact suggests that IA64 and AMD64 architectures are able to fulfill significantly higher throughput than the IA32, which is consistent with the SpecFPrate2000 benchmarks.
NASA Astrophysics Data System (ADS)
Guo, San-Dong
2016-05-01
To identify thermoelectric materials containing abundant, low-cost and non-toxic elements, we have studied the electronic structures and thermoelectric properties of (Mg2X)2/ (Mg2Y)2 (X, Y = Si, Ge, Sn) superlattices with state-of-the-art first-principles calculations using a modified Becke and Johnson (mBJ) exchange potential. Our results show that (Mg2Ge)2/ (Mg2Sn)2 and (Mg2Si)2/ (Mg2Sn)2 are semi-metals using mBJ plus spin-orbit coupling (mBJ + SOC), while (Mg2Si)2/ (Mg2Ge)2 is predicted to be a direct-gap semiconductor with a mBJ gap value of 0.46 eV and mBJ + SOC gap value of 0.44 eV. Thermoelectric properties are predicted by through solving the Boltzmann transport equations within the constant scattering time approximation. It is found that (Mg2Si)2/ (Mg2Ge)2 has a larger Seebeck coefficient and power factor than (Mg2Ge)2/ (Mg2Sn)2 and (Mg2Si)2/ (Mg2Sn)2 for both p-type and n-type doping. The detrimental influence of SOC on the power factor of p-type (Mg2X)2/ (Mg2Y)2 (X, Y = Si, Ge, Sn) is analyzed as a function of the carrier concentration, but there is a negligible SOC effect for n-type. These results can be explained by the influence of SOC on their valence and conduction bands near the Fermi level.
NASA Astrophysics Data System (ADS)
Masrour, R.; Hlil, E. K.
2016-08-01
Self-consistent ab initio calculations based on density-functional theory and using both full potential linearized augmented plane wave and Korring-Kohn-Rostoker-coherent potential approximation methods, are performed to investigate both electronic and magnetic properties of the Ga1-xMnxN system. Magnetic moments considered to lie along (001) axes are computed. Obtained data from ab initio calculations are used as input for the high temperature series expansions (HTSEs) calculations to compute other magnetic parameters such as the magnetic phase diagram and the critical exponent. The increasing of the dilution x in this system has allowed to verify a series of HTSEs predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from antiferromagnetic to ferromagnetic passing through the spins glace phase.
NASA Astrophysics Data System (ADS)
Govindarajan, M.; Karabacak, M.
2013-04-01
In this work, the vibrational spectral analysis was carried out by using FT-Raman and FT-IR spectroscopy in the range 100-4000 cm-1 and 400-4000 cm-1 respectively, for 4-hydroxypteridine (C6H4N4O, 4HDPETN) molecule. The potential energy curve shows that 4HDPETN molecule has two stable structures. The computational results diagnose the most stable conformer of the 4HDPETN as the S1 structure. The molecular structure, fundamental vibrational frequencies and intensities of the vibrational bands were interpreted with the aid of structure optimizations and normal coordinate force field calculations based density functional theory (DFT) and ab initio HF methods and different basis sets combination. The complete vibrational assignments of wavenumbers were made on the basis of potential energy distribution (PED). The results of the calculations were applied to simulated spectra of the title compound, which show excellent agreement with observed spectra. The scaled B3LYP/6-311++G(d,p) results show the best agreement with the experimental values over the other method. The energy and oscillator strength calculated by time-dependent density functional theory (TD-DFT) complements with the experimental findings. In addition, molecular electrostatic potential, nonlinear optical and thermodynamic properties of the title compound were performed. Mulliken and natural charges of the title molecule were also calculated and interpreted.
Muthu, S; Elamuruguporchelvi, E; Varghese, Anitha
2015-03-05
The solid phase FTIR and FT-Raman spectra of 2-[(5-nitro-1,3-thiazol-2-yl)carbamoyl]phenyl acetate (25N2LCPA) have been recorded 450-4000cm(-1) and 100-4000cm(-1) respectively. The normal coordinate analysis was carried out to confirm the precision of the assignments. DFT calculations have been performed giving energies, optimized structures, harmonic vibrational frequencies and IR intensities. The structure of the molecule was optimized and the structural characteristics were determined by density functional theory (DFT) using B3LYP method with 6-31+G(d,p) basis set. The detailed interpretation of the vibrational spectra has been carried out with aid of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology. The Vibrational frequencies are calculated in the above method and are compared with experimental frequencies which yield good agreement between observed and calculated frequencies. Stability of the molecule arising from hyper conjugative interactions, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. In addition, Frontiers molecular orbital and molecular electrostatic potential were computed by using Density Functional Theory (DFT) B3LYP/6-31+G(d,p) basis set. The calculated HOMO and LUMO energies show that charge transfer occurs in the molecule.
First-Principles Calculations of Electron Transfer in Organic Molecules
NASA Astrophysics Data System (ADS)
Pati, Ranjit; Karna, Shashi P.
2000-03-01
Suitably tailored organic structures are considered potential candidates as components in molecular electronic devices. A common molecular architecture for electronics consists of an electron donor (D) and an electron acceptor (A) moiety bonded together by a chemically inert bridging moiety, called spacer (S). The D-S-A combination constitutes the basic component equivalent of a solid state capacitor. A useful physical property that determines the applicability of molecular structures in moletronics is the electron transfer (ET) rate, which is related, in a two-state approximation, to the coupling matrix between the two electronic states representing the localization of electrons. In an effort to model potential organic structures, we have calculated the ET coupling matrix elements in a number of D-, S-, and A-type organic molecules with the use of ab initio Hartree-Fock method and two different basis sets, namely an STO-3G and a double zeta plus polarization (DZP). A number of important findings have emerged from this study: (i) The ET coupling matrix strongly depends upon the geometrical arrangement of the molecular fragment(s) in the architecture. (ii) In an oligomeric chain, the ET matrix decreases exponentially with molecular length (number of monomer units). (iii) In cyclic alkanes, the magnitude of the ET coupling matrix decreases with increasing size of fused rings.
Global nuclear-structure calculations
Moeller, P.; Nix, J.R.
1990-04-20
The revival of interest in nuclear ground-state octupole deformations that occurred in the 1980's was stimulated by observations in 1980 of particularly large deviations between calculated and experimental masses in the Ra region, in a global calculation of nuclear ground-state masses. By minimizing the total potential energy with respect to octupole shape degrees of freedom in addition to {epsilon}{sub 2} and {epsilon}{sub 4} used originally, a vastly improved agreement between calculated and experimental masses was obtained. To study the global behavior and interrelationships between other nuclear properties, we calculate nuclear ground-state masses, spins, pairing gaps and {Beta}-decay and half-lives and compare the results to experimental qualities. The calculations are based on the macroscopic-microscopic approach, with the microscopic contributions calculated in a folded-Yukawa single-particle potential.
Theoretical electronic structure of structurally modified graphene
NASA Astrophysics Data System (ADS)
Dvorak, Marc David
Graphene has emerged as a promising replacement for silicon in next-generation electronics and optoelectronic devices. If graphene is to be used in semiconductor devices, however, it must acquire an electronic band gap. Numerous approaches have been proposed to control the band gap of graphene, including the periodic patterning of defects. However, the mechanism for band gap opening and the associated physics in graphene patterned with defects remain unclear. Using both analytic theory and first-principles calculations, we show that periodic patterning of defects on graphene can open a large and tunable band gap, induce strong absorption peaks at optical wavelengths, and host a giant band gap quantum spin Hall phase. First, a geometric rule is analytically derived for the arrangements of defects that open a band gap in graphene, with one ninth of all possible patterns opening a band gap. Next, we perform ab-initio density functional calculations to compare the effects of structural vacancies, hexagonal BN dopants, and passivants on the electronic structure of graphene. Qualitatively, these three types of structural defects behave the same, with only slight differences in their resulting band structures. By adjusting the shape of structural defects, we show how to move the Dirac cones in reciprocal space in accordance with the tight-binding model for the anisotropic honeycomb lattice, while the fundamental mechanism for band gap opening remains the same. To quantitatively predict the band gap and optical properties of these materials, we employ many-body perturbation theory with Green's functions (GW/Bethe-Salpeter equation) to directly include electron-electron and electron-hole interactions. Structurally modified graphene shows a strong renormalization of the fundamental band gap over single particle descriptions, and a strong electron-hole interaction as indicated by strong exciton binding energies (> 0.5 eV). Finally, we show that structurally modified graphene
NASA Astrophysics Data System (ADS)
Sinha, Leena; Karabacak, Mehmet; Narayan, V.; Cinar, Mehmet; Prasad, Onkar
2013-05-01
Gabapentin (GP), structurally related to the neurotransmitter GABA (gamma-aminobutyric acid), mimics the activity of GABA and is also widely used in neurology for the treatment of peripheral neuropathic pain. It exists in zwitterionic form in solid state. The present communication deals with the quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of GP using density functional (DFT/B3LYP) method with 6-311++G(d,p) basis set. In view of the fact that amino acids exist as zwitterions as well as in the neutral form depending on the environment (solvent, pH, etc.), molecular properties of both the zwitterionic and neutral form of GP have been analyzed. The fundamental vibrational wavenumbers as well as their intensities were calculated and compared with experimental FT-IR and FT-Raman spectra. The fundamental assignments were done on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanical (SQM) method. The electric dipole moment, polarizability and the first hyperpolarizability values of the GP have been calculated at the same level of theory and basis set. The nonlinear optical (NLO) behavior of zwitterionic and neutral form has been compared. Stability of the molecule arising from hyper-conjugative interactions and charge delocalization has been analyzed using natural bond orbital analysis. Ultraviolet-visible (UV-Vis) spectrum of the title molecule has also been calculated using TD-DFT method. The thermodynamic properties of both the zwitterionic and neutral form of GP at different temperatures have been calculated.
NASA Astrophysics Data System (ADS)
Shein, I. R.; Ivanovskii, A. L.
2012-08-01
Structural, electronic properties and relative stability of quasi-two-dimensional (2D) free-standing planar nano-block (NBs) structures Tin+1Al0.5Cn and Tin+1Cn (n = 1 and 2), which can be prepared using the recently developed procedure of exfoliation of corresponding NBs from MAX phases, were examined within first principles calculations in comparison with parent MAX phases Ti3AlC2 and Ti2AlC. We found that in general Tin+1Cn and Tin+1Al0.5Cn NBs retain the atomic geometries of the corresponding blocks of the MAX phases, but some structural distortions for the NBs occur owing to the lowering of the coordination number for atoms in the external Ti sheets of the nano-block structures. Our analysis based on their cohesive and formation energies reveals that the stability of the nano-block structures increases with index n (or, in other words, with a growth of the number of Ti-C bonds), the Al-containing NBs becoming more stable than the "pure" Ti-C NBs. Our data show that the magnetization of the simulated planar nano-block structures can be expected; so, for the Ti3C2 nano-block the most stable will be the spin configuration, where within each external Ti sheet the spins are coupled ferromagnetically together with antiferromagnetic ordering between opposite external titanium sheets of this nano-block.
NASA Astrophysics Data System (ADS)
Orimoto, Yuuichi; Gu, Feng Long; Imamura, Akira; Aoki, Yuriko
2007-06-01
Elongation method was applied to determine the electronic structures of B-type poly(dG)•poly(dC) DNA at the ab initio molecular orbital level as a first step toward the calculation of aperiodic DNA. The discrepancy in total energy between the elongation method and a conventional calculation was negligibly small in the order of 10-8hartree/at. for 14 G-C base pair model. The local density of states for 10 G-C base pair model estimated by the elongation method well reproduced the results by the conventional calculation. It was found that the band gap of the whole system is mainly due to the energy difference between the valence band of guanine and the conduction band of cytosine. Moreover, the electron transfer path through stacking G-C base pairs rather than sugar-phosphate backbones has been confirmed by the authors' calculations.
Zhang, Guiling; Ma, Jing; Wen, Jin
2007-10-11
The packing structures and packing effects on excitation energies of oligomers of polyfuran (PFu), polypyrrole (PPy), polycyclopentidene (PCp), polythiophene (PTh), polyphosphole (PPh), and polysilole (PSi) are comparatively studied by employing molecular dynamics (MD) simulations and time-dependent density functional theory (TDDFT) calculations. The dependence of packing structures on the main group of heteroatoms in the five-membered heterocyclic oligomers is exhibited from MD simulations. The planarity of backbones and the population of pi-stacked structures increase with the heteroatoms going from group 14 to group 16; i.e., PCp < PPy < PFu; PSi < PPh < PTh. The polymers with the third row elements, PSi and PPh, tend to have larger chain flexibilities in the packing systems than those with the second row elements, PCp and PPy, respectively. On the basis of the second-order Møller-Plesset perturbation (MP2) and natural bond orbital (NBO) calculations of the pi-stacked pairs, the difference in pi-stack orientations, head-to-tail vs head-to-head, between various packing systems is rationalized by individual interchain bond orbital interactions involved with heteroatoms. The packing systems with higher row elements tend to have narrower band gaps. The band gaps are closely related to the chain torsions driven by interchain interactions. The noticeable chain distortions in the packing systems of PCp, PSi, and PPh lead to the significant increase of band gaps in comparison with those appraised from periodic boundary conditions (PBC) calculations on their planar isolated chains.
Electronic structure of Ca, Sr, and Ba under pressure.
NASA Technical Reports Server (NTRS)
Animalu, A. O. E.; Heine, V.; Vasvari, B.
1967-01-01
Electronic band structure calculations phase of Ca, Sr and Ba over wide range of atomic volumes under pressure electronic band structure calculations for fcc phase of Ca, Sr and Ba over wide range of atomic volumes under pressure electronic band structure calculations for fcc phase of Ca, Sr and Ba over wide range of atomic volumes under pressure
Singh, Satendra Pal; Kim, Minseuk; Park, Woon Bae; Lee, Jin-Woong; Sohn, Kee-Sun
2016-10-17
A solid-state combinatorial chemistry approach, which used the A-Ge-O (A = Li, K, Rb) system doped with a small amount of Mn(4+) as an activator, was adopted in a search for novel red-emitting phosphors. The A site may have been composed of either a single alkali metal ion or of a combination of them. This approach led to the discovery of a novel phosphor in the above system with the chemical formula Li3RbGe8O18:Mn(4+). The crystal structure of this novel phosphor was solved via direct methods, and subsequent Rietveld refinement revealed a trigonal structure in the P3̅1m space group. The discovered phosphor is believed to be novel in the sense that neither the crystal structure nor the chemical formula matches any of the prototype structures available in the crystallographic information database (ICDD or ICSD). The measured photoluminescence intensity that peaked at a wavelength of 667 nm was found to be much higher than the best intensity obtained among all the existing A2Ge4O9 (A = Li, K, Rb) compounds in the alkali-germanate system. An ab initio calculation based on density function theory (DFT) was conducted to verify the crystal structure model and compare the calculated value of the optical band gap with the experimental results. The optical band gap obtained from diffuse reflectance measurement (5.26 eV) and DFT calculation (4.64 eV) results were in very good agreement. The emission wavelength of this phosphor that exists in the deep red region of the electromagnetic spectrum may be very useful for increasing the color gamut of LED-based display devices such as ultrahigh-definition television (UHDTV) as per the ITU-R BT.2020-2 recommendations and also for down-converter phosphors that are used in solar-cell applications.
Wen, Jing; Zhang, Xitian; Gao, Hong
2015-02-15
Many conflicting electron microscopy data for In{sub 2}O{sub 3}(ZnO){sub m} indicate that it may have the polymorphous and polytypoid structures. We investigate their stabilities based on four controversial models. The calculated results confirm that the models with the zigzag feature are more stable than the others and it is possible to form different zigzag configurations in the samples as observed in the experiments. The dynamic process of eliminating the dangling bonds and the requirements of maximizing the symmetry and the distances between the In atoms in the slabs can be regarded as the dominant rules to stabilize the system, but the statistical equilibrium processes have the chances to transform it from the ground state structures to the other model structures. The study of the electronic structures based on the plane and zigzag models reveals that their band gaps and effective masses increase monotonically with m. The predicted band gaps are consistent with the experimental results. The anisotropic feature of electron effective mass tensor exhibited in the plane model differs from that of the zigzag one, which is so notable that can be employed to determine which model is more close to the actual structure of a given sample. The calculated results confirm the possibilities of the separation of conduction electrons and defects and the existence of the natural optimized transport channels in the layered structures, which demonstrate its advantage over ZnO to transport electrons and benefit its applications in the optoelectronic devices. - Graphical abstract: The conduction electrons are mainly distributed around the boundaries of the plane or zigzag shape. The optimized transport channels can be formed around the boundaries. - Highlights: • The formation mechanisms for the polytypoid structure of In{sub 2}O{sub 3}(ZnO){sub m} are revealed. • The predicted band gaps are consistent with the experimental results. • The natural optimized transport channels
Hoyer, Chad E.; Manni, Giovanni Li; Truhlar, Donald G. E-mail: gagliard@umn.edu; Gagliardi, Laura E-mail: gagliard@umn.edu
2014-11-28
The diatomic molecule Fe{sub 2} was investigated using restricted active space second-order perturbation theory (RASPT2). This molecule is very challenging to study computationally because predictions about the ground state and excited states depend sensitively on the choice of the quantum chemical method. For Fe{sub 2} we show that one needs to go beyond a full-valence active space in order to achieve even qualitative agreement with experiment for the dissociation energy, and we also obtain a smooth ground-state potential curve. In addition we report the first multireference study of Fe{sub 2}{sup +}, for which we predict an {sup 8}Σ{sub u}{sup −} ground state, which was not predicted by previous computational studies. By using an active space large enough to remove the most serious deficiencies of previous theoretical work and by explicitly investigating the interpretations of previous experimental results, this study elucidates previous difficulties and provides – for the first time – a qualitatively correct treatment of Fe{sub 2}, Fe{sub 2}{sup +}, and Fe{sub 2}{sup −}. Moreover, this study represents a record in terms of the number or active electrons and active orbitals in the active space, namely 16 electrons in 28 orbitals. Conventional CASPT2 calculations can be performed with at most 16 electrons in 16 orbitals. We were able to overcome this limit by using the RASPT2 formalism.
