Showing papers on "Electronic structure published in 1991"
TL;DR: In this paper, an open-shell complete basis set (CBS) model chemistry, based on the unrestricted Hartree-Fock (UHF) zero-order wave function, is defined to include corrections for basis set truncation errors.
Abstract: The major source of error in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. An open‐shell complete basis set (CBS) model chemistry, based on the unrestricted Hartree–Fock (UHF) zero‐order wave function, is defined to include corrections for basis set truncation errors. The total correlation energy for the first‐row atoms is calculated using the unrestricted Mo/ller–Plesset perturbation theory, the quadratic configuration interaction (QCI) method, and the CBS extrapolation. The correlation energies of the atoms He, Li, Be, B, C, N, O, F, and Ne, calculated using atomic pair natural orbital (APNO) basis sets, vary from 85.1% to 95.5% of the experimental correlation energies. However, extrapolation using the asymptotic convergence of the pair natural orbital expansions retrieves from 99.3% to 100.6% of the experimental correlation energies for these atoms. The total extrapolated energies (ESCF+Ecorrelation) are then in agreement with experiment to within ±0...
2,389 citations
TL;DR: In this paper, a chemical bonding model is developed which describes the arrangement of these sites and which accounts for many of the electronic and mechanical properties of amorphous carbon, including elastic modulus, hardness, wear rate, friction and film adhesion.
683 citations
TL;DR: Using density-functional calculations, a three-band spinless model Hamiltonian is suggested for the description of the Verwey transition using a Stoner model as well as from calculations within the framework of the local-spin-density approximation to the density- functional theory.
Abstract: Using density-functional calculations, we examine the electronic structure of magnetite in the spinel crystal structure in order to gain insight into the nature of the Verwey transition. The calculated cohesive and magnetic properties are in agreement with experimental results. The magnetic structure is analyzed using a Stoner model as well as from calculations within the framework of the local-spin-density approximation to the density-functional theory. The calculations show a minority-spin band at the Fermi energy consisting of ${\mathit{t}}_{2\mathit{g}}$ orbitals on the Fe(B) sublattice. These results suggest a three-band spinless model Hamiltonian for the description of the Verwey transition. The hopping integrals and the electron interaction parameters entering the model Hamiltonian are calculated using the ``constrained'' density-functional theory. The calculated parameters are consistent with the electronic origin of the Verwey transition.
610 citations
TL;DR: It is found that the CoO band gap is of an intermediate character, between Mott-Hubbard-like and charge-transfer-like, and the first ionization state of CoO is therefore of strongly mixed Co 3d and O 2p character.
Abstract: The electronic structure of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{Co}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$O (001\ensuremath{\le}x\ensuremath{\le}02), ${\mathrm{LiCoO}}_{2}$, and ${\mathrm{Co}}_{3}$${\mathrm{O}}_{4}$(1% Li) has been investigated using x-ray photoemission spectroscopy (XPS), bremsstrahlung isochromat spectroscopy (BIS), and x-ray-absorption spectroscopy The experimental results are compared with model cluster calculations We find that the CoO band gap is of an intermediate character, between Mott-Hubbard-like and charge-transfer-like The first ionization state of CoO is therefore of strongly mixed Co 3d and O 2p character Its local symmetry corresponds to $^{3}$${\mathit{T}}_{1\mathit{g}}$, similar to an intermediate-spin ${\mathrm{Co}}^{3+}$ state For x\ensuremath{\le}02 the local Co electronic structure is similar to that of CoO However, ${\mathrm{LiCoO}}_{2}$ has a strongly reduced Co-O interatomic distance, resulting in a ligand field strong enough to stabilize a ${\mathrm{Co}}^{3+}$ low-spin ground state ${\mathrm{LiCoO}}_{2}$ is an insulator with a gap of 27 eV From a comparison of the XPS and BIS CoO spectra to the cluster calculations, we find values for U (=53 eV), \ensuremath{\Delta} (=55 eV), and (pd\ensuremath{\sigma}) (=13 eV)
516 citations
TL;DR: In this article, the luminescence behavior of Pt(II) diimine complexes in dilute monomeric environments and in linear chain solid state structures is discussed. But, the authors do not consider the effect of the crystal lattice on the photophysical parameters of the monomer due to electronic interactions between nearest neighbors.
