Extended Huckel Approximation Research Articles

Total-energy Assisted Tight-binding Method Based on Local Density Approximation of Density Functional Theory

A novel tight-binding method is developed, based on the extended Huckel approximation and charge self-consistency, with referring the band structure and the total energy of the local density approx...

Electrical transport of zig-zag and folded graphene nanoribbons

ABSTRACTIn recent years, there has been much interest in modelling graphene nanoribbons as they have great potential for use in molecular electronics. We have employed the NEGF formalism to determine the conductivity of graphene nanoribbons in various configurations. The electronic structure calculations were performed within the framework of the Extended Huckel Approximation. Both zigzag and armchair nanoribbons have been considered. In addition, we have also computed the transmission and conductance using the non-equilibrium Greens function formalism for these structures. We also investigated the effect of defects by considering a zigzag nanoribbon with six carbon atoms removed. Finally, the effect of embedding boron nitride aromatic molecules in the nanoribbon has been considered. The results of our calculations are compared with that obtained from recent work carried out using tight-binding model Hamiltonians.

Simulation of the structure and electronic configuration of Pd n (C60) m complexes

The structure and electronic configuration of Pd(C60)2, Pd2(C60)2, Pd3(C60)3, and Pd6(C60)3 complexes have been simulated in terms of the density functional theory within the Perdew—Burke—Ernzerhof (PBE) approximation. The results of the calculation of the Pd6(C60)3 trimer have been used to simulate the structure of the quasi-one-dimensional polymer [C60Pd3]n molecule. For this macromolecule, the oneelectron energy levels have been calculated. It has been found that the band gap is 0.6 eV. The calculations have been performed using the crystal orbital method in the extended Huckel approximation. The possibility of using the obtained results for evaluating the catalytic properties of the studied complexes has been discussed.

First-Principles Study of the Tilted Dirac Cone in α-(BEDT-TTF)2I3at Hydrostatic Pressures

-(BEDT-TTF)2I3 is a layered organic material, and an exotic material which has massless Dirac fermion dispersion at the Fermi level similarly to graphene. Its main difference from graphene is that it is a three-dimensional bulk material and that its Dirac cone is markedly tilted. Such nature shows interesting physical properties, which have been studied through experiments and theories. In organic conductors, the semi-empirical extendedHuckel approximation usually gives good parameters in the tight binding model at ambient pressure. One of the reasons why it can give a good Fermi surface is that organic conductors usually have simple and large Fermi surfaces. For -(BEDT-TTF)2I3, however, the electronic structure is very delicate, and small difference can change the electronic structure near the Fermi level greatly. Another possible source of error in the semi-empirical approximation is the fact that the correctness of the pressure dependence has not been well studied. First-principles study does not have such shortcomings. The superiority of the dispersion given by the first-principles theory to that given by the semi-empirical theory has been confirmed by Kobayashi et al. to explain the transport properties. In our previous study, we have shown that the conducting plane is the a b -plane and that the Dirac fermion dispersion exists in the P 1 symmetry near ambient pressure, where experiments have observed the charge ordered state in the P1 symmetry at low temperatures. The main difference between the phases of the P 1 and P1 symmetry is that the charges on the A and A0 molecules are not equivalent in the latter phase, which inevitably affects the electronic structures near the Fermi level. Therefore, we must confirm whether or not the Dirac fermion dispersion appears at about 2GPa, where theoretical analyses predict its existence. The pressure dependence of the Dirac fermion dispersion is also demanded in order to analyze the physical properties of such dispersion. Our calculation employs the exchange–correlation function in the generalized gradient approximation (PW91) in the density functional theory. Technical parameters in the calculation are written in refs. 15, 20, and 21. We calculate electronic structures in two cases; we use the unit cells from the X-ray measurement, and we determine the unit cells theoretically with the constraint of the P 1 symmetry. We list the parameters of the dispersion in the experimental unit cell, but we mainly discuss them in the theoretical unit cell. We stress that theoretical pressure is hydrostatic. We briefly compare theoretical cells with experimental cells. The theoretical lattice constants a, b, and c at ambient pressure are slightly larger than the experimental ones (3, 2, and 1%, respectively), while the differences in the angles , , and are less than 1%. The situation is also the same under pressure. The changes in a, b, and c are 5:5, 5:0, and 2:8% in the experiment, while they are 5:1, 4:6, and 3:5% in the theory. A question is sometimes posed whether or not the experimentally applied pressure is really hydrostatic. Considering this dependence on pressure, the experimental change is almost the same as the theoretical change. Therefore, we conclude that the experimental pressure is also hydrostatic. Next, we analyze the Dirac fermion dispersion. Previous calculations have shown that the electronic structure of this material is almost two-dimensional. Therefore, we concentrate on the electronic structure on the conducting a b -plane. (The axis with the superscript means the reciprocal axis.) Following Kobayashi et al. and Morinari et al., the tilted Dirac fermion dispersion is given by diagonalizing Hðkx; kyÞ 1⁄4 ðv0k0 þ v y 0kyÞ1þ vxkx x þ vyky y, where 1 is a unit 2 2matrix and x and y are Pauli matrices, kx ky 1⁄4 cosð 0Þ sinð 0Þ sinð 0Þ cosð 0Þ k0 x k0 y !

