Showing papers on "Electronic structure published in 1987"

Ab initio methods in quantum chemistry

Abstract: Matrix Formulated Direct MCSCF and MCSCF CI Methods The MCSCF Method Propagator Methods Analytical Derivative Methods in Quantum Chemistry Symmetry and Degeneracy in X and Density Functional Theory Modern Valence Bond Theory The Complete Active Space (CAS) SCF Method and its Applications in Electronic Structure Calculations Transition Metal Atoms and Dimers Weakly Bonded Systems.

Observation of intrinsic bistability in resonant tunneling structures.

Abstract: A simple quantum system, the semiconductor-based double-barrier resonant-tunneling structure, exhibits intrinsic bistability which we attribute to the feedback dependence of the energy of the electronic states in the well on the tunneling current density.

The electronic structure and chemistry of solids

Abstract: Introduction spectroscopic methods electronic energy levels and chemical bonding elementary band theory the effects of electron repulsion lattice distortions defects, impurities, and surfaces Appendix A: The Fermi-Dirac distribution function Appendix B: Brillouin zones and the reciprocal lattice

Ab initio electronic structure of anions

Abstract: In characterizing anion states either experimentally or theoretically different techniques are required for bound and temporary or metastable anions. Anions that lie energetically above the ground state of the neutral molecule (or atom) are called temporary anions since they are unstable with respect to electron detachment. Temporary anions generally have lifetimes in the range of 10-13-10-16 s and thus are difficult to study by using optical spectroscopic methods. Various types of electron-impact spectroscopy have proven particularly valuable for characterizing temporary anions.’I2 Some of these experimental methods are discussed in the accompanying paper3 of Jordan and Burrow. Because temporary anion states lie in the continuum of the neutral species plus free electron, they cannot be treated in general by means of a straightforward variational calculation. Techniques designed for their study are discussed later in this paper. Bound or stable anions lie energetically below the ground state of their parent neutral and can be studied by a variety of experimental methods including photodetachment spectroscopy* and laser photoelectron spectro~copy.~ The contribution by Wetzel and Brauman in this issue covers many of these experimental techniques. Theoretically they can be characterized by using traditional quantum chemical methods such as the self-consistent field (SCF), configuration interaction (CI), multiconfiguration self-consistent field (MCSCF), and manybody techniques, although extra care must be exercised for reasons outlined below. The ab initio theoretical study of stable atomic and molecular negative ions involves complications not encountered in analogous studies of neutral or cationic species. In particular, the diffuse spatial extent of the outermost orbitals of anions places additional requirements on any quantum chemical calculation. Within the conventional LCAO-MO finite atomic basis approach, adequate description of diffuse charge densities requires that diffuse atomic orbitals be added to the core-and-valence atomic basis sets. Because computational expense varies as a high power of the size of the atomic orbital basis set, this greatly increases the computer-time requirements for negative ion calculations. Diffuse charge densities correspond to weak binding energies and low average values of kinetic energy. The weak binding energies imply that any method used to compute the electron affinities (EA) must be very reliable. The low classical kinetic energies suggest that energy transfer from vibrational and rotational degrees of freedom to the electronic degrees of freedom may be more facile than in neutral or cationic species where the more rapidly moving electrons readily “track” the motion of the underlying nuclei. Thus, the neglect of nonadiabatic interactions, which is the cornerstone upon which the concept of the potential energy surface is predicated, is less likely to be reasonable for anions. For most classes of molecules, the calculation of accurate EA’S requires that the molecular orbital picture be corrected by including so-called electron correlation effeds. In the self-consistent field (SCF) Hartree-Fock molecular orbital model of electronic structure, each electron is allowed to “feel” the other electrons only via an average interaction potential. This potential consists of the averaged Coulomb and exchange interaction

Organic polymer ferromagnet

Abstract: Low-dimensional, especially one-dimensional (1D), organic compounds are known to exhibit the properties of semiconductors, molecular metals and superconductors1. The theoretical prediction of 1D organic ferromagnets and the principles of construction of polymer polyradicals with a ground state spin proportional to a number of monomeric units were proposed in 1977–782,3. For nearly two decades various hydrocarbons of high spin multiplicity have been studied as the model of an organic ferromagnet4–6. Advanced quantum chemistry techniques have been applied for theoretical investigations of the electronic structure of such compounds7–9. Here we present a study of an organic polymer ferromagnetic preparation with a transition metal impurity content well below the level likely to significantly affect its magnetic properties. We find that all the organic polymer ferromagnet samples can be conditionally subdivided into three groups: paramagnetic polymers7–9, spin glasses10 and real polymer ferromagnets.