362 citations
342 citations
TL;DR: The laterally confining potential of quantum dots on semiconductors is approximated by a two-dimensional harmonic-oscillator well and the discrete level diagram for two interacting electrons in this potential is calculated in the effective-mass approximation.
Abstract: The laterally confining potential of quantum dots on semiconductors is approximated by a two-dimensional harmonic-oscillator well. The discrete level diagram for two interacting electrons in this potential is calculated in the effective-mass approximation as a function of the dot size and the strength of a magnetic field directed perpendicularly to the dot plane.
317 citations
TL;DR: In this paper, the band gaps, band structure, and excited state (exciton) energies of CdS, GaAs, and GaP semiconductor clusters are calculated using pseudopotentials.
Abstract: The band gaps, band structure, and excited‐state (exciton) energies of CdS, GaAs, and GaP semiconductor clusters are calculated using pseudopotentials. In addition, the sensitivity of the exciton energies to the size, shape, crystal structure, and lattice constant of the unit cell are investigated. The calculated exciton energies of CdS clusters are in excellent agreement with experiment over a wide range of cluster sizes. Also, the exciton states of small CdS clusters are sensitive to whether their crystal structure is zinc blende or hexagonal. Such a sensitivity is absent in large CdS clusters. Furthermore, small GaAs clusters are shown to exhibit anomalous redshift of their absorption spectra, in sharp contrast to CdS and large GaAs clusters whose spectra always shift to blue with decreasing cluster size. Finally, the lowest‐energy non‐Franck–Condon transition in GaP clusters always shifts to blue with decreasing cluster size, whereas the higher‐energy Franck–Condon transition in small clusters exhibits the anomalous redshift. These novel findings reveal that (1) the optical spectroscopy of semiconductor clusters is strongly material and crystal structure dependent; (2) the spectroscopy of small clusters is dramatically different from those of large clusters and bulk; and (3) these effects cannot be explained, even qualitatively, using the effective‐mass approximation.
296 citations
276 citations
TL;DR: In this article, the results of extensive computer simulations of several sodium microclusters, using the Car-Parrinello method (unified density functional theory and molecular dynamics), were presented.
Abstract: We present the results of extensive computer simulations of several sodium microclusters, using the Car–Parrinello method (unified density‐functional theory and molecular dynamics). Dynamical simulated annealing strategies are adopted in the search for low‐energy minima of the potential energy surface. A detailed analysis of the results for both structural and electronic properties at temperatures in the 0–600 K range is carried out, which allows us for the first time to gain insight into the structural ‘‘growth’’ pattern, the extent of the validity of (spherical, spheroidal, and ellipsoidal) jellium models, and the effects of temperature. In particular, new and unforeseen structures are discovered for n=10, 13, 18, and 20 and we emphasize the constant presence of arrangements with local pentagonal symmetry for the low‐energy isomers as well as the similarity of the structural pattern with that of Lennard‐Jones systems. Shape transformations with increasing temperature are observed, ‘‘rigidity’’ and ‘‘nonrigidity’’ of the individual clusters examined, and the presence of distinct isomers is identified for the smaller ones. Closing of electronic shells is confirmed for Na8 and Na20 and—to a minor extent only—for Na18. Hybridization of cluster states of different angular momenta, which represents a deviation from the spherical shell model, is discovered in several cases and discussed in detail, also in correspondence with the presence of anisotropy of the electronic potential. In most cases, this hybridization is observed to increase with increasing temperature, in parallel with the increase of the eccentricity of the cluster shape. In spite of the relatively high atomic mobility, our results do not support a spherical liquid‐droplet picture for the atomic distribution.
242 citations
TL;DR: In this paper, the authors presented a detailed analysis of the image using the molecular orbital approach in through-space and through-bond tunneling processes using the generalized Landauer formula.
TL;DR: In this article, the authors give a reasonably complete account of their properties including energy levels, absorption and MCD line-shapes, relaxed excited states, kinetics and polarization of the emission, and the role of various perturbations: electron-lattice interaction, spin-orbit coupling and magnetic fields.