(nBu4N)[Ni(dmstfdt)2]: A Planar Nickel Coordination Complex with an Extended-TTF Ligand Exhibiting Metallic Conduction, Metal−Insulator Transition, and Weak Ferromagnetism

The 1:1 tetrabutylammonium salt of a nickel complex with an extended-TTF ligand, (nBu4N)[Ni(dmstfdt)2] (dmstfdt = dimethyldiselenadithiafulvalenedithiolate) (1) is a unique ambivalent molecular system exhibiting weakly metallic conduction above room temperature and a weak ferromagnetism at low temperature. Its X−ray structure consists of two independent [Ni(dmstfdt)2]- (A and B) and two nBu4N+ in the unit cell. The pseudo-planar anions are arranged in a zigzag -ABA‘B‘- manner along the molecular side-by-side direction with a dihedral angle of the molecular planes of 42.6°. The tight-binding band structure calculation, based on transfer integrals estimated by the extended Huckel approximation, presents both three-dimensional electron and hole Fermi surfaces, which is consistent with the observed weakly metallic conduction around room-temperature despite the 1:1 stoichiometry of the complex. At 147 K, 1 shows a sharp insulating transition associated with the localization of one electron on each nickel compl...

Theoretical approach to the design of supramolecular conjugated porphyrin polymers

The design of novel conjugated polymers mainly consisting of porphyrin and/or phthalocyanine rings is studied by the use of the tight-binding crystal orbital method based on the extended Huckel approximation. This paper focuses on the tuning of the electronic structures of these polymers by ethynyl bridging as well as various linkage modes. It has been clarified that the bridging sites (meso-to-meso, meso-to-β and β-to-β) and the bridging patterns (linear or zigzag) in the ethynyl-bridged porphyrin polymers are crucial for tuning the π-conjugation throughout the main chain. Furthermore, comparison of two-dimensional (2D) porphyrin polymer sheets and stacked porphyrin polymers demonstrates quite different electronic properties.

Ba2Cu18-xAs10: A New Mixed-Valent Ternary Copper-Rich Arsenide with Metallic Properties

The new barium copper pnictide Ba2Cu18-xAs10 (x ≈ 1.67) was obtained from a direct element combination reaction in a sealed graphite tube at 700 °C, and its structure was determined by single-crystal X-ray diffraction methods. It crystallizes in the trigonal space group P31c (No. 163) with a = 6.7329(1) A, c = 11.6747(2) A, and Z = 1. Ba2Cu18-xAs10 has a three-dimensional structure with straight bottle-shape parallel tunnels running along the c-direction. The Ba atoms reside in the tunnels. The [Cu18-xAs10]4- framework features Cu atom vacancies with x = 1.67. The compound is metallic with a room-temperature resistivity along the c-axis of 1.7 × 10-4 Ω·cm and a Seebeck coefficient of ∼+2.5 μV/K. Band structure calculations in the extended Huckel approximation suggest that the defects on the Cu sites enhance its metallic character.