High-resolution photoemission study of the electronic structure of the noble-metal (111) surfaces.

Abstract: High-resolution angle-resolved photoemission studies of the (111) surfaces of copper, silver, and gold are reported which investigate in detail the properties of the intrinsic surface states located in the projected sp-band gaps at the center of the surface Brillouin zones. Accurate two-dimensional energy dispersion relations are reported for each surface state and are quantified in terms of effective masses at the surface Brillouin-zone center. The masses for the three metals are found to be remarkably similar when normalized to the effective mass of the lower edge of the bulk continuum. The decay length of the surface state wave function into the surface was determined for all three surfaces. These results are expressed in terms of an effective mass of the complex dispersion relation within the projected band gap. In accord with our previous results on the copper state, these effective masses are found to be anomalously large by approximately a factor of 2 relative to expectations based on effective mass theory coupled to first-principles bulk band calculations. An explanation of this anomaly involving the nonorthogonality of effective-mass-theory-derived states is explored. All experimental results are compared to the predictions of recent self-consistent surface electronic structure calculations for these surfaces.

Electronic structure of ultrasmall quantum-well boxes.

Abstract: The electronic structure of interacting, few-electron systems confined in quasi-zero-dimensional, ultrasmall, quantum-well boxes has been calculated by use of the multielectron effective-mass Schr\"odinger equation. The configuration interaction method is used to include electron correlation. Correlation effects are dominant in large boxes; the electrons form a Wigner lattice. In smaller boxes subband spacing becomes dominant and the carriers become frozen in the lowest subbands. The calculations determine how and on what size scale this transition occurs.

.pi.-Electron properties of large condensed polyaromatic hydrocarbons

Abstract: Hueckel molecular orbital (HMO) theory has been used to calculate energy level densities, bond orders, electron distributions, free valence, resonance energies, and heats of formation for several homologous series of large, hexagonally symmetric benzenoid polyaromatic molecules with well-defined edge structures containing up to 2300 carbon atoms. When extrapolated to the infinite limit, values for all properties converge to reasonable values. This is in contrast to several other ..pi..-electron theories that do not yield correct graphite limits. Carbon atoms at the edge of such large molecules are predicted to behave like those in small polynuclear aromatic molecules, with properties strongly dependent on local structure. Regardless of edge structure, interior carbons several bond lengths from an edge have properties similar to those in an infinite graphite sheet. Edge structure has a larger influence on heats of formation than that predicted by group additivity methods. Only a weak correlation was found between the energy of the highest occupied molecular orbital and the reactivity of the most reactive position.

Electronic structure and crystallography of MoTe2 and WTe2

Abstract: Calculations of electronic structure have been undertaken using ab initio tight-binding (TB) method comparing the group VIb ditellurides WTe2 and MoTe2 in their various guises. The group VIb ditellurides show deviation from a simple band model which predicts semiconducting behaviour due to a trigonal prismatic crystal-field splitting of a filled nonbonding dz2 orbital. For WTe2 and beta -MoTe2 the metal atom is displaced from the centre of an octahedron of Te atoms, and metal-metal chains with bond lengths only slightly longer than the elemental metals run along the layers. The reduced Madelung term for this configuration compensates for loss of the lone dz2-based band and thus results in semi-metallic crystals. Mo but not W occurs with a trigonal prismatic coordination ( beta -MoTe2). beta -MoTe2 differs from WTe2 only slightly-in the stacking of layers; a low-temperature polymorph is believed to stack identically to WTe2.

