Local Density Functional Theory Research Articles

The Study on the Filling of Atoms in a Carbon Nanotube

The filling of Pb, C, Al and S atoms into Carbon nanotubes is studied by use of the discrete variational method within the local density functional theory. It is found that Lead atom inclines to enter into tubes, while Carbon atom rather stays at the mouth of nanotubes, which is in good agreement with experiments. The effect of nanotube length and diameter on the filling is discussed.

First-principles study of electric-field gradients at the Cd site for neutral hydrogen-cadmium complexes in crystalline silicon

In the present work, a first-principles investigation of electric-field gradients (EFG's) at the Cd site for possible neutral complexes of Cd-H in crystalline silicon has been performed. This was motivated by experimental results observed by perturbed angular correlation spectroscopy, and with the aim of trying to establish the validity of the configurational models proposed in the literature. We use the full-potential linear-muffin-tin-orbital method, within the local-density functional theory, and the supercell approach. We have allowed local relaxations of the Cd and Si nearest and next-nearest neighbors for different positions of H in the high and low electron-density regions. We have found that H in an intrabond site is unstable (saddle point), and that a bridgelike configuration is the energetically favorable one, where the Cd atom and the nearest silicon suffer relaxations along the [111] direction. The antibond positions of low electronic density regions were also studied, and have been found to be also local minima. The calculated EFG's in these configurations that locally minimize the total energy of the supercell (H at bridgelike and H in antibond sites to Cd and ${\mathrm{Si}}_{1}$) give quadrupolar coupling constants which are in agreement with the experimental results.

Ab-Initio Study of Electronic Structure of Dilute Si Alloys Containing I–B Impurity Atoms

We have made a first principles calculation of the electronic structure and charge density of the dopants I–B group impurities Au, Ag and Cu in crystalline silicon by employing a full potential self-consistent linear muffin tin orbital (LMTO) method along with the local density functional theory. The scalar relativistic effects have also been included. A supercell containing 54 atoms, Si53X with X as impurity has been considered for these investigations. The states lying deep into the energy gap show a fast decaying behaviour and the corresponding wave-functions of the impurities lying in the neighbouring supercells show quite small overlap. The impurities lying in the adjacent supercells are the sixth neighbours among themselves in the direction of the lattice vectors of the fcc lattice. In all cases, impurity bands arising from the p-like orbitals appear in the gap region although some mixing with the d-like states is also observed. This p-like impurity state is a triply degenerate T state at the Γ-point. The impurity bands are peaked at 0.65, 0.61 and 0.56 eV for Au, Ag and Cu, respectively. The d-type impurity states appear in the middle of the valence band. The results in the valance band region are in good agreement with the available experimental data.

Site preferences and formation energies of substitutional Si, Nb, Mo, Ta, and W solid solutions inL10Ti-Al

Modifications to alloy chemistry are often used to tailor the intrinsic flow behavior of structural materials. Models of solution strengthening, high-temperature yield stress and creep must relate the effects of chemistry to the mechanisms which influence these material properties. In ordered alloys, additional information regarding the crystallographic site occupancy of substitutional solid solutions is required. The energy of intrinsic and substitutional point defects in ${L1}_{0}$ TiAl are calculated within a first principles, local density functional theory framework. We calculate the relaxed structures and energies of vacancies, antisites, and substitutional defects using a plane-wave pseudopotential method. The results of these total energy calculations are incorporated into a simple thermodynamic model in order to determine the density of point defects as a function of temperature and stoichiometry. Defect densities are presented as a function of alloy chemistry and temperature for binary TiAl and ternary additions (Si, Nb, Mo, Ta, and W). Defect formation energies are calculated using the derived chemical potentials of each atomic species for several alloy compositions. The predicted site selection of the solutes are in excellent agreement with recent x-ray emission experiments using a quantitative statistical method for atom location by channelling enhanced microanalysis.