Stabilization of the New Antimonide Zr2V6Sb9 by V–V and Sb–Sb Bonding

The metal-rich antimonide Zr2V6Sb9 has been prepared by arc-melting of stoichiometric mixtures of Zr, V, and VSb2. Zr2V6Sb9 is the first example of a ternary ordered (filled) variant of the unusual V15Sb18 structure type. In addition to strong metal–antimony bonding, the crystal structure is significantly stabilized by bonding V–V and Sb–Sb interactions, whereas the Zr atoms do not form short metal–metal bonds. Band structure calculations using the Extended Huckel approximation reveal Zr2V6Sb9 being metallic, in agreement with the Pauli paramagnetism experimentally observed.

Spin Crossover in a One-Dimensional Iron Chain

The electronic structures of a triazole-bridged Fe(II) chain of (MX3)n type are examined employing the extended Huckel approximation. Several aspects of the spin transition in the one-dimensional s...

Calculation of electronic tunneling matrix element in proteins: Comparison of exact and approximate one-electron methods for Ru-modified azurin

In recent years several theoretical methods have been developed for evaluation of the magnitude of electronic coupling between distant donor and acceptor complexes mediated by a protein molecule. Most detailed studies have been carried out within the one-electron tight-binding (extended Hückel) approximation for electronic structure of the protein medium. In this paper different approximate and exact one-electron methods such as perturbation theory, exact diagonalization, and method of tunneling currents are reviewed and results of calculations are compared for three HisX-Ru-modified azurin molecules, where X=122, 124, and 126. These systems have been recently synthesized and studied experimentally by Gray and co-workers. The calculations show that perturbation theory results are in excellent agreement with exact calculations if the symmetry of the zeroth-order wave functions of the donor and acceptor metal ions are chosen correctly. A simple computational procedure for construction of such correct zeroth-order functions is proposed.

Lattice effects on the dHvA oscillations and magnetic breakdown in quasi-two dimensional organic conductors

We describe a theoretical approach to investigate the effect of the lattice potential on the dHvA oscillations and magnetic breakdown in quasi-two dimensional organic metals such as α-(BEDT-TTF) 2KHg(SCN) 4. We construct a tight binding Hamiltonian based on the extended Huckel approximation to describe these systems which can then be numerically solved to obtain the energy spectrum as a function of magnetic field. Results on the field dependence of the chemical potential and thermodynamic potential are presented.

Theoretical investigation on the band structures of several Chevrel-phase compounds

The band structures of several Chevrel-phase solid compounds, MO6X8, PbMO6X8 and TIMo3X3(X = S, Se, Te), have been calculated by use of the tight-binding method within the extended Huckel approximation (EHT). The energy bands, the densities of states and the crystal orbital overlap populations of these compounds are discussed. The relationship between the bonding properties and the superconducting transition temperature (Tc) has also been studied qualitatively.

Electronic band structure and superconducting transition of κ-(BEDT–TTF)2I3

Crystals of κ-(BEDT–TTF)2I3 have been prepared electrochemically from a tetrahydrofuran (THF) solution containing the mixed supporting electrolytes {[(C4H9)4N]I3(95%)–[(C4H9)4N]AuI2(5%)} and also from a THF solution without [(C4H9)4N]Aul2. A resistivity drop indicating a superconducting transition was observed at 4.2 K (mid-point). The Tc decreased with increasing pressure (dTc/dP=–0.8 K kbar–1). The resistivity exhibited an anomaly around 170 K, which was coupled with the structural phase transition accompanied by the change of the space group (P21/c→P21). The crystal structure was determined at 295, 150 and 10 K. The extinction rule of the X-ray intensity data collected at 295 K confirmed the space group to be P21/c as reported previously. The (h 0 l) reflections at 150 and 10 K revealed the absence of c-glide symmetry. The low-temperature crystal structure analyses showed conformational change of the ethylene groups. The low-temperature band structure calculation based on a simple extended Huckel approximation gave Fermi surfaces consistent with de Haas–van Alphen experiments.

Effect of surface reconstruction on Fermi-level pinning in the Sn on Si(111) system

We have performed detailed photoemission studies on the Si(111)(√3×√3)R30°-Sn and Si(111)(2√3×2√3)R30°-Sn reconstructed surfaces. Band bending is observed in both cases resulting in n-type Schottky barrier heights of (0.66±0.09) and (0.99±0.10) eV, respectively. In parallel, theoretical calculations using the self-consistent tight binding method in the extended Huckel approximation have been performed to determine the electronic structure and barrier heights at these reconstructed surfaces. The difference in barrier heights is found to be in excellent agreement with experiment.