Electronic correlation contributions to structural energies
NASA Astrophysics Data System (ADS)
Haydock, Roger
2015-03-01
The recursion method is used to calculate electronic excitation spectra including electron-electron interactions within the Hubbard model. The effects of correlation on structural energies are then obtained from these spectra and applied to stacking faults. http://arxiv.org/abs/1405.2288 Supported by the Richmond F. Snyder Fund and Gifts.
NASA Astrophysics Data System (ADS)
Galván, Donald H.; Dilley, N. R.; Maple, M. B.; Posada-Amarillas, A.; Reyes-Serrato, Armando; Samaniego Reyna, J. C.
2003-09-01
Calculation of the band structure, total and projected density of states, crystal orbital population analysis (COOP), and Mulliken population analysis were performed for the filled skutterudites YbFe4Sb12, UFe4P12, and ThFe4P12. The calculated energy bands depict a semimetal behavior for YbFe4Sb12 and UFe4P12, and metallic behavior for ThFe4P12. Furthermore, the contributions from each orbital to the total DOS for each compound corroborate these findings. The bonding strength was derived from the COOP analysis between different pairs of atoms, considering nearest neighbor distances between 3.40 and 6.47 Å for YbFe4Sb12, 2.91 and 6.47 Å for UFe4P12, and 2.48 and 5.51 Å for ThFe4P12. Mulliken population analysis suggests ionic behavior for these compounds.
Chatterjee, Avisek; Zhao, Liyan; Zhang, Lei; Pradhan, Debabrata; Zhou, Xiaojing; Leung, K T
2008-09-14
X-ray photoelectron spectroscopy (XPS) has been used to investigate the core-level electronic structures of glycine (G) and its peptides, including glycyl-glycine (GG), diglycyl-glycine (GGG), and polyglycine (poly-G), in their powder forms. Increasing the number of G units in the peptides does not change the locations of the respective C 1s, N 1s, and O 1s features corresponding to different functional groups: -COO(-), -NH(3)(+), >CH(2), and -CONH-. The electronic structures of the zwitterions of these molecules have been calculated as isolated molecules and as molecules in an aqueous environment under the periodic boundary conditions by quantum-mechanical and molecular mechanics methods. In the case of glycine zwitterion, the binding energies of the C 1s, N 1s, and O 1s XPS features are found to be in reasonable accord with the respective orbital energies obtained by Hartree-Fock self-consistent-field calculations, within the context of Koopmans' approximation. However, considerably worse agreement in the binding energies is found for the larger zwitterions (with the specific conformations considered in this work), indicating the need for higher-level calculations. The present work shows that optimizing the zwitterion in an aqueous environment under the periodic boundary conditions by molecular mechanics could be a very cost-effective approach for calculating the electronic structures of large, complex biomolecular systems.
First-principles calculation on electronic structure and optical property of BaSi2O2N2:Eu2+ phosphor
NASA Astrophysics Data System (ADS)
Tong, Zhi-Fang; Wei, Zhan-Long; Xiao, Cheng
2017-04-01
The crystal structure, electronic structure and optical properties of BaSi2O2N2:Eu2+ with varying Eu doping concentrations are computed by the density functional theory (DFT) and compared with experimental results. The results show that the lattice parameters of primitive cells of Ba1-xSi2O2N2:Eux become smaller and Eu-N bond length shortens as Eu concentration increases. The band structure of Ba1-xSi2O2N2:Eux exhibits a direct optical band gap and it's propitious to luminescence. The energy differences from the lowest Eu 5d state to the lowest Eu 4f state decrease with increasing Eu concentrations. The analysis of simulative absorption spectra indicates that the electron transition from Eu 4f states to 5d states of both Eu and Ba atoms contributes to the absorption of Ba1-xSi2O2N2:Eux. Under the coupling effect between Eu and Ba, Ba in BaSi2O2N2 exhibits longer wavelength absorption and increases absorption efficiency. The emission wavelength is deduced by measuring energy differences from the lowest Eu 5d state to the lowest Eu 4f state, and the result is in good agreement with experimental value within experimental Eu2+ doping range.
NASA Astrophysics Data System (ADS)
Roscioli, Joseph R.; Hammer, Nathan I.; Johnson, Mark A.; Diri, Kadir; Jordan, Kenneth D.
2008-03-01
We report a combined photoelectron and vibrational spectroscopy study of the (H2O)7- cluster anions in order to correlate structural changes with the observed differences in electron binding energies of the various isomers. Photoelectron spectra of the (H2O)7-ṡArm clusters are obtained over the range of m =0-10. These spectra reveal the formation of a new isomer (I') for m >5, the electron binding energy of which is about 0.15eV higher than that of the type I form previously reported to be the highest binding energy species [Coe et al., J. Chem. Phys. 92, 3980 (1990)]. Isomer-selective vibrational predissociation spectra are obtained using both the Ar dependence of the isomer distribution and photochemical depopulation of the more weakly (electron) binding isomers. The likely structures of the isomers at play are identified with the aid of electronic structure calculations, and the electron binding energies, as well as harmonic vibrational spectra, are calculated for 28 low-lying forms for comparison with the experimental results. The HOH bending spectrum of the low binding type II form is dominated by a band that is moderately redshifted relative to the bending origin of the bare water molecule. Calculations trace this feature primarily to the bending vibration localized on a water molecule in which a dangling H atom points toward the electron cloud. Both higher binding forms (I and I') display the characteristic patterns in the bending and OH stretching regions signaling electron attachment primarily to a water molecule in an AA binding site, a persistent motif found in non-isomer-selective spectra of the clusters up to (H2O)50-.
Matyus, Edit; Reiher, Markus
2012-07-14
We elaborate on the theory for the variational solution of the Schroedinger equation of small atomic and molecular systems without relying on the Born-Oppenheimer paradigm. The all-particle Schroedinger equation is solved in a numerical procedure using the variational principle, Cartesian coordinates, parameterized explicitly correlated Gaussian functions with polynomial prefactors, and the global vector representation. As a result, non-relativistic energy levels and wave functions of few-particle systems can be obtained for various angular momentum, parity, and spin quantum numbers. A stochastic variational optimization of the basis function parameters facilitates the calculation of accurate energies and wave functions for the ground and some excited rotational-(vibrational-)electronic states of H{sub 2}{sup +} and H{sub 2}, three bound states of the positronium molecule, Ps{sub 2}, and the ground and two excited states of the {sup 7}Li atom.
NASA Astrophysics Data System (ADS)
Zhou, Bo; Su, Qing; He, De-Yan
2009-11-01
Using a first-principles approach based on density functional theory, this paper studies the electronic and dynamical properties of β-V2O5. A smaller band gap and much wider split-off bands have been observed in comparison with α-V2O5. The Raman- and infrared-active modes at the Γ point of the Brillouin zone are evaluated with LO/TO splitting, where the symbol denotes the longitudinal and transverse optical model. The nonresonant Raman spectrum of a β-V2O5 powder sample is also computed, providing benchmark theoretical results for the assignment of the experimental spectrum. The computed spectrum agrees with the available experimental data very well. This calculation helps to gain a better understanding of the transition from α- to β-V2O5.
Electronic structure and polarizability of metallic nanoshells
NASA Astrophysics Data System (ADS)
Prodan, E.; Nordlander, P.
2002-01-01
An efficient method for the calculation of the electronic structure of metallic nanoshells is developed. The method is applied to a large nanoshell (of 10 nm in diameter) containing more than 2.5×10 4 conduction electrons. The calculations show that the density of states of the nanoshell is relatively bulk-like. The frequency dependent polarizability is calculated and shown to display strong confinement effects and features similar to what is predicted by semi-classical electrodynamic theory.
Reshak, Ali Hussain; Khenata, R.; Auluck, S.
2011-08-15
From the refined atomic positions obtained by Belmal et al. (2004) using X-ray diffraction for Li{sub 0.50}Co{sub 0.25}TiO(PO{sub 4}), we have performed a structural optimization by minimizing the forces acting on the atoms keeping the lattice parameters fixed at the experimental values. With this relaxed (optimized) geometry we have performed a comprehensive theoretical study of electronic properties and dispersion of the linear optical susceptibilities using the full potential linear augmented plane wave (FP-LAPW) method. The generalized gradient approximation (GGA) exchange-correlation potential was applied. In addition, the Engel-Vosko generalized gradient approximation (EVGGA) was used for comparison with GGA because it is known that EVGGA approach yields better band splitting compared to the GGA. We have calculated the band structure, and the total and partial densities of states. The electron charge densities and the bonding properties were analyzed and discussed. The complex dielectric optical susceptibilities were discussed in detail. - Graphical abstract: It is shown that P is tetrahedrally coordinated by four O ions. Highlights: > Comprehensive theoretical study of electronic and optical properties was performed. > Using X-ray diffraction data we have performed a structural optimization. > The electron charge densities and the bonding properties were analyzed and discussed. > Fermi surface was analyzed since it is useful for predicting thermal, magnetic, and optical properties. > The density of states at E{sub F} and the electronic specific heat coefficient were calculated.
Instructional Approach to Molecular Electronic Structure Theory
ERIC Educational Resources Information Center
Dykstra, Clifford E.; Schaefer, Henry F.
1977-01-01
Describes a graduate quantum mechanics projects in which students write a computer program that performs ab initio calculations on the electronic structure of a simple molecule. Theoretical potential energy curves are produced. (MLH)
Computational Chemistry Using Modern Electronic Structure Methods
ERIC Educational Resources Information Center
Bell, Stephen; Dines, Trevor J.; Chowdhry, Babur Z.; Withnall, Robert
2007-01-01
Various modern electronic structure methods are now days used to teach computational chemistry to undergraduate students. Such quantum calculations can now be easily used even for large size molecules.
Sala, Matthieu; Kirkby, Oliver M; Guérin, Stéphane; Fielding, Helen H
2014-02-21
There have been a number of recent experimental investigations of the nonadiabatic relaxation dynamics of aniline following excitation to the first three singlet excited states, 1(1)ππ*, 1(1)π3s/πσ* and 2(1)ππ*. Motivated by differences between the interpretations of experimental observations, we have employed CASSCF and XMCQDPT2 calculations to explore the potential energy landscape and relaxation pathways of photoexcited aniline. We find a new prefulvene-like MECI connecting the 1(1)ππ* state with the GS in which the carbon-atom carrying the amino group is distorted out-of-plane. This suggests that excitation above the 1(1)π3s/πσ* vertical excitation energy could be followed by electronic relaxation from the 1(1)ππ* state to the ground-electronic state through this MECI. We find a MECI connecting the 1(1)π3s/πσ* and 1(1)ππ* states close to the local minimum on 1(1)π3s/πσ* which suggests that photoexcitation to the 1(1)π3s/πσ* state could be followed by relaxation to the 1(1)ππ* state and to the dissociative component of the 1(1)π3s/πσ* state. We also find evidence for a new pathway from the 2(1)ππ* state to the ground electronic state that is likely to pass through a three-state conical intersection involving the 2(1)ππ*, 1(1)π3s/πσ* and 1(1)ππ* states.
NASA Astrophysics Data System (ADS)
Li, Jia; Zhang, Shengli; Huang, Shiping; Wang, Peng; Tian, Huiping
2013-02-01
R3ZnH5 (R=K, Rb, Cs) series have been investigated with respect to the crystal structure, electronic and thermodynamic properties using first-principle methods based on density functional theory with generalized gradient approximation. The optimized structures and atomic coordinates are in good agreement with the experimental data. The strong covalent interactions are obtained between Zn and H atoms in the 18-electron [ZnH4]2- complex, while an ionic interaction is found between [ZnH4]2- and R atom. The formation enthalpies show that the formations of R3ZnH5 hydrides are all exothermic at 298 K. The vibration free energies of R3ZnH5 show that the thermodynamic stabilities of R3ZnH5 hydrides decrease with the increasing diameter of R atom. Two possible decomposition reactions of R3ZnH5 series have been suggested in our work. One (reaction one) is that R3ZnH5 hydrides decomposes to elements directly, and the other (reaction two) is that R3ZnH5 hydrides decomposes to RH hydride. The results show that the first decomposition reaction is more favorable one. The spontaneous decomposition reaction of K3ZnH5 hydrides occur upon 465 K via reaction one, and 564 K via reaction two, respectively.
NASA Astrophysics Data System (ADS)
Hiadsi, S.; Bouafia, H.; Sahli, B.; Abidri, B.; Bouaza, A.; Akriche, A.
2016-08-01
This study presents a theoretical prediction of the structural, mechanical, electronic and thermal properties of the zinc-based Perovskites (AgZnF3 and KZnF3) within the framework of Density Functional Theory (DFT) using All-electron self consistent Full Potential Augmented Plane Waves plus local orbital FP-(L)APW + lo method. To make our work comparable and reliable, several functional were used for the exchange-correlation potential. Also, this study intends to provide a basis and an improvement for updating either the values already predicted by other previous work (by using obsolete functional) or to predict them for the first time. GGA-PBE and GGA-PBEsol were used to predict the structural properties of AgZnF3 and KZnF3 Perovskites such as lattice parameter, bulk modulus and its pressure derivative and the cohesive energy. For these properties, the found values are in very good agreement; also those found by GGA-PBEsol are closer to other available previous and experimental results. The electronic properties of these materials are investigated and compared to provide a consolidated prediction by using the modified Becke Johnson potential TB-mBJ with other functional; the values found by this potential are closer to the available proven results and show that these materials exhibit an indirect gap from R to Γ point. The charge densities plot for [110] direction and QTAIM (Quantum Theory of Atoms in Molecules) theory indicate that ionic character is predominate for (K, Ag, Zn)sbnd F bonds. Finally, the effect of temperature and pressure on the unit cell volume, the heat capacity CV and entropy were studied using the quasi-harmonic Debye model.
Wang, Xianlong E-mail: pbeckman@brynmawr.edu; Mallory, Frank B.; Mallory, Clelia W.; Odhner, Hosanna R.; Beckmann, Peter A. E-mail: pbeckman@brynmawr.edu
2014-05-21
We report ab initio density functional theory electronic structure calculations of rotational barriers for t-butyl groups and their constituent methyl groups both in the isolated molecules and in central molecules in clusters built from the X-ray structure in four t-butyl aromatic compounds. The X-ray structures have been reported previously. We also report and interpret the temperature dependence of the solid state {sup 1}H nuclear magnetic resonance spin-lattice relaxation rate at 8.50, 22.5, and 53.0 MHz in one of the four compounds. Such experiments for the other three have been reported previously. We compare the computed barriers for methyl group and t-butyl group rotation in a central target molecule in the cluster with the activation energies determined from fitting the {sup 1}H NMR spin-lattice relaxation data. We formulate a dynamical model for the superposition of t-butyl group rotation and the rotation of the t-butyl group's constituent methyl groups. The four compounds are 2,7-di-t-butylpyrene, 1,4-di-t-butylbenzene, 2,6-di-t-butylnaphthalene, and 3-t-butylchrysene. We comment on the unusual ground state orientation of the t-butyl groups in the crystal of the pyrene and we comment on the unusually high rotational barrier of these t-butyl groups.
Wan, Chuan; Hu, Mary Y.; Jaegers, Nicholas R.; Shi, Dachuan; Wang, Huamin; Gao, Feng; Qin, Zhaohai; Wang, Yong; Hu, Jian Zhi
2016-10-13
The metal-support interaction in γ-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) 27Al MAS NMR spectroscopy, 2D MQMAS, 1H-27Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site, three different tetrahedral Al sites, and three octahedral Al sites. It is found that the penta-coordinated Al (AlP) site density decreases monotonically with an increased WOX loading while the octahedral Al (AlO) site density increases concurrently. This suggests that the Alp sites are the preferred surface anchoring positions for the WOX species. Importantly, the AlP site isotropic chemical shift observed for the unsupported γ-Al2O3 at about 38 ppm migrates into the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the Alp site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed cluster models is carried out to accurately interpret the dramatic chemical shift changes from which the detailed anchoring mechanisms are obtained. It is found that tungsten dimers and monomers are the preferred supported surface species on γ-Al2O3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.
Using electron microscopy to calculate optical properties of biological samples.
Wu, Wenli; Radosevich, Andrew J; Eshein, Adam; Nguyen, The-Quyen; Yi, Ji; Cherkezyan, Lusik; Roy, Hemant K; Szleifer, Igal; Backman, Vadim
2016-11-01
The microscopic structural origins of optical properties in biological media are still not fully understood. Better understanding these origins can serve to improve the utility of existing techniques and facilitate the discovery of other novel techniques. We propose a novel analysis technique using electron microscopy (EM) to calculate optical properties of specific biological structures. This method is demonstrated with images of human epithelial colon cell nuclei. The spectrum of anisotropy factor g, the phase function and the shape factor D of the nuclei are calculated. The results show strong agreement with an independent study. This method provides a new way to extract the true phase function of biological samples and provides an independent validation for optical property measurement techniques.