Electronic structure of f 1 actinide complexes. Part 3 of . Quasi-relativistic density functional calculations of the optical transition energies of PaX 62− (X=F, Cl, Br, I)

The four f→f transition energies of the single 5f-based electron of PaX 6 2− (X=F, Cl, Br, I) have been calculated using quasi-relativistic local density functional theory. Excellent agreement (<200 cm −1) between theory and experiment is obtained for PaCl 6 2−, PaBr 6 2− and PaI 6 2− by variation of the value of α in the X α exchange-only functional. In contrast, more sophisticated calculational methods including non-local corrections fail to reproduce the experiments well. The PaF 6 2− results are less impressive (up to 1000 cm −1 discrepancy), possibly due to non-aufbau orbital occupations for certain values of α. The values of α employed lie in the range 0.79–0.85, somewhat higher than the most widely used value of 0.7. The theoretical basis for using such values is discussed.

Energetics of3dImpurities on the (001) Surface of Iron

We present a detailed ab initio study of the alloying process in the dilute limit for $3d$ atoms on the Fe(001) surface. The calculations are based on local density functional theory and apply a KKR-Green's function method for impurities on surfaces. For practically all $3d$ transition metal impurities on Fe(001) we find a strong tendency for a direct exchange mechanism into the first surface layer. The early $3d$ impurities V, Cr, and Mn strongly repel each other on neighboring positions within the first layer, while Ni and Cu atoms show a moderate repulsion. The ab initio results are in good agreement with STM studies for Cr/Fe(001) and present valuable predictions for all $3d$/Fe(001) systems.

Density functional study for estimating Brønsted acid site strength in isomorphously substituted ZSM-5

We use local density functional theory to study the Bransted acid strength of a series of clusters that model a catalytic acid site within zeolite ZSM-5. We consider models of the formula (A) 3Si-OH-T(A) 3 where A = OH and OSi(OH) 3; T = Si, Al, B, Ga or Fe. We have studied the effect of replacing Si with trivalent metal cations on the structure and acid strength of the zeolite ZSM-5. Our results also show the effect of cluster size on the calculated properties. It is suggested that a more accurate descriptor of the decreased electron density around the hydroxyl hydrogen would be the electrostatic potential calculated on the total electron density surface. We studied the molecular electrostatic potential maps for the cluster models to test this fact. It is observed that the calculation results predict that acid strength increases in the order B-ZSM-5<Fe-ZSM-5<Ga-ZSM-5<Al-ZSM-5, which shows a good agreement with experimental results. The results show a prospective trend to predict properties of metal substituted zeolites which a special emphasis on Brønsted acid strength of zeolites.

Magnetism of UT 2Si 2 compounds: Effect of the orbital polarization correction

Two different ways to deal with correlations in localized f-electron systems in the framework of local spin density functional theory are compared, using new calculations for the series of uranium inter-metallic compounds UT 2 Si 2 (T=Cr,Mn,Fe,Co,Ni,Cu,Ru,Rh,Pd,Os). The strong localization and hence atomic like behaviour of the Uranium 5f band states is taken into account by incorporation of Hund's second rule through an orbital polarization correction (OPC) field. The two OPC methods tested are the L 2 method and the Orbital Hartree Fock method. Results for electronic and magnetic ground-state properties are compared with previous band structure calculations accounting for spin-orbit coupling only, as well as with experimental data. Both methods are found to yield a considerable improvement of the theoretical value of the magnetic moments over standard spinpolarized relativistic density functional calculations. Furthermore, other aspects such as the impact of the OPC methods on the calculated density of states are discussed.

Electronic Structure and Elastic Properties of Strongly Correlated Metal Oxides from First Principles: LSDA + U, SIC-LSDA and EELS Study of UO2 and NiO

We compare experimentally observed electron energy loss spectra (EELS) of uranium dioxide UO 2 and nickel monoxide NiO with the results of ab-initio calculations carried out by using a method combining the local spin density approximation and the Hubbard U term (the LSDA + U method). We show that by taking better account of strong Coulomb correlations between electrons in the 5f shell of uranium ions in UO 2 and in the 3d shell of nickel ions in NiO it is possible to arrive at a better description of electron energy loss spectra, cohesive energies and elastic constants of both oxides compared with local spin density functional theory. For NiO we also compare the LSDA + U results and EELS spectra with a self-interaction corrected LSDA calculation.