The electronic structure of Si(100) and As/Si(100) surfaces

The electronic structure of the arsenic terminated silicon (100) surface is calculated using the tight binding method in the extended Huckel approximation. The results are compared with the reconstructed silicon (100) surface, and the passivation of the silicon surface is discussed in context of the authors results.

Dynamics of the chemisorption of hydrogen on the Fe(001) surface

The binding energy curves of the H–Fe(001) system have been computed within a tight-binding approach in the extended Huckel approximation, and have been used to build a LEPS potential-energy surface for H2–Fe(001). The results indicate that the adsorption is exothermic with respect both to atomic and molecular hydrogen. The LEPS potential was then employed in stochastic quasi-classical trajectory calculations to study the dynamics of H2 adsorption. We have found that the dissociative adsorption probability Pa increases for increasing values of the collision and total energies, according to steep S-shaped curves; the H2 translational energy is more effective than the rovibrational component in overcoming the adsorption barrier; Pa decreases on increasing the polar angle of incidence, and a good trend is observed for normal energy scaling: Pa is significantly influenced by the corrugation of the (001) surface.

Tight-binding study of the adsorption of hydrogen on Pt, Au and Pt–Au (111) surfaces

The results of a tight-binding study of the adsorption of hydrogen on Pt, Au and Pt–Au (111) surfaces using the Extended Huckel approximation are presented and discussed. It is shown that, in general, the bond between the adsorbed H atom and the Pt–Au alloy surface is weaker than that observed for pure metals. Therefore, the Pt-rich alloys can be catalytically more active than pure Pt because the weakening of the H–surface bond, due to the presence of inert Au atoms, makes the H atoms on the surface more mobile and more reactive with other adsorbed species. In pure Pt and pure Au, the top site is more stable than the bridge and hollow sites, while for Pt-rich and Au-rich alloys, the different sites show the same stability.

119Sn electric field gradients in model clusters of chalcogenide glasses

Calculations of 119Sn electric field gradients (EFG) have been performed using the Extended Huckel approximation on characteristic molecular clusters simulating possible types of sites in chalcogenide glasses. The motivation for these calculations derives from theoretical concepts on varying near neighbor relationships in these types of glasses, and from recent 119Sn Mossbauer experiments on Sn-doped Gex(Se or S)1−x bulk glasses which reveal three types (A, B and C) of chemically inequivalent sites, with distinct values and composition dependences for their isomer shifts and quadrupole splittings. The model clusters chosen for the calculations were the ethane-like (Ge2Se3)n quasi-one-dimensional chains of varying lengths which have been proposed as possible sources of the B site. In addition, calculations were also carried out on several additional types of clusters, in order to help in interpreting the results for the chains. We find that the magnitude of the quadrupole splitting in isolated linear ethane-like chains is very small, and almost independent of the particular site along the chain at which Sn replaces Ge. It therefore seems unlikely that such isolated linear clusters would be the source of the B sites. These sites are more likely to be related to distortions of the ethane-like clusters into non-linear configurations, as well as interactions with neighboring clusters, as forced by the constraints of the packing in the structure of the glass.

Towards an understanding of stacked base interactions: non-equilibrium phase transitions as a probable model

The distances of stacked bases of DNA in different conformations can-not be calculated by equilibrium potentials, since they exhibit not more than only one minimum. However, a double potential minimum is obtained by considering the most simple semiclassical model of excitons that are coupled adiabatically to lattice vibrations, hence forming polaritons. By use of Danilov's extended Huckel approximation of DNA excitons, stacked base distances can be calculated that agree fairly well with the experimental data. The errors of this approach are small as compared to the differences of the distances in various conformations. Consequently, this model may work as a first base of understanding molecular interactions in DNA.

Chemical bonding at the Si–metal interface: Si–Ni and Si–Cr

Chemical bonding at the interface of a near-noble-metal (Ni) and a transition metal (Cr) with Si is examined through synchrotron radiation photoelectron spectroscopy studies of in situ formed interfaces, of cleaved bulk silicides, and of disordered surfaces prepared by sputter etching of the silicides. Interpretation of these experimental results is guided by parallel linear combination of atomic orbitals (LCAO) (extended Huckel approximation) calculations of stoichiometric Ni and Cr silicides.