Using electron microscopy to calculate optical properties of biological samples
Wu, Wenli; Radosevich, Andrew J.; Eshein, Adam; Nguyen, The-Quyen; Yi, Ji; Cherkezyan, Lusik; Roy, Hemant K.; Szleifer, Igal; Backman, Vadim
2016-01-01
The microscopic structural origins of optical properties in biological media are still not fully understood. Better understanding these origins can serve to improve the utility of existing techniques and facilitate the discovery of other novel techniques. We propose a novel analysis technique using electron microscopy (EM) to calculate optical properties of specific biological structures. This method is demonstrated with images of human epithelial colon cell nuclei. The spectrum of anisotropy factor g, the phase function and the shape factor D of the nuclei are calculated. The results show strong agreement with an independent study. This method provides a new way to extract the true phase function of biological samples and provides an independent validation for optical property measurement techniques. PMID:27896013
NASA Astrophysics Data System (ADS)
Das, Debashish; Ganguly, Shreemoyee; Sanyal, Biplab; Ghosh, Subhradip
2016-10-01
CoCr2O4 has attracted significant attention recently due to several interesting properties such as magnetostriction, magnetoelectricity etc. More recent experiments on Fe substituted CoCr2O4 observed a variety of novel phenomena such as the magnetic compensation accompanied by the occurrence of exchange bias, which reverses its sign. Understanding of such phenomena may lead to control the properties of these material in an efficient way to enhance its potential for multifunctional applications. In this paper, we study the fundamental understanding of Fe doping in modifying the structural and magnetic properties of CoCr2O4 with varying composition and substitution of Fe at different sublattices by first-principles density functional calculations. We have analysed in detail the effect of Fe substitution on crystal field and exchange splittings, magnetic moments and interatomic exchange parameters. It is also observed that with increasing concentration of Fe impurity, the system has a tendency towards forming an ‘inverse spinel’ structure as observed in experiments. Such tendencies are crucial to understand this system as it would lead to modifications in the magnetic exchange interactions associated with sites with different symmetry finally affecting the magnetic structure and the multiferrocity in turn.
NASA Astrophysics Data System (ADS)
Long, Run; English, Niall J.
2011-04-01
The electronic structures of Mg/Ca- and/or Mo/W- (mono- and co-) doped anatase TiO2 have been investigated via generalized Kohn-Sham theory with the Heyd-Scuseria-Ernzerhof hybrid functional for exchange-correlation {J. Heyd et al., [J. Chem. Phys. 118, 8207 (2003)], J. Heyd et al., [J. Chem. Phys. 124, 219906 (2006)], and J. Paier et al., [J. Chem. Phys. 125, 249901 (2006)]}, in the context of density functional theory. Gap narrowing is small for monodoping, which also creates impuritiy bands in the "forbidden gap," either as acceptor or donor states, limiting possible utility as visible-light photocatalysts. However, codoping of Mg/Ca and Mo/W not only induces appreciable gap narrowing, but also serves to passivate the impurity bands, which can harvest visible-light to a greater extent. Considering ionic radii, Mg and Mo should constitute the best cation-pair.
Liu, J. J.; Fu, X. L.; Chen, S. F.; Zhu, Y. F.
2011-11-07
The electronic structure and optical properties of Ag{sub 3}PO{sub 4} were studied by hybrid density functional theory. The results indicated that the band gap is 2.43 eV, which agrees well with the experimental value of 2.45 eV. The conduction bands of Ag{sub 3}PO{sub 4} are mainly attributable to Ag 5s and 5p states, while the valence bands are dominated by O 2p and Ag 4d states. The highest valence band edge potential was 2.67 V (vs. normal hydrogen electrode), which has enough driving force for photocatalytic water oxidation and pollutants degradation. The optical absorption spectrum showed that Ag{sub 3}PO{sub 4} is a visible light response photocatalyst.
Arockia Doss, M; Savithiri, S; Rajarajan, G; Thanikachalam, V; Anbuselvan, C
2015-12-05
FT-IR and FT-Raman spectra of 3-pentyl-2,6-di(furan-2-yl) piperidin-4-one (3-PFPO) were recorded in the solid phase. The structural and spectroscopic analyses of 3-PFPO were made by using B3LYP/HF level with 6-311++G(d, p) basis set. The fundamental vibrations are assigned on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method and PQS program. Comparison of the observed fundamental vibrational frequencies of 3-PFPO with calculated results by HF and DFT methods indicates that B3LYP is superior to HF method for molecular vibrational problems. The electronic properties such as excitation energies, oscillator strength, wavelengths and HOMO-LUMO energies were obtained by time-dependent DFT (TD-DFT) approach. The polarizability and first order hyperpolarizability of the title molecule were calculated and interpreted. The hyperconjugative interaction energy (E((2))) and electron densities of donor (i) and acceptor (j) bonds were calculated using NBO analysis. In addition, MEP and atomic charges of carbon, nitrogen, oxygen and hydrogen were calculated using B3LYP/6-311++G(d, p) level theory. Moreover, thermodynamic properties (heat capacities, entropy and enthalpy) of the title compound at different temperatures were calculated in gas phase.
Three-dimensional rf structure calculations
Cooper, R.K.; Browman, M.J.; Weiland, T.
1988-01-01
The calculation of three-dimensional rf structures is rapidly approaching adolescence, after having been in its infancy for the last four years. This paper will show the kinds of calculations that are currently being performed in the frequency domain and is a companion paper to one in which time-domain calculations are described. 13 refs., 14 figs.
Three-dimensional RF structure calculations
NASA Astrophysics Data System (ADS)
Cooper, R. K.; Browman, M. J.; Weiland, T.
1989-04-01
The calculation of three-dimensional rf structures is rapidly approaching adolescence, after having been in its infancy for the last four years. This paper will show the kinds of calculations that are currently being performed in the frequency domain and is a companion paper to one in which time-domain calculations are described.
NASA Astrophysics Data System (ADS)
Liu, Hong-Xia; Tang, Fu-Ling; Xue, Hong-Tao; Zhang, Yu; Cheng, Yu-Wen; Feng, Yu-Dong
2016-12-01
Using the first-principles plane-wave calculations within density functional theory, the perfect bi-layer and monolayer terminated WZ-CIS (100)/WZ-CdS (100) interfaces are investigated. After relaxation the atomic positions and the bond lengths change slightly on the two interfaces. The WZ-CIS/WZ-CdS interfaces can exist stably, when the interface bonding energies are -0.481 J/m2 (bi-layer terminated interface) and -0.677 J/m2 (monolayer terminated interface). Via analysis of the density of states, difference charge density and Bader charges, no interface state is found near the Fermi level. The stronger adhesion of the monolayer terminated interface is attributed to more electron transformations and orbital hybridizations, promoting stable interfacial bonds between atoms than those on a bi-layer terminated interface. Project supported by the National Natural Science Foundation of China (Grant Nos. 11164014 and 11364025) and the Gansu Science and Technology Pillar Program, China (Grant No. 1204GKCA057).
NASA Astrophysics Data System (ADS)
Huang, Wenchao; Wang, Xiaofang; Chen, Xiaoshuang; Lu, Wei; Damewood, L.; Fong, C. Y.
2015-03-01
First principles calculations with spin polarization based on density functional theory have been performed on half-Heusler alloys PtXBi, with X=Mn, Fe, Co and Ni, in three different atomic configurations (i.e. α, β, and γ phases). For each configuration, their optimized lattice constants are determined. Electronic and magnetic properties are also investigated. The differences reflect the atomic arrangements of the three phases and varied transition metal elements X. Meanwhile, the possibility of having the integer magnetic moment for each phase is explored. PtMnBi in α phase show half-metallic (HM) properties when its lattice constant is reduced from -3.0% to -11.2% with magnetic moment consistent with the values given by the modified Slater-Pauling rule. Additionally, we examined the effects of the spin-orbit (S-O) interaction on half-metal PtMnBi by comparing the relative shifts of the valence bands and the indirect semiconducting gap with respect to the spin polarized results.
Calculation of electron wave functions and refractive index of Ne
NASA Astrophysics Data System (ADS)
Zhu, Min; Liu, Wei; Zhang, Tao
2008-10-01
The radial wave functions of inner electron shell and outer electron shell of a Ne atom were obtained by the approximate analytical method and tested by calculating the ground state energy of the Ne atom. The equivalent volume of electron cloud and the refractive index of Ne were calculated. The calculated refractive index agrees well with the experimental result. Relationship between the refractive index and the wave function of Ne was discovered.
NASA Astrophysics Data System (ADS)
Barragan-Yani, Daniel; Albe, Karsten
2017-03-01
Structural and electronic properties of screw and 60∘-mixed glide and shuffle dislocations in the solar absorber materials CuInSe2 and CuGaSe2 are investigated by means of electronic structure calculations within density functional theory (DFT). Screw dislocations present distorted bonds but remain fully coordinated after structural relaxation. Relaxed 60∘-mixed dislocations, in contrast, exhibit dangling and "wrong," cation-cation or anion-anion bonds, which induce deep charge transition levels and are electrically active. Analysis of Bader charges and local density of states (LDOS) reveals that acceptor and donor levels are induced by α and β cores, respectively. Moreover, there is local charge accumulation in the surrounding of those cores which contain dangling or "wrong" bonds. Thus the apparently harmless nature of dislocations is not because they are electrically inactive, but can only be a result of passivation by segregating defects.
NASA Astrophysics Data System (ADS)
Chain, Fernando E.; Ladetto, María Florencia; Grau, Alfredo; Catalán, César A. N.; Brandán, Silvia Antonia
2016-02-01
In the present work, the structural, topological and vibrational properties of four members of the N-benzylamides series derived from Maca (Lepidium meyenii) whose names are, N-benzylpentadecanamide, N-benzylhexadecanamide, N-benzylheptadecanamide and N-benzyloctadecanamide, were studied combining the FTIR, FT-Raman and 1H and 13C-NMR spectroscopies with density functional theory (DFT) and ONION calculations. Furthermore, the N-benzylacetamide, N-benzylpropilamide and N-benzyl hexanamide derivatives were also studied in order to compare their properties with those computed for the four macamides. These seven N-benzylamides series have a common structure, C8H8NO-R, being R the side chain [-(CH2)n-CH3] with a variable n number of CH2 groups. Here, the atomic charges, molecular electrostatic potentials, stabilization energies, topological properties of those macamides were analyzed as a function of the number of C atoms of the side chain while the frontier orbitals were used to compute the gap energies and some descriptors in order to predict their reactivities and behaviors in function of the longitude of the side chain. Here, the force fields, the complete vibrational assignments and the corresponding force constants were only reported for N-benzylacetamide, N-benzyl hexanamide and N-benzylpentadecanamide due to the high number of vibration normal modes that present the remains macamides.
Suturina, Elizaveta A; Nehrkorn, Joscha; Zadrozny, Joseph M; Liu, Junjie; Atanasov, Mihail; Weyhermüller, Thomas; Maganas, Dimitrios; Hill, Stephen; Schnegg, Alexander; Bill, Eckhard; Long, Jeffrey R; Neese, Frank
2017-03-06
The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)2[Co(SPh)4] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)4](2-) upon structural variations, depending on the counterion and crystallization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)4](2-): (PPh4)2[Co(SPh)4] (1) and (NEt4)2[Co(SPh)4] (2). The characterization methods employed include multifield SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic circular dichroism (VTVH-MCD), and frequency domain Fourier transform THz-EPR (FD-FT THz-EPR). Notably, the paramagnetic Co(II) complex [Co(SPh)4](2-) shows strong axial magnetic anisotropy in 1, with D = -55(1) cm(-1) and E/D = 0.00(3), but rhombic anisotropy is seen for 2, with D = +11(1) cm(-1) and E/D = 0.18(3). Multireference ab initio CASSCF/NEVPT2 calculations enable interpretation of the remarkable variation of D and its dependence on the electronic structure and geometry.
Reboredo, F A; Hood, R Q; Kent, P C
2009-01-06
We develop a formalism and present an algorithm for optimization of the trial wave-function used in fixed-node diffusion quantum Monte Carlo (DMC) methods. The formalism is based on the DMC mixed estimator of the ground state probability density. We take advantage of a basic property of the walker configuration distribution generated in a DMC calculation, to (i) project-out a multi-determinant expansion of the fixed node ground state wave function and (ii) to define a cost function that relates the interacting-ground-state-fixed-node and the non-interacting trial wave functions. We show that (a) locally smoothing out the kink of the fixed-node ground-state wave function at the node generates a new trial wave function with better nodal structure and (b) we argue that the noise in the fixed-node wave function resulting from finite sampling plays a beneficial role, allowing the nodes to adjust towards the ones of the exact many-body ground state in a simulated annealing-like process. Based on these principles, we propose a method to improve both single determinant and multi-determinant expansions of the trial wave function. The method can be generalized to other wave function forms such as pfaffians. We test the method in a model system where benchmark configuration interaction calculations can be performed and most components of the Hamiltonian are evaluated analytically. Comparing the DMC calculations with the exact solutions, we find that the trial wave function is systematically improved. The overlap of the optimized trial wave function and the exact ground state converges to 100% even starting from wave functions orthogonal to the exact ground state. Similarly, the DMC total energy and density converges to the exact solutions for the model. In the optimization process we find an optimal non-interacting nodal potential of density-functional-like form whose existence was predicted in a previous publication [Phys. Rev. B 77 245110 (2008)]. Tests of the method are
Naumov, Pance; Sakurai, Kenji; Ishikawa, Tadahiko; Takahashi, Junichi; Koshihara, Shin-ya; Ohashi, Yuji
2005-08-18
The nitro-assisted proton transfer (NAPT), responsible for the photoactivity of ortho-nitrobenzylpyridines and a model for the nitro-based caged compounds, is studied along with the parent compound 2-(2',4'-dinitrobenzyl)pyridine (DNBP) with polarized optical spectroscopy and theoretical calculations. The transition dipole moments of a DNBP single-crystal identified oriented molecules of the long-lived enamine tautomer (NH), rather than of the aci-nitro tautomer (OH), as carriers of the photoinduced blue coloration. It is clarified that the blue second singlet transition owes to intramolecular charge transfer from the allyl-pyridinium part to the dinitrophenyl fragment of NH. The theoretical modeling of the ground-state potential energy surface showed that while NH and OH can interconvert by means of direct proton transfer, such a process between the initial form CH and either OH and NH would require significant rotation of the aromatic rings. In the ground state, OH is less stable but the kinetically preferred product over NH. Once created, regardless of whether via ground-state or excited-state routes, the aci-nitro group of OH undergoes energetically inexpensive rotation to deliver the proton to the nitrogen acceptor. The "softening" of the energy surface around OH due to its structural flexibility, that is, mediation of the proton transfer by the nitro group, is crucial to overcome the high barrier for a direct proton jump from CH to NH, even in cases of unfavorable donor-acceptor geometry. The very small structural change experienced by the surrounding of a molecule undergoing NAPT is promising for the design of photoactive systems which retain their crystallinity during a prolonged operation.
2006-04-03
2) Substituting a vinyl hydrogen with a fluorine presents an interesting situation for electrophilic reactions. The π-bond is less...reactive toward electrophiles due to the electron-withdrawing effect of the vinyl fluorine . Therefore, carbocations or radical cations are destabilized...NUMBER Distributions from Terminal Alkenes: The Effect of Electron-Withdrawing Fluorine Substituents on the Structure and Charge Localization of
NASA Astrophysics Data System (ADS)
Sun, Liang; Gao, Yimin; Yoshida, Katsumi; Yano, Toyohiko; Li, Yefei; Liu, Yangzhen
2017-01-01
Structural, electronic, elastic and thermal properties of Al4Si2C5 under constant pressure were investigated by using first-principles theory. The total volume of the cell decreased by almost 15.7% under 40 GPa which is smaller than that of Al4SiC4 (16%), while the linear compressibility along a- or b-axis direction showed better anti-deformation behavior than that of along c-axis direction. The peak heights of total density of state (TDOS) and partial density of state (PDOS) curves of Al4Si2C5 are slightly lowered with forced high pressure. Meanwhile, the mechanical properties of Al4Si2C5-like elastic constants and elastic moduli accelerate with the pressure increasing from 0 GPa to 40 GPa; the thermal expansion coefficient α increases rapidly at lower temperature and this tendency gradually approaches a linear increase when the temperature is above 1000 K. At particular temperature, α decreases continuously with the pressure accelerating. Heat capacity at constant volume (CV) with pressure was also evaluated, the results displayed that CV is sensitive with the temperature rather than the pressure. The elastic anisotropy and Debye temperature with pressure were successfully obtained and discussed.
Gagarin, S.G.; Teterin, Yu.A.; Plekhanov, Yu.V.
1986-05-01
The x-ray photoelectron spectra of the core 2p/sub 1/2/, 2p/sub 3/2/, 3s/sub 1/2/, 3p/sub 1/2/, and 3/p/sub 3/2/ electrons of nickel in the oxide NiO and in an aluminum - nickel catalyst have been discussed on the basis of the results of a self-consistent calculation of the electronic states of the NiO/sub 6//sup 10 -/ cluster in the spin-polarized variation of the X..cap alpha..-scattered-wave method with complete consideration of the relativistic properties of the core electrons in the angular-momentum-polarized variant. Partial consideration of the relativistic properties of the valence-band electrons (consideration of the Darwin terms and the dependence of the mass on the velocity during the averaging of the wave functions and the density with respect to the angular momentum) does not alter the basic conclusions of the work.
Electronic structure quantum Monte Carlo
NASA Astrophysics Data System (ADS)
Bajdich, Michal; Mitas, Lubos
2009-04-01
Quantum Monte Carlo (QMC) is an advanced simulation methodology for studies of manybody quantum systems. The QMC approaches combine analytical insights with stochastic computational techniques for efficient solution of several classes of important many-body problems such as the stationary Schrödinger equation. QMC methods of various flavors have been applied to a great variety of systems spanning continuous and lattice quantum models, molecular and condensed systems, BEC-BCS ultracold condensates, nuclei, etc. In this review, we focus on the electronic structure QMC, i.e., methods relevant for systems described by the electron-ion Hamiltonians. Some of the key QMC achievements include direct treatment of electron correlation, accuracy in predicting energy differences and favorable scaling in the system size. Calculations of atoms, molecules, clusters and solids have demonstrated QMC applicability to real systems with hundreds of electrons while providing 90-95% of the correlation energy and energy differences typically within a few percent of experiments. Advances in accuracy beyond these limits are hampered by the so-called fixed-node approximation which is used to circumvent the notorious fermion sign problem. Many-body nodes of fermion states and their properties have therefore become one of the important topics for further progress in predictive power and efficiency of QMC calculations. Some of our recent results on the wave function nodes and related nodal domain topologies will be briefly reviewed. This includes analysis of few-electron systems and descriptions of exact and approximate nodes using transformations and projections of the highly-dimensional nodal hypersurfaces into the 3D space. Studies of fermion nodes offer new insights into topological properties of eigenstates such as explicit demonstrations that generic fermionic ground states exhibit the minimal number of two nodal domains. Recently proposed trial wave functions based on Pfaffians with
A time-dependent embedding calculation of surface electron emission.