TL;DR: The electronic structure of MnO has been investigated using high-energy spectroscopies and an experimental gap of 3.9 eV is found.
Abstract: The electronic structure of MnO has been investigated using high-energy spectroscopies. An experimental gap of 3.9 eV is found. By comparing the experimental results to a configuration-interaction ...
TL;DR: In this article, the supershell effect was observed in sodium clusters with up to 3,000 constituent atoms and the existence of both triangular and square orbits, with slightly different periodicities of their magic numbers, should lead to a "quantum beating" effect that imposes a low-frequency envelope on the periodic variation in cluster stability with increasing size.
Abstract: ATOMIC clusters of sodium and other simple metals are known to exhibit a shell structure, giving rise to enhanced stability at certain 'magic numbers' of constituent atoms1–3. Balian and Bloch have shown4 that such shells are the likely result of particularly stable electronic structures: electrons in a spherical cavity (approximating the potential in the clusters) follow semiclassical triangular or square orbits, leading to a shell structure similar to that in atoms5, and stable configurations occur at magic numbers proportional to the cube root of the number of electrons. Balian and Bloch4 also predicted that the existence of both triangular and square orbits, with slightly different periodicities of their magic numbers, should lead to a 'quantum beating' effect that imposes a low-frequency envelope on the periodic variation in cluster stability with increasing size, in effect creating an additional 'supershell' structure. Here we report the observation of this supershell effect in sodium clusters with up to 3,000 constituent atoms.
TL;DR: First-principles calculations of the total energies of O(10) structures are used to define a multispin Ising Hamiltonian, whose ground-state structures can be systematically searched by using methods of lattice theories, establishing a direct and systematic link between the electronic structure and phase stability.
Abstract: While as elemental solids, Al, Ni, Cu, Rh, Pd, Pt, and Au crystallize in the face-centered-cubic (fcc) structure, at low temperatures, their 50%-50% compounds exhibit a range of structural symmetries: CuAu has the fcc-based L1o structure, CuPt has the rhombohedral L1& structure, and CuPd and A1Ni have the body-centered-cubic B2 structure, while CuRh does not exist (it phase separates into Cu and Rh). Phenomenological approaches attempt to rationalize this type of structural selectivity in terms of classical constructs such as atomic sizes, electronegativities, and electron/atom ratios. More recently, attempts have been made at explaining this type of selectivity in terms of the (quantum-mechanical) electronic structure, e.g. , by contrasting the self-consistently calculated total electron+ion energy of various ordered structures. Such calculations, however, normally select but a small, O(10) subset of "intuitive structures" out of the 2 possible configurations of two types of atoms on a fixed lattice with X sites, searching for the lowest energy. We use instead first-principles calculations of the total energies of O(10) structures to define a multispin Ising Hamiltonian, whose ground-state structures can be systematically searched by using methods of lattice theories. Extending our previous work on semiconductor alloys [S.-H. Wei, L. G. Ferreira, and A. Zunger, Phys. Rev. B 41, 8240 (1990)], this is illustrated here for the intermetallic compounds A1Ni, CuRh, CuPd, CuPt, and CuAu, for which the correct ground states are identified out of -65000 configurations, through the combined use of the densityfunctional formalism (to extract Ising-type interaction energies) with a simple configurational-search strategy (to find ground states). This establishes a direct and systematic link between the electronic structure and phase stability.
TL;DR: In this paper, a series of donor-acceptor acetylene compounds were synthesized in which systematic changes in both the conjugation length and the donor acceptor strength were made, and the effect of these structural changes on the spectroscopic and electronic properties of the molecules and ultimately on the measured second-order molecular hyperpolarizabilities (beta) was investigated.