The electrostatic interactions in van der Waals complexes involving aromatic molecules

Abstract: The minima in the electrostatic energy, for accessible orientations, have been located for the s‐tetrazine and benzene dimers and the 1:1 complexes of s‐tetrazine with hydrogen chloride, water, acetylene, and benzene, and of benzene with acetylene, anthracene, and perylene. The minima give reasonably successful predictions of the structures of these van der Waals molecules, demonstrating the importance of the electrostatic interactions in these systems. The electrostatic energy was calculated using sets of distributed multipoles obtained from ab initio wave functions of the monomers. This method is contrasted with empirical point charge and central multipole models for the electrostatic energy. It is shown that the simple models for the electrostatic interactions can give qualitatively misleading results for aromatic systems.

Band structure and chemical bonding in transition metal carbides and nitrides

Abstract: The electronic structure and chemical bonding of the nitrides and carbides of Ti, V, Zr, and Nb are studied The augmented plane wave method is used and results are discussed in terms of energy bands and density of states (AIP)

Experimental electronic structure studies of La2−xSrxCuO4

Abstract: X-ray induced photoelectron spectroscopy, bremsstrahlung-isochromat spectroscopy, and electron energy-loss spectroscopy were used to investigate the electronic structure of the high-temperature superconductor La2−xSrxCuO4 In general, good agreement was obtained with band structure calculations of the tetragonal phase of La2CuO4 Near the Fermi energyEF the systems shows forx=0 a smeared out gap or a region of very low density of states ∼2eV wide Upon replacement of La by Sr, the gap is filled with defect states, the intensity of which is proportional tox The experimental data are not consistent with the closing of a Peierls gap alone

Medium effects on the molecular electronic structure. I. The formulation of a theory for the estimation of a molecular electronic structure surrounded by an anisotropic medium

Abstract: A theory has been developed for estimation of the electronic structure of molecules embedded in an anisotropic (inhomogeneous) medium. It is assumed that the medium surrounding a solute molecule is composed of more than two polarizable dielectrics with different dielectric constants and that the dielectrics contact each other through arbitrary shaped boundaries. Then, the charge distribution in the solute molecule interacts with the dielectrics through a reaction field. The boundary element method is introduced to obtain a numerical solution to the Poisson equation with necessary boundary conditions. The expressions for the total energy of the molecule and the Helmholtz free energy of the system are given under the Hartree–Fock–Roothaan approximation. The Fock matrix is derived by taking into account nonlinearity of the Schrodinger equation in wave function. The present theory is generally applicable to the problem of medium effects when the medium surrounding a molecule is inhomogeneous and could be approximated by a multidielectric system.

The electronic structure and superexchange interactions in transition-metal compounds

Abstract: In this paper, we discuss the electronic structure of transition-metal compounds in light of a new theoretical approach using an Anderson impurity Hamiltonian. We arrive at conclusions concerning t...

Plasmons and high-temperature superconductivity in alloys of copper oxides.

Abstract: The two-dimensional character of the electronic structure of M-N-Cu-O alloys, with M = La,Y and N = Ba,Sm,..., is shown to favor the formation of ''acoustic'' plasmons at energies above the acoustic phonons. Providing that the Fermi energy intersects a small pocket of electrons (or holes) in addition to the expected occupation of a primary electron (or hole) band, the plasma oscillations of the secondary charge carriers may provide a mechanism for room-temperature superconductivty.

Bulk and surface electronic structure of hexagonal boron nitride.

Abstract: Accurate full-potential self-consistent linearized augmented-plane-wave (FLAPW) calculations have been carried out for hexagonal boron nitride. The resulting energy-band structure indicates that this material is an indirect-gap insulator and shows the existence of two unoccupied interlayer bands, similar to those found in graphite and graphite intercalation compounds. Chemical bonding is mainly covalent, with a small charge transfer towards the nitrogen atoms. Moreover, model-potential calculations, based on first-principles FLAPW wave functions and potentials, have been used to study slabs of thickness up to 35 layers. Contrary to the case of graphite, our results do not provide evidence of surface states associated with the interlayer bands.