Inglesfield, J E
2011-08-03
The Dirac-Frenkel variational principle is used to derive the embedding method for solving the time-dependent Schrödinger equation. Embedding allows the time evolution of the wavefunction to be calculated explicitly in a limited region of space, the region of physical interest, the embedding potential ensuring that the wavefunction satisfies the correct boundary conditions for matching on to the rest of the system. This is applied to a study of the excitation of electrons at a metal surface, represented by a one-dimensional model potential for Cu(111). Time-dependent embedding potentials are derived for replacing the bulk substrate, and the image potential and vacuum region outside the surface, so that the calculation of electron excitation by a surface perturbation can be restricted to the surface itself. The excitation of the Shockley surface state and a continuum bulk state is studied, and the time structure of the resulting currents analysed. There is a distinction between emission from the localized surface state, where the charge is steadily depleted, and the extended continuum state, where the current emitted into the vacuum is compensated by current approaching the surface from the bulk. The time taken for the current to arrive outside the surface is studied.
NASA Astrophysics Data System (ADS)
Boettger, Jonathan C.; Ray, Asok K.
2000-07-01
The fluorite structure light-actinide dioxides, uranium dioxide and plutonium dioxide, are both known to be prototypical Mott-Hubbard insulators, with band gaps produced by strong Coulomb correlation effects that are not adequately accounted for in traditional density functional theory (DFT) calculations. Indeed, DFT electronic structure calculations for these two actinide dioxides have been shown to incorrectly predict metallic behavior. The highly-correlated electron effects exhibited by the actinide dioxides, combined with the large relativistic effects (including spin-orbit coupling) expected for any actinide compound, provide an extreme challenge for electronic structure theorists. For this reason, few fully-self-consistent DFT calculations have been carried out for the actinide dioxides, in general, and only one for plutonium dioxide. In that calculation, the troublesome 5f electrons were treated as core electrons, and spin-orbit coupling was ignored.
NASA Astrophysics Data System (ADS)
Ricca, Chiara; Ringuedé, Armelle; Cassir, Michel; Adamo, Carlo; Labat, Frédéric
2016-05-01
The structural, electronic and surface properties of the mixed lithium-sodium (LiNaCO3) and lithium-potassium (LiKCO3) carbonates were studied through periodic calculations performed at the density functional theory (DFT) level, using three different exchange-correlation functionals. The hybrid functional PBE0 was found to be the best one to describe both geometric and electronic features of bulk LiNaCO3 and LiKCO3. Polar (001) and non-polar (110) low index surfaces were taken into account, the first one being found the most stable in both cases, after reconstruction. Both introduction of vacancies (R1) and octopolar terminations (R2) of (001), exposing Li ((001)Li) or Na ((001)Na) were described in detail. The computed stability order for the reconstructed surfaces in gas phase is: (001)R1Na > > (001)R1Li > (001)R2Na ≈ (001)R2Li. The obtained information, in particular regarding the electronic and surface properties, could be used in future to help understanding the role of mixed carbonates as component of oxide-carbonate electrolytes for low temperature solid oxide fuel cells (LT-SOFCs) applications, especially as reasonable starting points for dynamics calculations of liquid molten carbonates based systems.
Electronic Structure of Small Lanthanide Containing Molecules
NASA Astrophysics Data System (ADS)
Kafader, Jared O.; Ray, Manisha; Topolski, Josey E.; Chick Jarrold, Caroline
2016-06-01
Lanthanide-based materials have unusual electronic properties because of the high number of electronic degrees of freedom arising from partial occupation of 4f orbitals, which make these materials optimal for their utilization in many applications including electronics and catalysis. Electronic spectroscopy of small lanthanide molecules helps us understand the role of these 4f electrons, which are generally considered core-like because of orbital contraction, but are energetically similar to valence electrons. The spectroscopy of small lanthanide-containing molecules is relatively unexplored and to broaden this understanding we have completed the characterization of small cerium, praseodymium, and europium molecules using photoelectron spectroscopy coupled with DFT calculations. The characterization of PrO, EuH, EuO/EuOH, and CexOy molecules have allowed for the determination of their electron affinity, the assignment of numerous anion to neutral state transitions, modeling of anion/neutral structures and electron orbital occupation.
Calculation of electron-impact ionization of potassium
NASA Astrophysics Data System (ADS)
Bray, I.; Fursa, D. V.; Stelbovics, A. T.
2009-11-01
We calculate electron-impact ionization of potassium at a broad range of energies for the case where it is the valence electron that is ejected. The convergent close-coupling method is used to calculate the total and fully differential cross sections. The unusual shape of the total ionization spin asymmetries measured by Baum et al. [1] is explained. However, agreement with the fully differential cross section measurements of Murray [2] is somewhat mixed.
Calculating Buckling And Vibrations Of Lattice Structures
NASA Technical Reports Server (NTRS)
Anderson, M. S.; Durling, B. J.; Herstrom, C. L.; Williams, F. W.; Banerjee, J. R.; Kennedy, D.; Warnaar, D. B.
1989-01-01
BUNVIS-RG computer program designed to calculate vibration frequencies or buckling loads of prestressed lattice structures used in outer space. For buckling and vibration problems, BUNVIS-RG calculates deadload axial forces caused in members by any combination of externally-applied static point forces and moments at nodes, axial preload or prestrain in members, and such acceleration loads as those due to gravity. BUNVIS-RG is FORTRAN 77 computer program implemented on CDC CYBER and VAX computer.
Enhancing Scalability of Parallel Structured AMR Calculations
Wissink, A M; Hysom, D; Hornung, R D
2003-02-10
This paper discusses parallel scaling performance of large scale parallel structured adaptive mesh refinement (SAMR) calculations in SAMRAI. Previous work revealed that poor scaling qualities in the adaptive gridding operations in SAMR calculations cause them to become dominant for cases run on up to 512 processors. This work describes algorithms we have developed to enhance the efficiency of the adaptive gridding operations. Performance of the algorithms is evaluated for two adaptive benchmarks run on up 512 processors of an IBM SP system.
Benchmark Calculations of Electron-Impact Differential Cross Sections
Bray, I.; Bostock, C. J.; Fursa, D. V.; Hines, C. W.; Kadyrov, A. S.; Stelbovics, A. T.
2011-05-11
The calculation of electron-atom excitation and ionization cross section is considered in both the non-relativistic and relativistic scattering theory. We consider electron collisions with H, He, Cs, and Hg. Differential cross sections for elastic scattering and ionization are presented.
Zhao, Zong-Yan; Liu, Qing-Lu; Dai, Wen-Wu
2016-01-01
Six BiOX1−xYx (X, Y = F, Cl, Br, and I) solid solutions have been systematically investigated by density functional theory calculations. BiOCl1−xBrx, BiOBr1−xIx, and BiOCl1−xIx solid solutions have very small bowing parameters; as such, some of their properties increase almost linearly with increasing x. For BiOF1−xYx solid solutions, the bowing parameters are very large and it is extremely difficult to fit the related calculated data by a single equation. Consequently, BiOX1−xYx (X, Y = Cl, Br, and I) solid solutions are highly miscible, while BiOF1−xYx (Y = Cl, Br, and I) solid solutions are partially miscible. In other words, BiOF1−xYx solid solutions have miscibility gaps or high miscibility temperature, resulting in phase separation and F/Y inhomogeneity. Comparison and analysis of the calculated results and the related physical–chemical properties with different halogen compositions indicates that the parameters of BiOX1−xYx solid solutions are determined by the differences of the physical–chemical properties of the two halogen compositions. In this way, the large deviation of some BiOX1−xYx solid solutions from Vegard’s law observed in experiments can be explained. Moreover, the composition ratio of BiOX1−xYx solid solutions can be measured or monitored using optical measurements. PMID:27549344
NASA Astrophysics Data System (ADS)
Zhao, Zong-Yan; Liu, Qing-Lu; Dai, Wen-Wu
2016-08-01
Six BiOX1‑xYx (X, Y = F, Cl, Br, and I) solid solutions have been systematically investigated by density functional theory calculations. BiOCl1‑xBrx, BiOBr1‑xIx, and BiOCl1‑xIx solid solutions have very small bowing parameters; as such, some of their properties increase almost linearly with increasing x. For BiOF1‑xYx solid solutions, the bowing parameters are very large and it is extremely difficult to fit the related calculated data by a single equation. Consequently, BiOX1‑xYx (X, Y = Cl, Br, and I) solid solutions are highly miscible, while BiOF1‑xYx (Y = Cl, Br, and I) solid solutions are partially miscible. In other words, BiOF1‑xYx solid solutions have miscibility gaps or high miscibility temperature, resulting in phase separation and F/Y inhomogeneity. Comparison and analysis of the calculated results and the related physical–chemical properties with different halogen compositions indicates that the parameters of BiOX1‑xYx solid solutions are determined by the differences of the physical–chemical properties of the two halogen compositions. In this way, the large deviation of some BiOX1‑xYx solid solutions from Vegard’s law observed in experiments can be explained. Moreover, the composition ratio of BiOX1‑xYx solid solutions can be measured or monitored using optical measurements.
NASA Astrophysics Data System (ADS)
Mizutani, U.; Asahi, R.; Sato, H.; Takeuchi, T.
2006-12-01
The mechanism for the stability of the Al8V5γ -brass containing 52 atoms in its cubic unit cell has been investigated by means of first-principles full-potential linearized augmented plane wave (FLAPW) and linearized muffin-tin orbital-atomic sphere approximation (LMTO-ASA) electronic structure calculations. The LMTO-ASA identified a deep valley at 0.5eV above the Fermi level in its density of states (DOS) as arising from orbital hybridizations between V 3d and Al 3p states. On the other hand, the FLAPW revealed the V 3d states mediated resonance of electrons with different sets of lattice planes. The resonance involved is found to be substantial not only at ∣G∣2=18 or {330} and {411} zones but also at those in the range 14⩽∣G∣2⩽30 . A comparison with the electronic structure of the CsCl-type AlV compound proved that the V 3d states mediated resonance occurs only in Al8V5 but not in AlV compound. The V 3d states mediated resonance is proved to result in a significant suppression of the sp -partial DOS over the energy range from the Fermi level up to +2.2eV . A gain in the electronic energy has been attributed to the formation of highly condensed bonding states below the Fermi level, again caused by the V 3d states mediated resonance. It is also proposed that the Al8V5 is stabilized at e/a=1.94 rather than 21/13 as is expected from the Hume-Rothery electron concentration rule.
Dorofeeva, Olga V; Vogt, Natalja; Vogt, Jürgen; Popik, Mikhail V; Rykov, Anatolii N; Vilkov, Lev V
2007-07-19
The molecular structure of 1,3-dihydroxyacetone (DHA) has been studied by gas-phase electron diffraction (GED), combined analysis of GED and microwave (MW) data, ab initio, and density functional theory calculations. The equilibrium re structure of DHA was determined by a joint analysis of the GED data and rotational constants taken from the literature. The anharmonic vibrational corrections to the internuclear distances (re-ra) and to the rotational constants (B(i)e-B(i)0) needed for the estimation of the re structure were calculated from the B3LYP/cc-pVTZ cubic force field. It was found that the experimental data are well reproduced by assuming that DHA consists of a mixture of three conformers. The most stable conformer of C2v symmetry has two hydrogen bonds, whereas the next two lowest energy conformers (Cs and C1 symmetry) have one hydrogen bond and their abundance is about 30% in total. A combined analysis of GED and MW data led to the following equilibrium structural parameters (re) of the most abundant conformer of DHA (the uncertainties in parentheses are 3 times the standard deviations): r(C=O)=1.215(2) A, r(C-C)=1.516(2) A, r(C-O)=1.393(2) A, r(C-H)=1.096(4) A, r(O-H)=0.967(4) A, angleC-C=O=119.9(2) degrees, angleC-C-O=111.0(2) degrees, angleC-C-H=108.2(7) degrees, angleC-O-H=106.5(7) degrees. These structural parameters reproduce the experimental B(i)0 values within 0.05 MHz. The experimental structural parameters are in good agreement with those obtained from theoretical calculations. Ideal gas thermodynamic functions (S degrees (T), C degrees p(T), and H degrees (T)-H degrees (0)) of DHA were calculated on the basis of experimental and theoretical molecular parameters obtained in this work. The enthalpy of formation of DHA, -523+/-4 kJ/mol, was calculated by the atomization procedure using the G3X method.
Li, Shenggang; Zhai, Hua-Jin; Wang, Lai-Sheng; Dixon, David A.
2009-09-28
We report a comparative study of reduced transition metal oxide clusters, M₃O₈⁻ (M = Cr, W) anions and their neutrals, via anion photoelectron spectroscopy (PES) and density functional theory (DFT) and molecular orbital theory (CCSD(T)) calculations. Well-resolved PES spectra are obtained for M₃O₈⁻ (M = Cr, W) at 193 and 157 nm photon energies. Different PES spectra are observed for M = Cr versus M = W. ExtensiveDFT and CCSD(T) calculations are performed to locate the ground and low-lying excited states for the neutrals and anions. The ground states of Cr₃O₈ and Cr₃O₈⁻ are predicted to be the ³B₂ and ⁴B₂ states of a C₂v structure, respectively, revealing ferromagnetic spin coupling for Cr 3d electrons. In contrast, the ground states of W₃O₈ and W₃O₈⁻ are predicted to be the ¹A' state (Cs symmetry) and the ²A₁ state (C₂v symmetry), respectively, showing metal-metal d-d bonding in the anion. The current cluster geometries are in qualitative agreement with prior DFT studies at the PBE level for M = Cr and the B3LYP level for M = W. The BP86 and PW91 functionals significantly outperform the B3LYP functional for the Cr species, in terms of relative energies, electron detachment energies, and electronic excitation energies, whereas the B3LYP functional is better for the W species. Accurate heats of formation for the ground states of M₃O₈ are calculated from the clustering energies and the heats of formation of MO₂ and MO₃. The energetics have been used to predict redox reaction thermochemistry.
Electronic structure interpolation via atomic orbitals.
Chen, Mohan; Guo, G-C; He, Lixin
2011-08-17
We present an efficient scheme for accurate electronic structure interpolation based on systematically improvable optimized atomic orbitals. The atomic orbitals are generated by minimizing the spillage value between the atomic basis calculations and the converged plane wave basis calculations on some coarse k-point grid. They are then used to calculate the band structure of the full Brillouin zone using the linear combination of atomic orbitals algorithms. We find that usually 16-25 orbitals per atom can give an accuracy of about 10 meV compared to the full ab initio calculations, and the accuracy can be systematically improved by using more atomic orbitals. The scheme is easy to implement and robust, and works equally well for metallic systems and systems with complicated band structures. Furthermore, the atomic orbitals have much better transferability than Shirley's basis and Wannier functions, which is very useful for perturbation calculations.
First Principles Structure Calculations Using the General Potential Lapw Method
NASA Astrophysics Data System (ADS)
Wei, Su-Huai
We have developed a completely general first principles self-consistent full-potential linearized-augmented-plane -wave (LAPW) method program within the density functional formalism to calculate electronic band structure, total energy, pressure and other quantities. No symmetry assumptions are used for the crystal structure. Shape unrestricted charge densities and potentials are calculated inside muffin -tin (MT) spheres as well as in the interstitial regions. All contributions to the Hamiltonian matrix elements are completely taken into account. The core states are treated fully relativistically using the spherical part of the potential only. Scalar relativistic effects are included for the band-states, and spin-orbit coupling is included using a second variation procedure. Both core states and valence states are treated self-consistently, the frozen core approximation is not required. The fast Fourier transformation method is used wherever it is applicable, and this greatly improves the efficiency. This state-of-the-art program has been tested extensively to check the accuracy and convergence properties by comparing calculated electronic band structures, ground state properties, equations of state and cohesive energies for bulk W and GaAs with other theoretical calculations and experimental results. It has been successfully applied to calculate and predict structural and metal-insulator phase transitions for close-packed crystal BaSe and BaTe and the geometric structure of the d-band metal W(001) surface. The results are in generally good agreement with experiment.
NASA Astrophysics Data System (ADS)
Pourmatin, Hossein; Dayal, Kaushik
2016-10-01
We consider the scattering of incident plane-wave electrons from a defect in a crystal modeled by the time-harmonic Schrödinger equation. While the defect potential is localized, the far-field potential is periodic, unlike standard free-space scattering problems. Previous work on the Schrödinger equation has been almost entirely in free-space conditions; a few works on crystals have been in one-dimension. We construct absorbing boundary conditions for this problem using perfectly matched layers in a tight-binding formulation. Using the example of a point defect in graphene, we examine the efficiency and convergence of the proposed absorbing boundary condition.
Electron photon verification calculations using MCNP4B
Gierga, D.P.; Adams, K.J.
1998-07-01
MCNP4B was released in February 1997 with significant enhancements to electron/photon transport methods. These enhancements have been verified against a wide range of published electron/photon experiments, spanning high energy bremsstrahlung production to electron transmission and reflection. Three sets of bremsstrahlung experiments were simulated. The first verification calculations for bremsstrahlung production used the experimental results in Faddegon for 15 MeV electrons incident on lead, aluminum, and beryllium targets. The calculated integrated bremsstrahlung yields, the bremsstrahlung energy spectra, and the mean energy of the bremsstrahlung beam were compared with experiment. The impact of several MCNP tally options and physics parameters was explored in detail. The second was the experiment of O`Dell which measured the bremsstrahlung spectra from 10 and 20.9 MeV electrons incident on a gold/tungsten target. The final set was a comparison of relative experimental spectra with calculated results for 9.66 MeV electrons incident on tungsten based on the experiment of Starfelt and Koch. The transmission experiments of Ebert were also studied, including comparisons of transmission coefficients for 10.2 MeV electrons incident on carbon, silver, and uranium foils. The agreement between experiment and simulation was usually within two standard deviations of the experimental and calculational errors.