Abstract: A series of donor-acceptor acetylene compounds was synthesized in which systematic changes in both the conjugation length and the donor-acceptor strength were made. The effect of these structural changes on the spectroscopic and electronic properties of the molecules and, ultimately, on the measured second-order molecular hyperpolarizabilities (beta) was investigated. It was found that increases in the donor-acceptor strength resulted in increases in the magnitude of beta. For this class of molecules, the increase is dominated by the energy of the intramolecular charge-transfer transition, while factors such as the ground to excited-state dipole moment change and the transition-moment integral are much less important. Increasing the conjugation length from one to two acetylene linkers did not result in an increase in the value of beta; however, beta increased sharply in going from two acetylenes to three. This increase is attributed to the superposition of several nearly isoenergetic excited states.
TL;DR: In this paper, selected configuration interaction (CI) calculations and second order perturbation theory are combined to systematically approach the full-CI limit, with negligible requirement for memory or disk space, being limited only by available cpu time.
Abstract: Selected configuration interaction (CI) calculations and second order perturbation theory are combined to systematically approach the full‐CI limit. The resulting algorithm has negligible requirement for memory or disk space, being limited only by available cpu time. Comparison is made to existing full‐CI benchmarks (DZ and DZP water, the oxygen atom and its anion, ammonia and the magnesium atom). In all cases the full‐CI result is recovered to better than 0.1 kcal/mol.
TL;DR: A first-principles molecular-dynamics study of pure amorphous silicon obtained by simulated quench from the melt, finding an average structural, dynamical, and electronic properties in good agreement with the available experimental data.
Abstract: We present a first-principles molecular-dynamics study of pure amorphous silicon obtained by simulated quench from the melt. A cooling rate of ${10}^{14}$ K/s is sufficient to recover a tetrahedral network starting from a well-equilibrated metallic liquid having average coordination larger than 6. Dramatic changes in physical properties are observed upon cooling. In particular, a gap forms in the electronic spectrum, indicating a metal-to-semiconductor transition. The as-quenched structure has average coordination very close to 4, but contains several coordination defects as well as a large fraction of distorted bonds. Subsequent annealing reduces the amount of strain and the number of defects present in our system. The average structural, dynamical, and electronic properties of our sample are in good agreement with the available experimental data. We report a detailed analysis of the structural relaxation processes accompanying annealing and compare our findings with recent experiments.
TL;DR: In this article, the authors observed the evolution of the electronic structure of doped polythiophene from monomer to polymer for chain lengths between two and nine, and extrapolated the results to infinite chain length to predict the positions of heretofore unobserved electronic transitions of bulk polythsiophene.
Abstract: Pentasil zeolites such as ZSM-5 and Na-β can be used as supporting matrices in which short-chain oligomers of polythiophene can be prepared, oxidatively doped to the conducting state, stabilized, and finally spectroscopically characterized. For the first time the evolution of the electronic structure of doped polythiophene from monomer to polymer has been observed directly for chain lengths between two and nine. Plots of the electronic absorption band energies for the polaron and bipolaron are found to be linear functions of inverse chain length. These results are extrapolated to infinite chain length to predict the positions of heretofore unobserved electronic transitions of bulk polythiophene. These extrapolations suggest that the lowest energy polaron and bipolaron levels of doped polythiophene are remarkably close in energy, implying that transient formation of polarons from bipolarons in energetically feasible and that this process could play a role in interchain charge hopping in this material.
TL;DR: In this paper, a linear combination of high-resolution photoemission and soft X-ray absorption spectroscopies was used to fit the electronic structure near the Fermi energy in the superconducting copper oxide compounds to constrain the possible mechanisms of superconductivity.
Abstract: HIGH-resolution photoemission and soft X-ray absorption spectroscopies have provided valuable information on the electronic structure near the Fermi energy in the superconducting copper oxide compounds1–4, helping to constrain the possible mechanisms of superconductivity. Here we describe the application of these techniques to KxC60, found recently to be superconducting below 19.3 K for x ≈ 3 (refs 5–7). The photoemission and absorption spectra as a function of x can be fitted by a linear combination of data from just three phases, C60, K3C60 and K6C60 indicating that there is phase separation in our samples. The photoemission spectra clearly show a well defined Fermi edge in the K3C60 phase with a density of states of 5.2 x 10-3 electrons eV-1 A-3 and an occupied-band width of 1.2 eV, suggesting that this phase may be a weakly coupled BCS-like (conventional) superconductor. The Cls absorption spectra show large non-rigid-band shifts between the three phases with half and complete filling, in the K3C60 and K6C60 phases respectively, of the conduction band formed from the lowest unoccupied molecular orbital of C60. These observations clearly demonstrate that the conduction band has C 2p character. The non-rigid-band shift coupled with the anomalous occupied-band width implies that there is significant mixing of the electronic states of K and C60 in the superconducting phase.