Can CH cations and nobel gases form stable chemical systems? The density functional theory and ab initio studies of the stability of CHHe + and CHHe 2+

The singlet and triplet structures ofCHHe + and doublet structure CHHe 2+ were investigated with hybrid, local and gradient-corrected density functional theory (DFT) methods, as well as HF, MP2, G1, G2 and CBS-Q ab initio methods. In all calculations, a 6–311++G(3df,3pd) basis set was used. High level ab initio calculations predict that singlet CHHe + was ∼26 kcal mol −1 more stable than the corresponding triplet CHHe +, while hybrid and gradient-corrected DFT methods estimate that the energy preference is around 16 kcal mol −1. Nevertheless, CHHe 2+ should be more stable than singlet CHHe + because the computed CHe bond dissociation energy for singlet CHHe 2+ is ~14 kcal mol −1 in comparison with ~1 kcal mol −1 for CHHe +. These results indicate that CHHe 2+ should be detectable in mass spectroscopic studies.

Ab initiomolecular-dynamics studies of doped magic clusters and their interaction with atoms

We present results of the atomic and electronic structure of icosahedral ${\mathrm{Al}}_{12}X$ $(X=\mathrm{S}\mathrm{i}$, Ge, and Sn) clusters using the ab initio molecular-dynamics method within the local density functional theory. Substitutional doping of a ${\mathrm{Al}}_{13}$ cluster by a tetravalent atom leads to a substantial gain in energy in all the cases studied. Tin is found to have a lower energy at a vertex site in contrast to the central site for Si and Ge, leading to surface segregation of Sn in these clusters. Also in the case of a ${\mathrm{Al}}_{13}\mathrm{Si}$ cluster, Si occupies the central site of a capped icosahedral structure. These results when interpreted in terms of the interaction of closed shell clusters with atoms leads to a relatively strong interaction of ${\mathrm{Al}}_{12}\mathrm{Si}$ with $\mathrm{Al}$ as compared to the weak interaction of rare gas atoms with other elements.

Ab-initio calculations of the 6D potential energy surfaces for the dissociative adsorption of H 2 on the (100) surfaces of Rh, Pd and Ag

Detailed investigations of the six-dimensional potential energy surface (PES) for the dissociative adsorption of a hydrogen molecule on the (100) surface of Rh, Pd and Ag are presented. The calculations are based on local density functional theory with generalized gradient corrections to the exchange-correlation functional, and have been performed using the Vienna ab-initio simulation package vasp. vasp works in a plane-wave basis and uses ultrasoft pseudopotentials. We show that adsorption on Rh(100) and Pd(100) is in general non-activated, but barriers exist along certain reaction channels. The “adiabatic” minimum-energy channel has been determined by a five-dimensional minimization of the total energy at a fixed height of the molecule. The variation of the covalent hydrogen-metal bond along this channel is studied using crystal orbital overlap populations and the electron localization function. On Ag(100), H 2 adsorption is strongly activated, with a pronounced variation of the barrier over the surface cell.

First-principles calculations of band offsets

The band offsets are calculated for both the Ga-As and interface bonds within first-principles local density functional theory. It is found that the band offsets are independent of the type of interfacial bond, while the valence band offset, VBO, decreases linearly with x as and the inferred conduction band offset CBO (the band-gap difference minus the valence band offset) decreases linearly as for .

Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study

We demonstrate how by taking better account of electron correlations in the $3d$ shell of metal ions in nickel oxide it is possible to improve the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin density functional theory.