Electronic transport in nanoscale structures
NASA Astrophysics Data System (ADS)
Lagerqvist, Johan
In this dissertation electronic transport in nanoscale structures is discussed. An expression for the shot noise, a fluctuation in current due to the discreteness of charge, is derived directly from the wave functions of a nanoscale system. Investigation of shot noise is of particular interest due to the rich fundamental physics involved. For example, the study of shot noise can provide fundamental insight on the nature of electron transport in a nanoscale junction. We report calculations of the shot noise properties of parallel wires in the regime in which the interwire distance is much smaller than the inelastic mean free path. The validity of quantized transverse momenta in a nanoscale structure and its effect on shot noise is also discussed. We theoretically propose and show the feasibility of a novel protocol for DNA sequencing based on the electronic signature of single-stranded DNA while it translocates through a nanopore. We find that the currents for the bases are sufficiently different to allow for efficient sequencing. Our estimates reveal that sequencing of an entire human genome could be done with very high accuracy in a matter of hours, e.g., orders of magnitude faster than present techniques. We also find that although the overall magnitude of the current may change dramatically with different detection conditions, the intrinsic distinguishability of the bases is not significantly affected by pore size and transverse field strength. Finally, we study the ability of water to screen charges in nanopores by using all-atom molecular dynamics simulations coupled to electrostatic calculations. Due to the short length scales of the nanopore geometry and the large local field gradient of a single ion, the energetics of transporting an ion through the pore is strongly dependent on the microscopic details of the electric field. We show that as long as the pore allows the first hydration shell to stay intact, e.g., ˜6 nearby water molecules, the electric field
Program Calculates Forces in Bolted Structural Joints
NASA Technical Reports Server (NTRS)
Buder, Daniel A.
2005-01-01
FORTRAN 77 computer program calculates forces in bolts in the joints of structures. This program is used in conjunction with the NASTRAN finite-element structural-analysis program. A mathematical model of a structure is first created by approximating its load-bearing members with representative finite elements, then NASTRAN calculates the forces and moments that each finite element contributes to grid points located throughout the structure. The user selects the finite elements that correspond to structural members that contribute loads to the joints of interest, and identifies the grid point nearest to each such joint. This program reads the pertinent NASTRAN output, combines the forces and moments from the contributing elements to determine the resultant force and moment acting at each proximate grid point, then transforms the forces and moments from these grid points to the centroids of the affected joints. Then the program uses these joint loads to obtain the axial and shear forces in the individual bolts. The program identifies which bolts bear the greatest axial and/or shear loads. The program also performs a fail-safe analysis in which the foregoing calculations are repeated for a sequence of cases in which each fastener, in turn, is assumed not to transmit an axial force.
NASA Astrophysics Data System (ADS)
Zilani, M. A. K.; Xu, H.; Liu, T.; Sun, Y. Y.; Feng, Y. P.; Wang, X.-S.; Wee, A. T. S.
2006-05-01
The electronic structure of cobalt-induced magic clusters grown on Si(111)-(7×7) is investigated by scanning tunneling microscopy, scanning tunneling spectroscopy, and real-space multiple-scattering calculations. Topographical images of a half unit cell of Si(111)-(7×7) with the cluster acquired at low bias voltages of ±0.4V show greatly reduced cluster heights; however, the heights of the corner adatoms are unchanged, indicative of the highly localized nature of the charge distribution. Spectroscopic studies of the clusters indicate a band gap of ˜0.8eV , suggesting localized nonmetallic behavior. The opening of such a band gap is suggested to be a stabilizing factor for the observed magic clusters. A 65-atom Co-Si cluster is constructed to calculate the momentum- and element-projected density of states. The calculated result identifies that the intense state below the Fermi level at -1.75V in the experimental spectroscopic curve is primarily due to localized 3d orbitals of Co atoms in the magic clusters.
NASA Astrophysics Data System (ADS)
Jong, Un-Gi; Yu, Chol-Jun; Ri, Jin-Song; Kim, Nam-Hyok; Ri, Guk-Chol
2016-09-01
Extensive studies have demonstrated the promising capability of the organic-inorganic hybrid halide perovskite CH3NH3PbI3 in solar cells with a high power conversion efficiency exceeding 20%. However, the intrinsic as well as extrinsic instabilities of this material remain the major challenge to the commercialization of perovskite-based solar cells. Mixing halides is expected to resolve this problem. Here, we investigate the effect of chemical substitution in the position of the halogen atom on the structural, electronic, and optical properties of mixed halide perovskites CH3NH3Pb (I1-xBrx) 3 with a pseudocubic phase using the virtual crystal approximation method within density functional theory. With an increase of Br content x from 0.0 to 1.0, the lattice constant decreases in proportion to x with the function of a (x )=6.420 -0.333 x (Å), while the band gap and the exciton binding energy increase with the quadratic function of Eg(x ) =1.542 +0.374 x +0.185 x2 (eV) and the linear function of Eb(x ) =0.045 +0.057 x (eV), respectively. The photoabsorption coefficients are also calculated, showing a blueshift of the absorption onsets for higher Br contents. We calculate the phase decomposition energy of these materials and analyze the electronic charge density difference to estimate the material stability. Based on the calculated results, we suggest that the best match between efficiency and stability can be achieved at x ≈0.2 in CH3NH3Pb (I1-xBrx) 3 perovskites.
NASA Astrophysics Data System (ADS)
Fujisawa, Jun-ichi; Hanaya, Minoru
2016-06-01
Interfacial charge-transfer (ICT) transitions between inorganic semiconductors and π-conjugated molecules allow direct charge separation without loss of energy. This feature is potentially useful for efficient photovoltaic conversions. Charge-transferred complexes of TiO2 nanoparticles with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its analogues (TCNX) show strong ICT absorption in the visible region. The ICT band was reported to be significantly red-shifted with extension of the π-conjugated system of TCNX. In order to clarify the mechanism of the red-shift, in this work, we systematically study electronic structures of the TiO2-TCNX surface complexes (TCNX; TCNE, TCNQ, 2,6-TCNAQ) by ionization potential measurements and density functional theory (DFT) calculations.
Dose calculation for electron therapy using an improved LBR method
Gebreamlak, Wondesen T.; Alkhatib, Hassaan A.; Tedeschi, David J.
2013-07-15
Purpose: To calculate the percentage depth dose (PDD) of any irregularly shaped electron beam using a modified lateral build-up ratio (LBR) method.Methods: Percentage depth dose curves were measured using 6, 9, 12, and 15 MeV electron beam energies for applicator cone sizes of 6 Multiplication-Sign 6, 10 Multiplication-Sign 10, 14 Multiplication-Sign 14, and 20 Multiplication-Sign 20 cm{sup 2}. Circular cutouts for each cone were prepared from 2.0 cm diameter to the maximum possible size for each cone. In addition, three irregular cutouts were prepared.Results: The LBR for each circular cutout was calculated from the measured PDD curve using the open field of the 14 Multiplication-Sign 14 cm{sup 2} cone as the reference field. Using the LBR values and the radius of the circular cutouts, the corresponding lateral spread parameter [{sigma}{sub R}(z)] of the electron shower was calculated. Unlike the commonly accepted assumption that {sigma}{sub R}(z) is independent of cutout size, it is shown that its value increases linearly with circular cutout size (R). Using this characteristic of the lateral spread parameter, the PDD curves of irregularly shaped cutouts were calculated. Finally, the calculated PDD curves were compared with measured PDD curves.Conclusions: In this research, it is shown that the lateral spread parameter {sigma}{sub R}(z) increases with cutout size. For radii of circular cutout sizes up to the equilibrium range of the electron beam, the increase of {sigma}{sub R}(z) with the cutout size is linear. The percentage difference of the calculated PDD curve from the measured PDD data for irregularly shaped cutouts was under 1.0% in the region between the surface and therapeutic range of the electron beam. Similar results were obtained for four electron beam energies (6, 9, 12, and 15 MeV)
Electron/Photon Verification Calculations Using MCNP4B
D. P. Gierga; K. J. Adams
1999-04-01
MCNP4BW was released in February 1997 with significant enhancements to electron/photon transport methods. These enhancements have been verified against a wide range of published electron/photon experiments, spanning high energy bremsstrahlung production to electron transmission and reflection. The impact of several MCNP tally options and physics parameters was explored in detail. The agreement between experiment and simulation was usually within two standard deviations of the experimental and calculational errors. Furthermore, sub-step artifacts for bremsstrahlung production were shown to be mitigated. A detailed suite of electron depth dose calculations in water is also presented. Areas for future code development have also been explored and include the dependence of cell and detector tallies on different bremsstrahlung angular models and alternative variance reduction splitting schemes for bremsstrahlung production.
Boron Fullerenes: An Electronic Structure Study
NASA Astrophysics Data System (ADS)
Sadrzadeh, Arta; Pupysheva, Olga; Boustani, Ihsan; Yakobson, Boris
2008-03-01
Using ab initio calculations, we study electronic structure and frequency modes of B80, a member of boron fullerene family made from boron isomorphs of carbon fullerenes with additional atoms in the centers of hexagons. We also investigate geometrical and electronic structural properties of double-rings with various diameters, which are important as building blocks of boron nanotubes, and as the most stable clusters among the studied isomers with no more than 36 atoms. Double-rings also appear as building blocks of B80. Furthermore, we investigate the possibility of further stabilizing some of fullerenes by depleting them.
Calculation of electrostatic fields in periodic structures of complex shape
NASA Technical Reports Server (NTRS)
Kravchenko, V. F.
1978-01-01
A universal algorithm is presented for calculating electrostatic fields in an infinite periodic structure consisting of electrodes of arbitrary shape which are located in mirror-symmetrical manner along the axis of electron-beam propagation. The method is based on the theory of R-functions, and the differential operators which are derived on the basis of the functions. Numerical results are presented and the accuracy of the results is examined.
Lattice QCD Calculation of Nucleon Structure
Liu, Keh-Fei; Draper, Terrence
2016-08-30
It is emphasized in the 2015 NSAC Long Range Plan that "understanding the structure of hadrons in terms of QCD's quarks and gluons is one of the central goals of modern nuclear physics." Over the last three decades, lattice QCD has developed into a powerful tool for ab initio calculations of strong-interaction physics. Up until now, it is the only theoretical approach to solving QCD with controlled statistical and systematic errors. Since 1985, we have proposed and carried out first-principles calculations of nucleon structure and hadron spectroscopy using lattice QCD which entails both algorithmic development and large-scale computer simulation. We started out by calculating the nucleon form factors -- electromagnetic, axial-vector, πNN, and scalar form factors, the quark spin contribution to the proton spin, the strangeness magnetic moment, the quark orbital angular momentum, the quark momentum fraction, and the quark and glue decomposition of the proton momentum and angular momentum. The first round of calculations were done with Wilson fermions in the `quenched' approximation where the dynamical effects of the quarks in the sea are not taken into account in the Monte Carlo simulation to generate the background gauge configurations. Beginning in 2000, we have started implementing the overlap fermion formulation into the spectroscopy and structure calculations. This is mainly because the overlap fermion honors chiral symmetry as in the continuum. It is going to be more and more important to take the symmetry into account as the simulations move closer to the physical point where the u and d quark masses are as light as a few MeV only. We began with lattices which have quark masses in the sea corresponding to a pion mass at ~ 300 MeV and obtained the strange form factors, charm and strange quark masses, the charmonium spectrum and the D_{s} meson decay constant f_{Ds}, the strangeness and charmness, the meson mass decomposition and the
The Band Structure of Polymers: Its Calculation and Interpretation. Part 3. Interpretation.
ERIC Educational Resources Information Center
Duke, B. J.; O'Leary, Brian
1988-01-01
In this article, the third part of a series, the results of ab initio polymer calculations presented in part 2 are discussed. The electronic structure of polymers, symmetry properties of band structure, and generalizations are presented. (CW)
Vogt, Natalja; Khaikin, Leonid S; Grikina, Olga E; Rykov, Anatolii N; Vogt, Jürgen
2008-08-21
Thymine is one of the nucleobases which forms the nucleic acid (NA) base pair with adenine in DNA. The study of molecular structure and dynamics of nucleobases can help to understand and explain some processes in biological systems and therefore it is of interest. Because the scattered intensities on the C, N, and O atoms as well as some bond lengths in thymine are close to each other the structural problem cannot been solved by the gas phase electron diffraction (GED) method alone. Therefore the rotational constants from microvawe (MW) studies and differences in the groups of N-C, C=O, N-H, and C-H bond lengths from MP2 (full)/cc-pVQZ calculations were used as supplementary data. The analysis of GED data was based on the C(s) molecular symmetry according to results of the structure optimizations at the MP2 (full) level using 6-311G (d,p), cc-pVTZ, and cc-pVQZ basis sets confirmed by vibrational frequency calculations with 6-311G (d,p) and cc-pVTZ basis sets. Mean-square amplitudes as well as harmonic and anharmonic vibrational corrections to the internuclear distances (r(e)-r(a)) and to the rotational constants (B(e)(k)-B(0)(k), where k = A, B, C) were calculated from the quadratic (MP2 (full)/cc-pVTZ) and cubic (MP2 (full)/6-311G (d,p)) force constants (the latter were used only for anharmonic corrections). The harmonic force field was scaled using published IR and Raman spectra of the parent and N1,N3-dideuterated species, which were for the first time completely assigned in the present work. The main equilibrium structural parameters of the thymine molecule determined from GED data supplemented by MW rotational constants and results of MP2 calculations are the following (bond lengths in Angstroms and bond angles in degrees with 3sigma in parentheses): r(e) (C5=C6) = 1.344 (16), r(e) (C5-C9) = 1.487 (8), r(e) (N1-C6) = 1.372 (3), r(e) (N1-C2) = 1.377 (3), r(e) (C2-N3) = 1.378 (3), r(e) (N3-C4) = 1.395 (3), r(e) (C2=O7) = 1.210 (1), r(e) (C4=O8) = 1.215 (1
Intensity modulation with electrons: calculations, measurements and clinical applications.
Karlsson, M G; Karlsson, M; Zackrisson, B
1998-05-01
Intensity modulation of electron beams is one step towards truly conformal therapy. This can be realized with the MM50 racetrack microtron that utilizes a scanning beam technique. By adjusting the scan pattern it is possible to obtain arbitrary fluence distributions. Since the monitor chambers in the treatment head are segmented in both x- and y-directions it is possible to verify the fluence distribution to the patient at any time during the treatment. Intensity modulated electron beams have been measured with film and a plane parallel chamber and compared with calculations. The calculations were based on a pencil beam method. An intensity distribution at the multileaf collimator (MLC) level was calculated by superposition of measured pencil beams over scan patterns. By convolving this distribution with a Gaussian pencil beam, which has propagated from the MLC to the isocentre, a fluence distribution at isocentre level was obtained. The agreement between calculations and measurements was within 2% in dose or 1 mm in distance in the penumbra zones. A standard set of intensity modulated electron beams has been developed. These beams have been implemented in a treatment planning system and are used for manual optimization. A clinical example (prostate) of such an application is presented and compared with a standard irradiation technique.
Lee, K.-S.; Koo, H.-J.; Whangbo, M.-H.
1999-05-03
The oxides A(3)M'MO(6) (M = Rh, Ir; A = Ca, Sr; M' = alkaline earth, Zn, Cd) of the K(4)CdCl(6) structure type consist of isolated (MO(6))(8)(-) octahedral anions and exhibit an antiferromagnetic ordering at low temperatures. The spin-spin interactions in these oxides, Ca(3)NaMO(6) (M = Ir, Ru), and Sr(3)NaRuO(6) were examined by calculating how strongly the t(2g)-block levels of adjacent (MO(6))((6+)(n)()())(-) (n = 1, 2) anions interact in the presence and absence of the intervening cations A(2+) and M' (n)()(+) (n = 1, 2). Our calculations show that the spin-spin interactions in these oxides are three-dimensional, and the superexchange interactions occur mainly through the short intrachain and interchain M-O.O-M linkages. When the M(n)()(+) cation is very small compared with the A(2+) cation, the intrachain interaction is substantially stronger than the interchain interaction. The opposite is found when the sizes of the M(n)()(+) and A(2+) cations become similar.
NASA Astrophysics Data System (ADS)
Halim, Shimaa Abdel; Ibrahim, Magdy A.
2017-02-01
New derivative of heteroannulated chromone identified as 5-methyl-8H-benzo[h]chromeno[2,3-b][1,6]naphthyridine-6(5H),8-dione (5, MBCND) was easily and efficiently synthesized from DBU catalyzed condensation reaction of 2-aminochromone-3-carboxaldehyde (1) with 4-hydroxy-1-methylquinolin-2(1H)-one (2). The same product 5 was isolated from condensation reaction of aldeyde 1 with 3-(4-hydroxy-1-methyl-2-oxo-1,2-dihydroquinolin-3-yl)-3-oxopropanoic acid (3) or ethyl 4-(4-hydroxy-1-methyl-2-oxo-1,2-dihydroquinolin-3-yl)-2,4-dioxobutanoate (4). Structure of compound (5, MBCND) was deduced based on their elemental analyses and spectral data (IR, 1H NMR and mass spectra). Density Functional Theory (DFT) calculations at the B3LYP/6-311G (d,p) level of theory have been carried out to investigate the equilibrium geometry of the novel compound (5, MBCND). Moreover, total energy, energy of HOMO and LUMO and Mullikan atomic charges were calculated. In addition, the dipole moment, theoretical study of the electronic structure, nonlinear optical properties (NLO), and natural bonding orbital (NBO) analysis and orientation have been performed and discussed. Also the electronic absorption spectra were measured in polar (methanol) as well as non polar (dioxane) solvents and the assignment of the observed bands has been discussed by TD-DFT calculations. The correspondences between calculated and experimental transitions energies are satisfactory.