Electronic structure of some 3D transition-metal pyrites

Abstract: Bremsstrahlung Isochromat spectra of FeS2, NiS2 NiS1.2Se0.8 and NiSe2 are reported. These are the first direct experimental evidence for a sharp antibonding p-like state above the Fermi level. A comparison is made with experimental results in the literature. For FeS2 band structure calculations are also presented; these are in good agreement with experiment. The authors show that it is necessary in this calculation to include 'empty spheres' when using a muffin-tin approximation. This is because the pyrite crystal structure is far from close packed. Furthermore, they briefly discuss the electrical and magnetic properties of the compounds.

Ground and excited states of group IVA diatomics from local‐spin‐density calculations: Model potentials for Si, Ge, and Sn

Abstract: LCGTO‐MP‐LSD results are reported for the spectroscopic constants and electronic structure of the diatomic molecules Si2, Ge2, Sn2, SiGe, SiSn, and GeSn in their low‐lying electronic states. For the homonuclear molecules we found that the ground state is 3Σ−g with the most important lower‐lying excited states being 3Πu, 1Πu, and 1Σ+g, respectively. Our results are in good agreement with the available experimental data and also in qualitative agreement with other theoretical studies. We present here the first theoretical study on the heteronuclear molecules, for which experimental data are not available. We found the 3Σ− state to be the lowest, followed by 3Π and 1Σ+ states. Model potentials (MP) are reported for the Si, Ge, and Sn atoms. The reliable results for molecules complement those for the atoms and show that the LSD model potentials presented here allow for an accurate description of chemical bonding and spectroscopic properties in the title molecules.

A perturbed electron droplet model for the electronic structure of small aluminum clusters

Abstract: The results of first principles total energy calculations for optimized structures of Aln(n=2–6) are presented in which the evolution of cluster orbital occupation with cluster size is found to be different than that predicted by the ‘‘electron droplet’’ or electronic shell model. We find that these variations may be understood in terms of an extension to the droplet model that introduces the structure of the exact electron–nuclear attraction potential as a perturbation on the smooth spherically symmetric potential of the droplet model. We discuss the limits in which this effect is significant, finding that its importance should diminish with increasing cluster size. The detailed calculations produce cluster ionization potentials that are almost all larger than for the Al atom (5.8 to 6.6 eV), in agreement with experiment. Electron affinities are larger as well, ranging from 0.1 to 2.1 eV. Bond lengths increase from 2.51 A for Al2 to 2.81 A for Al6, but are highly sensitive to cluster electronic state. Co...

The open‐shell coupled‐cluster method: Excitation energies and ionization potentials of H2O

Abstract: The open‐shell coupled cluster method is used to calculate directly several electronic excitation energies and ionization potentials of the water molecule. Correlation effects are included by summing single and double virtual excitations to infinite order. Triple excitations are treated approximately, to the lowest order they appear. Their contribution is significant, 0.2–0.4 eV for excitation energies and 0.5–0.7 eV for ionization potentials. The calculated energies are in good agreement (∼0.15 eV) with experiment.

Model potentials for molecular calculations. I. The sd‐MP set for transition metal atoms Sc through Hg

Abstract: Model potential parameters and valence orbitals were generated for the transition metal atoms Sc through Hg. They are named the spd-MPs and are supplementary to the sd-MPs presented in the preceding article. The outermost core np electrons were treated explicitly together with valence nd and (n + 1)s electrons, and the remaining electrons were replaced by a model potential. The model potential parameters and valence orbitals were determined in the same way as the sd-MPs. Major relativistic effects (via the mass velocity and Darwin terms) were also incorporated in the spd-MPs for the second-and third-row transition metal atoms. The results of numerical nonrelativistic Hartree-Fock (HF) calculations for the first-row transition metal atoms and of the quasirelativistic HF calculations with Cowan and Griffin's method for the second-row and third-row transition metal atoms were used as reference data in determination of the spd-MPs.