Phase stability of C15 MV2 (M=Zr, Hf or Ta): An electronic structure investigation

Abstract The phase stability of HfV2-based C15 Laves phases has been studied by specific heat measurements and by in-situ transmission electron microscopy at low temperatures. The results indicate that HfV2 undergoes a structural transformation at approximately 115 K and that Nb additions reduce the transformation temperature, eventually stabilizing the C15 structure. First-principles quantum-mechanical calculations based on the local-density functional theory have been performed to study the electronic structure and physical properties of C15 Laves phases MV2 (M = Zr, Hf or Ta). The density of states at the Fermi level N(E F) and the Fermi surface geometry were obtained in order to understand the low-temperature phase instability of HfV2 and ZrV2 and the corresponding low-temperature stability of TaV2. We propose that a large N(E F) and Fermi surface nesting, which give rise to phonon softening, are the physical reasons for the structural instability of HfV2 and ZrV2 at low temperatures. The relationships between the anomalous elasticity and structural instability of HfV2 and ZrV2 are also discussed.

A density functional study of small nickel clusters

Small nickel clusters up to the tetramer are investigated within the framework of the local spin density functional theory. Several competitive states are studied for the dimer. Both the geometry and the spin state are optimized for several starting symmetries in the case of the trimer and the tetramer. Moreover, all those calculations are followed by a vibrational analysis in order to discriminate between real minima and saddle points on the potential energy surface. It is found that Jahn–Teller deformations play an important role in determining transition-metal cluster geometries. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies are reported in this work. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 693–701, 1998

A density functional study of small nickel clusters

Small nickel clusters up to the tetramer are investigated within the framework of the local spin density functional theory. Several competitive states are studied for the dimer. Both the geometry and the spin state are optimized for several starting symmetries in the case of the trimer and the tetramer. Moreover, all those calculations are followed by a vibrational analysis in order to discriminate between real minima and saddle points on the potential energy surface. It is found that Jahn–Teller deformations play an important role in determining transition-metal cluster geometries. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies are reported in this work. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 693–701, 1998

Chemical bonding and magnetism in the germanides HT-UCo2Ge2, UGe3 and U3Co4Ge7 from local spin density functional calculations

The electronic and magnetic structures of HT-UCo2Ge2 (high temperature form), UGe3 and of the new intermetallic system U3Co4Ge7 are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) which was recently combined with the ASW method. From this we address the modifications of both chemical bonding characteristics and of magnetic polarisations of uranium and cobalt in HT-UCo2Ge2 and UGe3, these two compounds being considered as the building blocks of U3Co4Ge7.

Self-Consistent Calculations of the Ground State and the Capacitance of a 3D Si/SiO2 Quantum Dot

We perform self-consistent electronic structure calculations in the framework of inhomogeneously and anisotropically scaled local density functional theory of a fully 3D modeled Si/SiO2 quantum dot. Electrons are laterally confined in the semiconductor/ oxide heterojunction by a metallic gate atop of the device. Total charge densities, total free energies, chemical potentials for different numbers of electrons in the dot, and the differential capacitances for various dot sizes are calculated. We observe shell filling effects in the differential capacitance. The magic-numbers are governed by the six valley bandstructure of silicon, which leads to four fold degenerated single particle levels in the dot.

X-ray magnetic circular dichroism study of the induced spin polarization of Cu in Co/Cu and Fe/Cu multilayers

We present an x-ray magnetic circular dichroism (XMCD) study of Co/Cu and Fe/Cu multilayers, finding that the Cu atoms in these structures exhibit an induced magnetic moment in the d shell. The average Cu spin moment is shown to fall-off inversely with the thickness of the Cu layer. Further, for comparable Cu layer thicknesses, the Cu moments in Fe/Cu and Co/Cu multilayers are found to be nearly equal, despite the fact that the Cu layers in the Co/Cu multilayers are shown to be fee while those in the Fe/Cu structures are bcc. These observations suggest that the induced moment is primarily situated at the Co/Cu and Fe/Cu interfaces and is resultant from short range chemical hybridization between the ferromagnetic and Cu atoms. Results from a local spin density functional theory are presented and found to be in excellent agreement with experimental observations. These results indicate that the Cu d electrons play a central role in mediating the exchange coupling between successive ferromagnetic layers.