Calculation of electronic transport coefficients of Ag and Au plasma
Apfelbaum, E. M.
2011-12-15
The thermoelectric transport coefficients of silver and gold plasma have been calculated within the relaxation-time approximation. We considered temperatures of 10-100 kK and densities of {rho} < or approx. 1 g/cm{sup 3}. The plasma composition was calculated using a corresponding system of coupled mass action laws, including the atom ionization up to +4. For momentum cross sections of electron-atom scattering we used the most accurate expressions available. The results of our modeling have been compared with other researchers' data whenever possible.
Electronic structure of worm-eaten graphene
NASA Astrophysics Data System (ADS)
Negishi, Hayato; Takeda, Kyozaburo
2017-02-01
We theoretically study the electronic structure of graphenes having several kinds of imperfections such as atomic vacancies and heteroatom replacements. We consider 12 different configurations of vacancies and 39 different geometries of heteroatom replacements in order to approximately take into account the random conformations of imperfections. To systematically provide a perspective understanding of the defect π and σ states caused by atomistic voids and/or vacancies and heteroatom replacements, we have carried out a tight-binding (TB) calculation. We study the orbital hybridization to clarify the origin and formation of π and σ defect states arising from such imperfections. We also discuss the electronic structure around the Fermi level through the TB band calculation.
Atomic Structure Calculations from the Los Alamos Atomic Physics Codes
Cowan, R. D.
The well known Hartree-Fock method of R.D. Cowan, developed at Los Alamos National Laboratory, is used for the atomic structure calculations. Electron impact excitation cross sections are calculated using either the distorted wave approximation (DWA) or the first order many body theory (FOMBT). Electron impact ionization cross sections can be calculated using the scaled hydrogenic method developed by Sampson and co-workers, the binary encounter method or the distorted wave method. Photoionization cross sections and, where appropriate, autoionizations are also calculated. Original manuals for the atomic structure code, the collisional excitation code, and the ionization code, are available from this website. Using the specialized interface, you will be able to define the ionization stage of an element and pick the initial and final configurations. You will be led through a series of web pages ending with a display of results in the form of cross sections, collision strengths or rates coefficients. Results are available in tabular and graphic form.
Electronic structure of lithium tetraborate
NASA Astrophysics Data System (ADS)
Wooten, David J.
Due to many of its attributes, Li2B4O7 provides a possible material for incorporation as either a primary or companion material in future solid state neutron detectors. There is however a lack of fundamental characterization information regarding this useful material, particularly its electronic configuration. To address this, an investigation of Li2B4O7(110) and Li2B 4O7(100) was undertaken, utilizing photoemission and inverse photoemission spectroscopic techniques. The measured band gap depended on crystallographic direction with the band gaps ranging from 8.9+/-0.5 eV to 10.1+/-0.5 eV. The measurement yielded a density of states that qualitatively agreed with the theoretical results from model bulk band structure calculations for Li2B4O7; albeit with a larger band gap than predicted, but consistent with the known deficiencies of Local Density Approximation and Density Functional Theory calculations. The occupied states of both surfaces were extremely flat; to the degree that resolving periodic dispersion of the occupied states was inconclusive, within the resolution of the system. However, both surfaces demonstrated clear periodic dispersion within the empty states very close to theoretical Brillouin zone values. These attributes also translated to a lighter charge carrier effective mass in the unoccupied states. Of the two surfaces, Li2B4O 7(110) yielded the more consistent values in orthogonal directions for energy states. The presence of a bulk band gap surface state and image potential state in Li2B4O7(110) was indicative of a defect-free surface. The absence of both in the more polar, more dielectric Li2B4O7(100) was attributed to the presence of defects determined to be O vacancies. The results from Li2B 4O7(110) were indicative of a more stable surface than Li 2B4O7(100). In addition, Li 1s bulk and surface core level components were determined at the binding energies of -56.5+0.4 and -53.7+0.5 eV. Resonance features were observed along the [001
Structural, electronic and optical properties of carbonnitride
Cohen, Marvin L.
1996-01-31
Carbon nitride was proposed as a superhard material and a structural prototype, Beta-C3N4, was examined using several theoretical models. Some reports claiming experimental verifications have been made recently. The current status of the theory and experiment is reviewed, and a detailed discussion is presented of calculations of the electronic and optical properties of this material. These calculations predict that Beta-C3N4 will have a minimum gap which is indirect at 6.4 plus or minus 0.5 eV. A discussion of the possibility of carbon nitride nanotubes is also presented.
First-principles calculations of heat capacities of ultrafast laser-excited electrons in metals
NASA Astrophysics Data System (ADS)
Bévillon, E.; Colombier, J. P.; Recoules, V.; Stoian, R.
2015-05-01
Ultrafast laser excitation can induce fast increases of the electronic subsystem temperature. The subsequent electronic evolutions in terms of band structure and energy distribution can determine the change of several thermodynamic properties, including one essential for energy deposition; the electronic heat capacity. Using density functional calculations performed at finite electronic temperatures, the electronic heat capacities dependent on electronic temperatures are obtained for a series of metals, including free electron like, transition and noble metals. The effect of exchange and correlation functionals and the presence of semicore electrons on electronic heat capacities are first evaluated and found to be negligible in most cases. Then, we tested the validity of the free electron approaches, varying the number of free electrons per atom. This shows that only simple metals can be correctly fitted with these approaches. For transition metals, the presence of localized d electrons produces a strong deviation toward high energies of the electronic heat capacities, implying that more energy is needed to thermally excite them, compared to free sp electrons. This is attributed to collective excitation effects strengthened by a change of the electronic screening at high temperature.
Calculation of Electron Affinity and Partial Cross Sections of Hf^-
NASA Astrophysics Data System (ADS)
Pan, Lin; Beck, Donald
2008-05-01
We have calculated for the first time the electron affinity (EA) of Hf^-, using the relativistic configuration interaction method. Our calculations show Hf^- has only one bound state 5d^26s^26p J=5/2, which is a 6p attachment to the ground state of Hf I. By combining our valence stage result with the separate estimate for the modest core-valence contribution, the EA of Hf^- is about 0.114 eV. So far there have been only two experimental results [1,2] for the EA of Hf^-, but both gave only the limits. Our result falls within both of the limits. We also calculate the partial cross sections for photodetachment to the lower lying neutral thresholds. [1] M-J. Nadeau et al, Nucl. Instr. and Meth. B 123, 521 (1997) [2] Vernon T. Davis et al, Nucl. Instr. and Meth. B 241, 118 (2005)
Ab initio calculation of the electronic and optical properties of solid pentacene
NASA Astrophysics Data System (ADS)
Tiago, Murilo L.; Northrup, John E.; Louie, Steven G.
2003-03-01
The optical and electronic properties of crystalline pentacene are studied, using a first-principle Green’s-function approach. The quasiparticle energies are calculated within the GW approximation and the electron-hole excitations are computed by solving the Bethe-Salpeter equation. We investigate the role of polymorphism on the electronic energy gap and linear optical spectrum by studying two different crystalline phases: the solution-phase structure and the vapor-phase structure. Charge-transfer excitons are found to dominate the optical spectrum. Excitons with sizable binding energies are predicted for both phases.
Ab initio calculation of the electronic and optical properties of solid pentacene
Tiago, Murilo L.; Northrup, John E.; Louie, Steve G.
2002-11-01
The optical and electronic properties of crystalline pentacene are studied, using a first-principles Green's-function approach. The quasiparticle energies are calculated within the GW approximation and the electron-hole excitations are computed by solving the Bethe-Salpeter equation. We investigate the role of polymorphism on the electronic energy gap and linear optical spectrum by studying two different crystalline phases: the solution-phase structure and the vapor-phase structure. charge-transfer excitons are found to dominate the optical spectrum. Excitons with sizable binding energies are predicted for both phases.
Gradual changes in electronic properties from graphene to graphite: first-principles calculations.
Alzahrani, A Z; Srivastava, G P
2009-12-02
Calculations based on the first-principles pseudopotential plane-wave method and density functional theory are performed to investigate the electronic properties of graphene, bilayer graphene, multilayer graphene, and graphite. From an analysis of the electronic band structure close to the Fermi level, we have quantified the gradual change in the Fermi surface topology from the point-like structure for graphene to a warped triangular shape for graphite. We have also discussed the gradual change in the electron and hole effective masses and velocities as the system evolves from graphene to graphite.
Extended Mermin Method for Calculating the Electron Inelastic Mean Free Path
NASA Astrophysics Data System (ADS)
Da, B.; Shinotsuka, H.; Yoshikawa, H.; Ding, Z. J.; Tanuma, S.
2014-08-01
We propose an improved method for calculating electron inelastic mean free paths (IMFPs) in solids from experimental energy-loss functions based on the Mermin dielectric function. The "extended Mermin" method employs a nonlimited number of Mermin oscillators and allows negative oscillators to take into account not only electronic transitions, as is common in the traditional approaches, but also infrared transitions and inner shell electron excitations. The use of only Mermin oscillators naturally preserves two important sum rules when extending to infinite momentum transfer. Excellent agreement is found between calculated IMFPs for Cu and experimental measurements from elastic peak electron spectroscopy. Notably improved fits to the IMFPs derived from analyses of x-ray absorption fine structure measurements for Cu and Mo illustrate the importance of the contribution of infrared transitions in IMFP calculations at low energies.
Electronic Structure Principles and Aromaticity
ERIC Educational Resources Information Center
Chattaraj, P. K.; Sarkar, U.; Roy, D. R.
2007-01-01
The relationship between aromaticity and stability in molecules on the basis of quantities such as hardness and electrophilicity is explored. The findings reveal that aromatic molecules are less energetic, harder, less polarizable, and less electrophilic as compared to antiaromatic molecules, as expected from the electronic structure principles.
Electron Scattering and Nuclear Structure
ERIC Educational Resources Information Center
Trower, W. P.; Ficenec, J. R.
1971-01-01
Presents information about the nucleus gained by studies of electron scattering. Discusses what can be implied about the shape of the charge distribution, the nucleus positions, the vibrational modes of the nucleus, the momentum of the nucleus, and the granularity and core structures of the nucleus. (DS)
Structural and electronic properties for atomic clusters
NASA Astrophysics Data System (ADS)
Sun, Yan
We have studied the structural and electronic properties for different groups of atomic clusters by doing a global search on the potential energy surface using the Taboo Search in Descriptors Space (TSDS) method and calculating the energies with Kohn-Sham Density Functional Theory (KS-DFT). Our goal was to find the structural and electronic principles for predicting the structure and stability of clusters. For Ben (n = 3--20), we have found that the evolution of geometric and electronic properties with size reflects a change in the nature of the bonding from van der Waals to metallic and then bulk-like. The cluster sizes with extra stability agree well with the predictions of the jellium model. In the 4d series of transition metal (TM) clusters, as the d-type bonding becomes more important, the preferred geometric structure changes from icosahedral (Y, Zr), to distorted compact structures (Nb, Mo), and FCC or simple cubic crystal fragments (Tc, Ru, Rh) due to the localized nature of the d-type orbital. Analysis of relative isomer energies and their electronic density of states suggest that these clusters tend to follow a maximum hardness principle (MHP). For A4B12 clusters (A is divalent, B is monovalent), we found unusually large (on average 1.95 eV) HOMO-LUMO gap values. This shows the extra stability at an electronic closed shell (20 electrons) predicted by the jellium model. The importance of symmetry, closed electronic and ionic shells in stability is shown by the relative stability of homotops of Mg4Ag12 which also provides support for the hypothesis that clusters that satisfy more than one stability criterion ("double magic") should be particularly stable.
Graph-based linear scaling electronic structure theory
NASA Astrophysics Data System (ADS)
Niklasson, Anders M. N.; Mniszewski, Susan M.; Negre, Christian F. A.; Cawkwell, Marc J.; Swart, Pieter J.; Mohd-Yusof, Jamal; Germann, Timothy C.; Wall, Michael E.; Bock, Nicolas; Rubensson, Emanuel H.; Djidjev, Hristo
2016-06-01
We show how graph theory can be combined with quantum theory to calculate the electronic structure of large complex systems. The graph formalism is general and applicable to a broad range of electronic structure methods and materials, including challenging systems such as biomolecules. The methodology combines well-controlled accuracy, low computational cost, and natural low-communication parallelism. This combination addresses substantial shortcomings of linear scaling electronic structure theory, in particular with respect to quantum-based molecular dynamics simulations.
Graph-based linear scaling electronic structure theory.
Niklasson, Anders M N; Mniszewski, Susan M; Negre, Christian F A; Cawkwell, Marc J; Swart, Pieter J; Mohd-Yusof, Jamal; Germann, Timothy C; Wall, Michael E; Bock, Nicolas; Rubensson, Emanuel H; Djidjev, Hristo
2016-06-21
We show how graph theory can be combined with quantum theory to calculate the electronic structure of large complex systems. The graph formalism is general and applicable to a broad range of electronic structure methods and materials, including challenging systems such as biomolecules. The methodology combines well-controlled accuracy, low computational cost, and natural low-communication parallelism. This combination addresses substantial shortcomings of linear scaling electronic structure theory, in particular with respect to quantum-based molecular dynamics simulations.
Semiempirical band structure calculations on skutterudite-type compounds
NASA Astrophysics Data System (ADS)
Partik, M.; Lutz, H. D.
Semiempirical band structure calculations were performed on several skutterudite-type compounds by using the extended Hückel method. Starting with the molecular orbital calculations on isolated P4 and As4 rings, the reason for the band dispersions of the skutterudites was found to be the interactions between the nonmetal atoms. Both the intermolecular and the intramolecular interactions between the phosphorus atoms are stronger than those between the arsenic atoms. Hence, the dispersion of the bands in CoP3 is larger than that in CoAs3. The COOP (crystal orbital overlap population) integrals of the intramolecular P-P bonds reveal the relation between the valence electron count and the observed bond lengths. The P-P bonds in the skutterudite-type compounds like TP3 (T=Co, Rh, Ir) become stronger by reduction as in NiP3 and weaker by oxidation as in RT4X12 (X=P, As, Sb; R=alkaline earth or rare earth metals) because the bands near the Fermi level are bonding. The electronic reason for the geometric distortion of the Ge2Y2 (Y=S, Se) units of mixed skutterudites TGe1.5Y1.5 is caused by an electron pair gap on germanium, which corresponds to low electron density perpendicular to the ring plane on the germanium atoms.
Effective one-electron approaches to calculate high harmonic generation
NASA Astrophysics Data System (ADS)
Rohringer, Nina; Santra, Robin
2006-05-01
The single-active electron approach (SAE) is frequently applied to calculate high harmonic generation in atoms and consists in solving a one-particle Schr"odinger equation in an appropriate model potential. As an ad hoc approach it is difficult to be systematically improved. Starting with the time-dependent configuration interaction singles (TDCIS) technique we derive a new class of effective one-electron approaches. The resulting one-electron equations are in general non-local and non-unitary. A local approximation to TDCIS can be derived by restricting the total many-body Hamiltonian to a local mean-field Hamiltonian (those usually used in SAE calculations). The resulting equations are similar to traditional SAE approaches but include an additional term which destroys the unitarity of the time-evolution. We show that this correction term is essential and improves on traditional SAE approaches. Numerical tests show that this improved SAE method gives dipole-moments in better agreement with exact results than time-dependent Hartree Fock. The test system is a one-dimensional model of helium which allows for a straightforward numerical solution and therefore provides a benchmark to assess the quality of the different approximations.
Electronic structure investigation of biphenylene films
NASA Astrophysics Data System (ADS)
Totani, R.; Grazioli, C.; Zhang, T.; Bidermane, I.; Lüder, J.; de Simone, M.; Coreno, M.; Brena, B.; Lozzi, L.; Puglia, C.
2017-02-01
Photoelectron Spectroscopy (PS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy have been used to investigate the occupied and empty density of states of biphenylene films of different thicknesses, deposited onto a Cu(111) crystal. The obtained results have been compared to previous gas phase spectra and single molecule Density Functional Theory (DFT) calculations to get insights into the possible modification of the molecular electronic structure in the film induced by the adsorption on a surface. Furthermore, NEXAFS measurements allowed characterizing the variation of the molecular arrangement with the film thickness and helped to clarify the substrate-molecule interaction.
Radial Moment Calculations of Coupled Electron-Photon Beams
FRANKE,BRIAN C.; LARSEN,EDWARD W.
2000-07-19
The authors consider the steady-state transport of normally incident pencil beams of radiation in slabs of material. A method has been developed for determining the exact radial moments of 3-D beams of radiation as a function of depth into the slab, by solving systems of 1-D transport equations. They implement these radial moment equations in the ONEBFP discrete ordinates code and simulate energy-dependent, coupled electron-photon beams using CEPXS-generated cross sections. Modified P{sub N} synthetic acceleration is employed to speed up the iterative convergence of the 1-D charged particle calculations. For high-energy photon beams, a hybrid Monte Carlo/discrete ordinates method is examined. They demonstrate the efficiency of the calculations and make comparisons with 3-D Monte Carlo calculations. Thus, by solving 1-D transport equations, they obtain realistic multidimensional information concerning the broadening of electron-photon beams. This information is relevant to fields such as industrial radiography, medical imaging, radiation oncology, particle accelerators, and lasers.
The electronic structure of nonpolyhex carbon nanotubes.
László, István
2004-01-01
Generalizing the folding method to any periodic two-dimensional planar carbon structures we have calculated the corresponding electronic structures in the framework of the one orbital one site tight-binding (Bloch-Hückel) method by solving the eigenvalue problems in a numerical way. We discussed the metallic or the nonmetallic behavior of the nanotubes by applying the folding vectors of parameters (m, n). We extended the topological coordinate method to two-dimensional periodic planar structures as well. Nearly regular hexagonal, pentagonal, and heptagonal polygons were obtained. The curvatures of the final relaxed structures can be read from the sizes of the polygons. Thus relying only on the topological information we could describe the shape of the tubular structures and their conductivity behaviors.
Linear Scaling Electronic Structure Methods with Periodic Boundary Conditions
Gustavo E. Scuseria
2008-02-08
The methodological development and computational implementation of linear scaling quantum chemistry methods for the accurate calculation of electronic structure and properties of periodic systems (solids, surfaces, and polymers) and their application to chemical problems of DOE relevance.
Electronic and structural properties of functional nanostructures
NASA Astrophysics Data System (ADS)
Yang, Teng
In this Thesis, I present a study of electronic and structural properties of functional nanostructures such as MoSxIy nanowires, self-assembled monolayer on top of metallic surfaces and structural changes induced in graphite by photo excitations. MoSxI y nanowires, which can be easily synthesized in one step, show many advantages over conventional carbon nanotubes in molecular electronics and many other applications. But how to self-assemble them into desired pattern for practical electronic network? Self-assembled monolayers of polymers on metallic surfaces may help to guide pattern formation of some nanomaterials such as MoSxIy nanowires. I have investigated the physical properties of these nanoscale wires and microscopic self-assembly mechanisms of patterns by total energy calculations combined with molecular dynamics simulations and structure optimization. First, I studied the stability of novel Molybdenum chaicohalide nanowires, a candidate for molecular electronics applications. Next, I investigated the self-assembly of nanoparticles into ordered arrays with the aid of a template. Such templates, I showed, can be formed by polymer adsorption on surfaces such as highly ordered pyrolytic graphite and Ag(111). Finally, I studied the physical origin of of structural changes induced in graphite by light in form of a femtosecond laser pulse.
Structure refinement from precession electron diffraction data.
Palatinus, Lukáš; Jacob, Damien; Cuvillier, Priscille; Klementová, Mariana; Sinkler, Wharton; Marks, Laurence D
2013-03-01
Electron diffraction is a unique tool for analysing the crystal structures of very small crystals. In particular, precession electron diffraction has been shown to be a useful method for ab initio structure solution. In this work it is demonstrated that precession electron diffraction data can also be successfully used for structure refinement, if the dynamical theory of diffraction is used for the calculation of diffracted intensities. The method is demonstrated on data from three materials - silicon, orthopyroxene (Mg,Fe)(2)Si(2)O(6) and gallium-indium tin oxide (Ga,In)(4)Sn(2)O(10). In particular, it is shown that atomic occupancies of mixed crystallographic sites can be refined to an accuracy approaching X-ray or neutron diffraction methods. In comparison with conventional electron diffraction data, the refinement against precession diffraction data yields significantly lower figures of merit, higher accuracy of refined parameters, much broader radii of convergence, especially for the thickness and orientation of the sample, and significantly reduced correlations between the structure parameters. The full dynamical refinement is compared with refinement using kinematical and two-beam approximations, and is shown to be superior to the latter two.
Calculation of electron scattering from the ground state of ytterbium
Bostock, Christopher J.; Fursa, Dmitry V.; Bray, Igor
2011-05-15
We report on the application of the convergent close-coupling method, in both relativistic and nonrelativistic formulations, to electron scattering from ytterbium. Angle-differential and integrated cross sections are presented for elastic scattering and excitation of the states (6s6p){sup 3}P{sub 0,1,2}, (6s6p){sup 1}P{sub 1}{sup o}, (6s7p){sup 1}P{sub 1}{sup o}, and (6s5d){sup 1}D{sub 2}{sup e} for a range of incident electron energies. We also present calculations of the total cross section, and angle-differential Stokes parameters for excitation of the (6s6p){sup 3}P{sub 1}{sup o} state from the ground state. A comparison is made with the relativistic distorted-wave method and experiments.
Relativistic collision rate calculations for electron-air interactions
Graham, G.; Roussel-Dupre, R.
1992-12-16
The most recent data available on differential cross sections for electron-air interactions are used to calculate the avalanche, momentum transfer, and energy loss rates that enter into the fluid equations. Data for the important elastic, inelastic, and ionizing processes are generally available out to electron energies of 1--10 kev. Prescriptions for extending these cross sections to the relativistic regime are presented. The angular dependence of the cross sections is included where data is available as is the doubly differential cross section for ionizing collisions. The collision rates are computed by taking moments of the Boltzmann collision integrals with the assumption that the electron momentum distribution function is given by the Juettner distribution function which satisfies the relativistic H- theorem and which reduces to the familiar Maxwellian velocity distribution in the nonrelativistic regime. The distribution function is parameterized in terms of the electron density, mean momentum, and thermal energy and the rates are therefore computed on a two-dimensional grid as a function of mean kinetic energy and thermal energy.
Calculation of Cross Sections in Electron-Nuclear Dynamics
NASA Astrophysics Data System (ADS)
Cabrera-Trujillo, R.; Sabin, John R.; Deumens, E.; Öhrn, Y.
In this work, we present an overview of the study of total and differential cross section calculations within the electron-nuclear dynamics (END). END is a method to solve the time-dependent Schrödinger equation in a non-adiabatic approach to direct dynamics. The method takes advantage of a coherent state representation of the molecular wave function. A quantum-mechanical Lagrangian formulation is employed to approximate the Schrödinger equation, via the time-dependent variational principle, to a set of coupled first-order differential equations in time for the END. We obtain the final wave function for the system allowing the determination of collisional properties of interest, as for example, deflection functions, charge exchange probabilities and amplitudes, and differential cross sections. We discuss the use and selection of basis sets for both the electronic description of the colliding systems as well as for their importance in the description of electron capture. As quantum effects are important in many cases and lacking for classical nuclei, we discuss the Schiff methodology and its advantages over other traditional methods for including semiclassical corrections. Time-lapse rendering of the dynamics of the participating electrons and atomic nuclei provides for a detailed view of dynamical and reactive processes. Comparison to experimental and other theoretical results is provided where appropriate data are available.
Relativistic collision rate calculations for electron-air interactions
Graham, G.; Roussel-Dupre, R.
1993-12-01
The most recent data available on differential cross sections for electron-air interactions are used to calculate the avalanche, momentum transfer, and energy loss rates that enter into the fluid equations. Data for the important elastic, inelastic, and ionizing processes are generally available out to electron energies of 1--10 keV. Prescriptions for extending these cross sections to the relativistic regime are presented. The angular dependence of the cross sections is included where data are available as is the doubly differential cross section for ionizing collisions. The collision rates are computed by taking moments of the Boltzmann collision integrals with the assumption that the electron momentum distribution function is given by the Juettner distribution function which satisfies the relativistic H- theorem and which reduces to the familiar Maxwellian velocity distribution in the nonrelativistic regime. The distribution function is parameterized in terms of the electron density, mean momentum, and thermal energy and the rates are therefore computed on a two dimensional grid as a function of mean kinetic energy and thermal energy.
Multiple scattering calculations of relativistic electron energy loss spectra
NASA Astrophysics Data System (ADS)
Jorissen, K.; Rehr, J. J.; Verbeeck, J.
2010-04-01
A generalization of the real-space Green’s-function approach is presented for ab initio calculations of relativistic electron energy loss spectra (EELS) which are particularly important in anisotropic materials. The approach incorporates relativistic effects in terms of the transition tensor within the dipole-selection rule. In particular, the method accounts for relativistic corrections to the magic angle in orientation resolved EELS experiments. The approach is validated by a study of the graphite CK edge, for which we present an accurate magic angle measurement consistent with the predicted value.
Real-space method for highly parallelizable electronic transport calculations
NASA Astrophysics Data System (ADS)
Feldman, Baruch; Seideman, Tamar; Hod, Oded; Kronik, Leeor
2014-07-01
We present a real-space method for first-principles nanoscale electronic transport calculations. We use the nonequilibrium Green's function method with density functional theory and implement absorbing boundary conditions (ABCs, also known as complex absorbing potentials, or CAPs) to represent the effects of the semi-infinite leads. In real space, the Kohn-Sham Hamiltonian matrix is highly sparse. As a result, the transport problem parallelizes naturally and can scale favorably with system size, enabling the computation of conductance in relatively large molecular junction models. Our use of ABCs circumvents the demanding task of explicitly calculating the leads' self-energies from surface Green's functions, and is expected to be more accurate than the use of the jellium approximation. In addition, we take advantage of the sparsity in real space to solve efficiently for the Green's function over the entire energy range relevant to low-bias transport. We illustrate the advantages of our method with calculations on several challenging test systems and find good agreement with reference calculation results.
Turowski, Marcus; Amotchkina, Tatiana; Ehlers, Henrik; Jupé, Marco; Ristau, Detlev
2014-02-01
The electronic and optical properties of TiO2 atomic structures representing simulated thin films have been investigated using density functional theory. Suitable model parameters and system sizes have been identified in advance by validation of the results with experimental data. Dependencies of the electronic band gap and the refractive index have been calculated as a function of film density. The results of the performed calculations have been compared to characterized optical properties of titania single layers deposited using different coating techniques. The modeled dependencies are consistent with experimental observations, and absolute magnitudes of simulated values are in agreement with measured optical data.
NASA Astrophysics Data System (ADS)
Thatribud, Abdulmutta; Pengpan, Teparksorn
2014-09-01
In this work, band gaps of the delafossite Cu-based transparent conducting oxides CuMO2 (M =B,Al,Ga,In) are calculated by density functional theory (DFT) implemented with many-body perturbation theory (MBPT) based on quasiparticle self-consistent GW approximation (QPscGW) and with Tran-Blaha's modified Becke-Johnson functional (DFT-TB09). Their band gaps are explicitly improved from DFT within local density approximation (LDA). Their optical absorption spectra are also calculated by solving Bethe-Salpeter equation (BSE) that includes the electron-hole correlation effect; they show strong excitonic peaks.
Electronic structure theory of the superheavy elements
NASA Astrophysics Data System (ADS)
Eliav, Ephraim; Fritzsche, Stephan; Kaldor, Uzi
2015-12-01
High-accuracy calculations of atomic properties of the superheavy elements (SHE) up to element 122 are reviewed. The properties discussed include ionization potentials, electron affinities and excitation energies, which are associated with the spectroscopic and chemical behavior of these elements, and are therefore of considerable interest. Accurate predictions of these quantities require high-order inclusion of relativity and electron correlation, as well as large, converged basis sets. The Dirac-Coulomb-Breit Hamiltonian, which includes all terms up to second order in the fine-structure constant α, serves as the framework for the treatment; higher-order Lamb shift terms are considered in some selected cases. Electron correlation is treated by either the multiconfiguration self-consistent-field approach or by Fock-space coupled cluster theory. The latter is enhanced by the intermediate Hamiltonian scheme, allowing the use of larger model (P) spaces. The quality of the calculations is assessed by applying the same methods to lighter homologs of the SHEs and comparing with available experimental information. Very good agreement is obtained, within a few hundredths of an eV, and similar accuracy is expected for the SHEs. Many of the properties predicted for the SHEs differ significantly from what may be expected by straightforward extrapolation of lighter homologs, demonstrating that the structure and chemistry of SHEs are strongly affected by relativity. The major scientific challenge of the calculations is to find the electronic structure and basic atomic properties of the SHE and assign its proper place in the periodic table. Significant recent developments include joint experimental-computational studies of the excitation spectrum of Fm and the ionization energy of Lr, with excellent agreement of experiment and theory, auguring well for the future of research in the field.
QED Based Calculation of the Fine Structure Constant
Lestone, John Paul
2016-10-13
Quantum electrodynamics is complex and its associated mathematics can appear overwhelming for those not trained in this field. Here, semi-classical approaches are used to obtain a more intuitive feel for what causes electrostatics, and the anomalous magnetic moment of the electron. These intuitive arguments lead to a possible answer to the question of the nature of charge. Virtual photons, with a reduced wavelength of λ, are assumed to interact with isolated electrons with a cross section of πλ^{2}. This interaction is assumed to generate time-reversed virtual photons that are capable of seeking out and interacting with other electrons. This exchange of virtual photons between particles is assumed to generate and define the strength of electromagnetism. With the inclusion of near-field effects the model presented here gives a fine structure constant of ~1/137 and an anomalous magnetic moment of the electron of ~0.00116. These calculations support the possibility that near-field corrections are the key to understanding the numerical value of the dimensionless fine structure constant.
Ab initio calculations of correlated electron dynamics in ultrashort pulses
NASA Astrophysics Data System (ADS)
Feist, Johannes
2010-03-01
The availability of ultrashort and intense light pulses on the femtosecond and attosecond timescale promises to allow to directly probe and control electron dynamics on their natural timescale. A crucial ingredient to understanding the dynamics in many-electron systems is the influence of electron correlation, induced by the interelectronic repulsion. In order to study electron correlation in ultrafast processes, we have implemented an ab initio simulation of the two-electron dynamics in helium atoms. We solve the time-dependent Schr"odinger equation in its full dimensionality, with one temporal and five spatial degrees of freedom in linearly polarized laser fields. In our computational approach, the wave function is represented through a combination of time-dependent close coupling with the finite element discrete variable representation, while time propagation is performed using an Arnoldi-Lanczos approximation with adaptive step size. This approach is optimized to allow for efficient parallelization of the program and has been shown to scale linearly using up to 1800 processor cores for typical problem sizes. This has allowed us to perform highly accurate and well- converged computations for the interaction of ultrashort laser pulses with He. I will present some recent results on using attosecond and femtosecond pulses to probe and control the temporal structure of the ionization process. This work was performed in collaboration with Stefan Nagele, Renate Pazourek, Andreas Kaltenb"ack, Emil Persson, Barry I. Schneider, Lee A. Collins, and Joachim Burgd"orfer.
Photodissociation of CCH: classical trajectory calculations involving seven electronic states.
Apaydin, Gökşin; Fink, William H; Jackson, William M
2004-11-15
The photodissociation dynamics of ethynyl radical, C(2)H, involving seven electronic states is studied by classical trajectory calculations. Initial values of the trajectories are selected based on relative absorption intensities calculated by Mebel et al. The energies and the derivatives are interpolated by three-dimensional cubic spline interpolator using an extended data pool. Mean square errors and standard deviations in interpolation of energies for 450 data points are found to be in the range 3.1 x 10(-6)-1.4 x 10(-5) and 1.7 x 10(-3)-3.8 x 10(-3) hartrees, respectively. The photofragments of C(2) and H are produced mainly in the X (1)Sigma(g) (+), a (3)Pi(u), b (3)Sigma(g) (-), c (3)Sigma(u) (+), A (1)Pi(u), B (1)Delta(g) electronic states of C(2) as product. The avoided crossings do not appear to be in the main dissociation pathways. The internal distributions are in good accord with the experimental results where comparison is possible, suggesting that the fragmentation mechanism of C(2)H(2) into C(2) and H is a two step process involving C(2)H radical as an intermediate with a life time long enough to allow complete collection of the phase space in the experiments.
Photodissociation of CCH: Classical trajectory calculations involving seven electronic states
NASA Astrophysics Data System (ADS)
Apaydın, Gökşin; Fink, William H.; Jackson, William M.
2004-11-01
The photodissociation dynamics of ethynyl radical, C2H, involving seven electronic states is studied by classical trajectory calculations. Initial values of the trajectories are selected based on relative absorption intensities calculated by Mebel et al. The energies and the derivatives are interpolated by three-dimensional cubic spline interpolator using an extended data pool. Mean square errors and standard deviations in interpolation of energies for 450 data points are found to be in the range 3.1×10-6-1.4×10-5 and 1.7×10-3-3.8×10-3 hartrees, respectively. The photofragments of C2 and H are produced mainly in the X 1Σg+, a 3Πu, b 3Σg-, c 3Σu+, A 1Πu, B 1Δg electronic states of C2 as product. The avoided crossings do not appear to be in the main dissociation pathways. The internal distributions are in good accord with the experimental results where comparison is possible, suggesting that the fragmentation mechanism of C2H2 into C2 and H is a two step process involving C2H radical as an intermediate with a life time long enough to allow complete collection of the phase space in the experiments.
Structural phase transition and electronic properties of NdBi
Sahu, Ashvini K.; Patiya, Jagdish; Sanyal, Sankar P.
2015-06-24
The structural and electronic properties of NdBi from an electronic structure calculation have been presented. The calculation is performed using self-consistent tight binding linear muffin tin orbital (TB-LMTO) method within the local density approximation (LDA). The calculated equilibrium structural parameters are in good agreement with the available experimental results. It is found that this compound shows metallic behavior under ambient condition and undergoes a structural phase transition from the NaCl structure to the CsCl structure at the pressure 20.1 GPa. The electronic structures of NdBi under pressure are investigated. It is found that NdBi have metallization and the hybridizations of atoms in NdBi under pressure become stronger.
Electronic structure of Mn and Fe oxides
NASA Astrophysics Data System (ADS)
Harrison, Walter
2008-03-01
We present a clear, simple tight-binding representation of the electronic structure and cohesive energy (energy of atomization) of MnO, Mn2O3, and MnO2, in which the formal charge states Mn^2+, Mn^3+, and Mn^4+, respectively, occur. It is based upon localized cluster orbitals for each Mn and its six oxygen neighbors. This approach is fundamentally different from local-density theory (or LDA+U), and perhaps diametrically opposite to Dynamical Mean Field Theory. Electronic states were calculated self-consistently using existing parameters [1], but it is found that the charge density is quite insensitive to charge state, so that the starting parameters are adequate. The cohesive energy per Mn is dominated by the transfer of two s electrons to oxygen p states, the same for all three compounds. The differing transfer of majority d electrons to oxygen p states, and the coupling between them, accounts for the observed variation in cohesion in the series. The same description applies to the perovskites, such as LaxSr1-xMnO3, and can be used for FeO, Fe2O3 (and FeO2), Because the formulation is local, it is equally applicable to impurities, defects and surfaces. [1] Walter A. Harrison, Elementary Electronic Structure, World Scientific (Singapore, 1999), revised edition (2004).
First-Principles Mobility Calculations and Atomic-Scale Interface Roughness in Nanoscale Structures
Evans, Matthew H; Zhang, Xiaoguang; Joannopoulos, J. D.; Pantelides, Sokrates T
2005-01-01
Calculations of mobilities have so far been carried out using approximate methods that suppress atomic-scale detail. Such approaches break down in nanoscale structures. Here we report the development of a method to calculate mobilities using atomic-scale models of the structures and density functional theory at various levels of sophistication and accuracy. The method is used to calculate the effect of atomic-scale roughness on electron mobilities in ultrathin double-gate silicon-on-insulator structures. The results elucidate the origin of the significant reduction in mobility observed in ultrathin structures at low electron densities.
Actinide electronic structure and atomic forces
NASA Astrophysics Data System (ADS)
Albers, R. C.; Rudin, Sven P.; Trinkle, Dallas R.; Jones, M. D.
2000-07-01
We have developed a new method[1] of fitting tight-binding parameterizations based on functional forms developed at the Naval Research Laboratory.[2] We have applied these methods to actinide metals and report our success using them (see below). The fitting procedure uses first-principles local-density-approximation (LDA) linear augmented plane-wave (LAPW) band structure techniques[3] to first calculate an electronic-structure band structure and total energy for fcc, bcc, and simple cubic crystal structures for the actinide of interest. The tight-binding parameterization is then chosen to fit the detailed energy eigenvalues of the bands along symmetry directions, and the symmetry of the parameterization is constrained to agree with the correct symmetry of the LDA band structure at each eigenvalue and k-vector that is fit to. By fitting to a range of different volumes and the three different crystal structures, we find that the resulting parameterization is robust and appears to accurately calculate other crystal structures and properties of interest.
NASA Astrophysics Data System (ADS)
Chen, Z. J.; Xiao, H. Y.; Zu, X. T.; Gao, F.
2008-11-01
The electronic structures and defect formation energies for a series of stannate pyrochlores Ln2Sn2O7 (Ln=La, Pr, Nd, Sm, Gd, Tb, Ho, Er, Lu, and Y) have been investigated using the first-principles total energy calculations. The calculated results show that Ln-site cation ionic radius, x-O48f, lattice constant and the covalency of the ⟨Sn-O48f⟩ bond have a significant affect on the defect formation energies. The cation-antisite defect has the lowest formation energy, as compared with that of other defects, indicating that cation disorder causes local oxygen disordering. The present studies suggest that Lu2Sn2O7 is the most resistant to ion beam-induced amorphization. The electronic structure calculations reveal that Ln2Sn2O7 compounds have direct band gaps of 2.64-2.95 eV at the Γ point in the Brillouin zone.
Relativistic Calculating the Spectral Lines Hyperfine Structure Parameters for Heavy Ions
Khetselius, O. Yu.
2008-10-22
The energies and constants of the hyperfine structure, derivatives of the one-electron characteristics on nuclear radius, nuclear electric quadrupole, magnetic dipole moments for some Li-like multicharged ions are calculated.
Structural Dynamics of Electronic Systems
NASA Astrophysics Data System (ADS)
Suhir, E.
2013-03-01
The published work on analytical ("mathematical") and computer-aided, primarily finite-element-analysis (FEA) based, predictive modeling of the dynamic response of electronic systems to shocks and vibrations is reviewed. While understanding the physics of and the ability to predict the response of an electronic structure to dynamic loading has been always of significant importance in military, avionic, aeronautic, automotive and maritime electronics, during the last decade this problem has become especially important also in commercial, and, particularly, in portable electronics in connection with accelerated testing of various surface mount technology (SMT) systems on the board level. The emphasis of the review is on the nonlinear shock-excited vibrations of flexible printed circuit boards (PCBs) experiencing shock loading applied to their support contours during drop tests. At the end of the review we provide, as a suitable and useful illustration, the exact solution to a highly nonlinear problem of the dynamic response of a "flexible-and-heavy" PCB to an impact load applied to its support contour during drop testing.
Electronic structure investigations of quasicrystals
NASA Astrophysics Data System (ADS)
Rotenberg, E.; Theis, W.; Horn, K.
2004-08-01
We present a review of the determination of density of states (DOS) of quasicrystals using valence band photoemission spectroscopy. The absence of fine or spiky structure in the angle-integrated DOS of quasicrystals suggests the possibility of delocalized electronic states. These were confirmed with angle-resolved photoemission studies, which clearly establish the presence of dispersing features attributed to momentum-dependent bandstructure. Such dispersing states are observed not only for deeper-lying sp states, but also for d-derived bands near the Fermi level. Data from three different high symmetry surfaces of decagonal Al-Ni-Co, an ideal model system, are presented. We find that only a few dominant reciprocal lattice vectors are sufficient to describe the quasiperiodic potential, and the implications for electronic properties are discussed.
Electronic instrumentation for smart structures
NASA Astrophysics Data System (ADS)
Blanar, George J.
1995-04-01
The requirements of electronic instrumentation for smart structures are similar to those of data acquisition systems at our national particle physics laboratories. Modern high energy and heavy ion physics experiments may have tens of thousands of channels of data sources producing data that must be converted to digital form, compacted, stored and interpreted. In parallel, multiple sensors distributed in and around smart structures generate either binary or analog signals that are voltage, charge, or time like in their information content. In all cases, they must be transmitted, converted and preserved into a unified digital format for real-time processing. This paper will review the current status of practical large scale electronic measurement systems with special attention to architectures and physical organization. Brief surveys of the current state of the art will include preamplifiers and amplifiers, comparators and discriminators, voltage or charge analog-to-digital converters, time internal meters or time-to-digital converters, and finally, counting or scalar systems. The paper will conclude by integrating all of these ideas in a concept for an all-digital readout of a smart structure using the latest techniques used in physics research today.
Razafinjanahary, H.; Rogemond, F.; Chermette, H.
1994-08-15
The MS-LSD method remains a method of interest when rapidity and small computer resources are required; its main drawback is some lack of accuracy, mainly due to the muffin-tin distribution of the potential. In the case of large clusters or molecules, the use of an empty sphere to fill, in part, the large intersphere region can improve greatly the results. Calculations bearing on C{sub 60} has been undertaken to underline this trend, because, on the one hand, the fullerenes exhibit a remarkable possibility to fit a large empty sphere in the center of the cluster and, on the other hand, numerous accurate calculations have already been published, allowing quantitative comparison with results. The author`s calculations suggest that in case of added empty sphere the results compare well with the results of more accurate calculations. The calculated electron affinity for C{sub 60} and C{sub 60}{sup {minus}} are in reasonable agreement with experimental values, but the stability of C{sub 60}{sup 2-} in gas phase is not found. 35 refs., 3 figs., 5 tabs.
NASA Astrophysics Data System (ADS)
Belkacem, Ali; Slaughter, Daniel
2015-05-01
Understanding electron-driven chemical reactions is important for improving a variety of technological applications such as materials processing and the important role they play in the radiation damage in bulk matter. Furthermore, dissociative electron attachment often exhibits site-selective bond cleavage, which holds promise for prediction and precise control of electron-driven chemical reactions. Recent dynamical studies of these reactions have demonstrated that an understanding of anion dissociation dynamics beyond simple one-dimensional models is crucial in interpreting the measured fragment angular distributions. We combine ion fragment momentum imaging experiments with electron attachment entrance amplitude calculations to interrogate the non-Born-Oppenheimer dynamics of dissociative electron attachment in polyatomic molecules. We will report recent experimental developments in molecules of technological interest including methanol, methane and uracil. Work supported by Chemical Sciences, Geosciences and Biosciences division of BES/DOE.
Liu, Wei; Tan, Zhenyu; Zhang, Liming; Champion, Christophe
2017-03-01
In this work, direct DNA damage induced by low-energy electrons (sub-keV) is simulated using a Monte Carlo method. The characteristics of the present simulation are to consider the new mechanism of DNA damage due to dissociative electron attachment (DEA) and to allow determining damage to specific bases (i.e., adenine, thymine, guanine, or cytosine). The electron track structure in liquid water is generated, based on the dielectric response model for describing electron inelastic scattering and on a free-parameter theoretical model and the NIST database for calculating electron elastic scattering. Ionization cross sections of DNA bases are used to generate base radicals, and available DEA cross sections of DNA components are applied for determining DNA-strand breaks and base damage induced by sub-ionization electrons. The electron elastic scattering from DNA components is simulated using cross sections from different theoretical calculations. The resulting yields of various strand breaks and base damage in cellular environment are given. Especially, the contributions of sub-ionization electrons to various strand breaks and base damage are quantitatively presented, and the correlation between complex clustered DNA damage and the corresponding damaged bases is explored. This work shows that the contribution of sub-ionization electrons to strand breaks is substantial, up to about 40-70%, and this contribution is mainly focused on single-strand break. In addition, the base damage induced by sub-ionization electrons contributes to about 20-40% of the total base damage, and there is an evident correlation between single-strand break and damaged base pair A-T.
Configuration interaction calculations of the vertical electronic spectrum of silane.
Chantranupong, L.; Hirsch, G.; Buenker, R. J.; Dillon, M. A.; Environmental Research; Univ. Wuppertal
1993-01-01
Ab initio multireference single- and double-excitation configuration interaction (MRD-CI) calculations are reported for a large series of the lowest-lying electronic states of silane SiH{sub 4}. The transition energies computed with and without the multi-reference Davidson correction are found to agree within 0.2 eV, which is an indication that the full CI level for the AO basis employed is approached to this degree of accuracy. These results are found to be in good agreement with the CIPSI values reported earlier by Larrieu et al., but lie as much as 1.0-1.5 eV above their Davidson-corrected MRSD-CI transition energies
Electron paramagnetic resonance calculations for hydrogenated Si surfaces
NASA Astrophysics Data System (ADS)
Rohrmüller, M.; Schmidt, W. G.; Gerstmann, U.
2017-03-01
Electron paramagnetic resonance (EPR) signatures, more specifically the elements of the electronic g tensor, are calculated within density functional theory for hydrogenated Si(111), Si(001), Si(113), Si(114), Si (11 2 ¯) , and Si(110) surfaces. Thereby both perturbation theory and a more sophisticated Berry phase technique are applied. Specific defects on different surface orientations are shown to reproduce the resonances at g ¯=2.0043 and g ¯=2.0052 measured for hydrogenated microcrystalline silicon: The latter value is argued here to originate from regions with low hydrogen coverage; the resonance at g ¯=2.0043 is shown to appear in positions with dihydride environment, where an H atom is directly bound to the silicon dangling-bond atoms. A third group of EPR signals with considerably larger g ¯ values between 2.006 and 2.009 is predicted for highly symmetric dangling bonds resembling single dangling-bond defects in silicon bulk material. As the exact value depends strongly on local strain, this type of defect can explain a less intense signal with large g strain observed in microcrystalline as well as in amorphous material.
Dramatic changes in electronic structure revealed by fractionally charged nuclei
NASA Astrophysics Data System (ADS)
Cohen, Aron J.; Mori-Sánchez, Paula
2014-01-01
Discontinuous changes in the electronic structure upon infinitesimal changes to the Hamiltonian are demonstrated. These are revealed in one and two electron molecular systems by full configuration interaction (FCI) calculations when the realm of the nuclear charge is extended to be fractional. FCI electron densities in these systems show dramatic changes in real space and illustrate the transfer, hopping, and removal of electrons. This is due to the particle nature of electrons seen in stretched systems and is a manifestation of an energy derivative discontinuity at constant number of electrons. Dramatic errors of density functional theory densities are seen in real space as this physics is missing from currently used approximations. The movements of electrons in these simple systems encapsulate those in real physical processes, from chemical reactions to electron transport and pose a great challenge for the development of new electronic structure methods.
Zhao Dongqiu; Huang Xiaowei; Tian Baoli; Zhou Shaomin; Li Yuncai; Du Zuliang
2011-04-18
The effect of electronegative difference between nitrogen and oxygen on electronic properties of N-doped anatase TiO{sub 2} has been studied using first-principles calculations. The results indicate that the valence band maximum (VBM) shifts to high energy by 0.27 eV and the band gap states composed of N 2p, O 2p, and Ti 3d states are formed through the three states entering into the gap after N doping. The interactions of three states widen and delocalize the band gap states. The raised VBM and the wide band gap states can improve the visible light photocatalytic activity.
Pu electronic structure and photoelectron spectroscopy
Joyce, John J; Durakiewicz, Tomasz; Graham, Kevin S; Bauer, Eric D; Moore, David P; Mitchell, Jeremy N; Kennison, John A; Martin, Richard L; Roy, Lindsay E; Scuseria, G. E.
2010-01-01
The electronic structure of PuCoGa{sub 5}, Pu metal, and PuO{sub 2} is explored using photoelectron spectroscopy. Ground state electronic properties are inferred from temperature dependent photoemission near the Fermi energy for Pu metal. Angle-resolved photoemission details the energy vs. crystaJ momentum landscape near the Fermi energy for PuCoGa{sub 5} which shows significant dispersion in the quasiparticle peak near the Fermi energy. For the Mott insulators AnO{sub 2}(An = U, Pu) the photoemission results are compared against hybrid functional calculations and the model prediction of a cross over from ionic to covalent bonding is found to be reasonable.
Structural and electronic properties of fluorographene.
Samarakoon, Duminda K; Chen, Zhifan; Nicolas, Chantel; Wang, Xiao-Qian
2011-04-04
The structural and electronic characteristics of fluorinated graphene are investigated based on first-principles density-functional calculations. A detailed analysis of the energy order for stoichiometric fluorographene membranes indicates that there exists prominent chair and stirrup conformations, which correlate with the experimentally observed in-plane lattice expansion contrary to a contraction in graphane. The optical response of fluorographene is investigated using the GW-Bethe-Salpeter equation approach. The results are in good conformity with the experimentally observed optical gap and reveal predominant charge-transfer excitations arising from strong electron-hole interactions. The appearance of bounded excitons in the ultraviolet region can result in an excitonic Bose-Einstein condensate in fluorographene.
Analysis of boron carbides' electronic structure
NASA Technical Reports Server (NTRS)
Howard, Iris A.; Beckel, Charles L.
1986-01-01
The electronic properties of boron-rich icosahedral clusters were studied as a means of understanding the electronic structure of the icosahedral borides such as boron carbide. A lower bound was estimated on bipolaron formation energies in B12 and B11C icosahedra, and the associated distortions. While the magnitude of the distortion associated with bipolaron formation is similar in both cases, the calculated formation energies differ greatly, formation being much more favorable on B11C icosahedra. The stable positions of a divalent atom relative to an icosahedral borane was also investigated, with the result that a stable energy minimum was found when the atom is at the center of the borane, internal to the B12 cage. If incorporation of dopant atoms into B12 cages in icosahedral boride solids is feasible, novel materials might result. In addition, the normal modes of a B12H12 cluster, of the C2B10 cage in para-carborane, and of a B12 icosahedron of reduced (D sub 3d) symmetry, such as is found in the icosahedral borides, were calculated. The nature of these vibrational modes will be important in determining, for instance, the character of the electron-lattice coupling in the borides, and in analyzing the lattice contribution to the thermal conductivity.
Electronic properties of tantalum pentoxide polymorphs from first-principles calculations
Lee, J.; Lu, W.; Kioupakis, E.
2014-11-17
Tantalum pentoxide (Ta{sub 2}O{sub 5}) is extensively studied for its attractive properties in dielectric films, anti-reflection coatings, and resistive switching memory. Although various crystalline structures of tantalum pentoxide have been reported, its structural, electronic, and optical properties still remain a subject of research. We investigate the electronic and optical properties of crystalline and amorphous Ta{sub 2}O{sub 5} structures using first-principles calculations based on density functional theory and the GW method. The calculated band gaps of the crystalline structures are too small to explain the experimental measurements, but the amorphous structure exhibits a strong exciton binding energy and an optical band gap (∼4 eV) in agreement with experiment. We determine the atomic orbitals that constitute the conduction band for each polymorph and analyze the dependence of the band gap on the atomic geometry. Our results establish the connection between the underlying structure and the electronic and optical properties of Ta{sub 2}O{sub 5}.
QED calculations in heavy many-electron atoms and one-electron quasi-molecules
NASA Astrophysics Data System (ADS)
Tupitsyn, I. I.; Safronova, M. S.; Kozlov, M. G.; Porsev, S. G.; Shabaev, V. M.
2016-05-01
Construction of simple one-electron approach to one-loop QED operator is an important task for the relativistic quantum theory of atoms and molecules. In this work we used two modifications of the model QED potential approach to calculations of the Lamb shift in many-electron atoms and one-electron quasi-molecules. The model potential is constructed as a sum of local and nonlocal (separable) potentials. The nonlocal part of the model potential was introduced to reproduce exactly the diagonal elements and also off-diagonal elements of the one-loop ab initio QED operator. The one-particle model QED operator was introduced in the Dirac-Fock and CI+MBPT relativistic calculations of the heavy and super-heavy atoms and in the calculations of the diatomic quasi-molecules. The comparison of the data obtained in different approaches to the one-loop QED operator is presented. Model QED potential is applied to calculate Lamb shift in the U91+- U92+ dimer. The results are compared with Ref..
Monte Carlo calculation of multi-electron effects on synchrotron radiation
Wang, C.
1993-07-01
The phase space distribution and time structure of an electron beam have fundamental influences on synchrotron radiation properties. These influences are due to the superposition of radiation from all electrons, each following a different trajectory. When the radiation wavelength is longer than the electron bunch length, coherent superposition occurs and results in the observed coherent synchrotron radiation. Usually the wavelength we use is much shorter, so incoherent superposition occurs and the emittance e