TL;DR: In this article, an efficient formulation of the analytic energy gradient for the single and double excitation coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations is presented.
Abstract: An efficient formulation of the analytic energy gradient for the single and double excitation coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations is presented. The formulation has a small computational cost, and the algebraic manipulations may be applied generally to the analytic gradient of Moller-Plesset perturbation theory energies. The new formulation has been implemented in an efficient set of programs that utilize highly vectorized algorithms and has been used to investigate the equilibrium structures, harmonic vibrational frequencies, IR intensities, and energy separation of cis- and trans-HONO.
TL;DR: The first application of valence-band photoemission to a quantum-dot system and the spectra are qualitatively similar to those obtained for bulk cadmium sulfide, but show a shift in the valences-band maximum with size.
Abstract: We report the first application of valence-band photoemission to a quantum-dot system. Photoemission spectra of cadmium sulfide quantum dots, ranging in size from 12 to 35 A radius, were obtained using photon energies of 20 to 70 eV. The spectra are qualitatively similar to those obtained for bulk cadmium sulfide, but show a shift in the valence-band maximum with size.
TL;DR: In this article, the ground state of diπ−cyclooctatetraene cerium (cerocene) was predicted to be in a 4f1 configuration corresponding to Ce3⊕(C8H1.5■8)2.
Abstract: Quantum chemical calculations are presented which predict that in the ground state of di‐π‐cyclooctatetraene cerium (cerocene) the Ce ion is almost entirely in a 4f1 configuration corresponding to Ce3⊕(C8H1.5■8)2. The 4f electron forms with an electron of the ligand e2u highest‐occupied molecular orbital a 4f1e32u singlet in close analogy to a Kondo ion in a metal. Due to coupling of the 4f1e32u with the 4f0e42u configuration, the latter corresponding to Ce4⊕(C8H2≤8)2, the splitting between the ground state singlet and the first excited triplet is of the order 0.5 eV. The self‐consistent‐field and multiconfiguration self‐consistent‐field parts of the calculations are done by employing recently developed pseudopotentials for cerium using basis sets of up to 626 basis functions. The correlation energy is accounted for by means of various correlation‐energy density functionals and also by limited coupled electron‐pair approximation calculations. Similar results are found in both cases.
TL;DR: Techniques to make quantum wires, and quantum wells of controlled size and shape, from compound semiconductor materials, are discussed and some of the properties of these structures are described.
Abstract: Structures in which electrons are confined to move in two dimensions (quantum wells) have led to new physical discoveries and technological applications. Modification of these structures to confine the electrons to one dimension (quantum wires) or release them in the third dimension, are predicted to lead to new electrical and optical properties. This article discusses techniques to make quantum wires, and quantum wells of controlled size and shape, from compound semiconductor materials, and describes some of the properties of these structures.
TL;DR: In this paper, accurate ab initio calculations have been performed to investigate the structures and energies of the negative ions of Si2-Si10, and the effects of polarization functions, diffuse functions and electron correlation have been included in these calculations.
Abstract: Accurate ab initio calculations have been performed to investigate the structures and energies of the negative ions of Si2–Si10. The effects of polarization functions, diffuse functions, and electron correlation have been included in these calculations. In most cases, there is a good correspondence between the ground state structures of the negative ions and those of the corresponding neutral species. Adiabatic electron affinities are computed and compared with recent experimental measurements. Si3, Si5, Si8, and Si9 are found to have electron affinities which are larger than their neighbors. This result is interpreted using our previous calculations on the low‐lying states of the corresponding neutral species.
TL;DR: The spin-polarized band calculations for the iron nitrides, Fe 3 N, Fe 4 N and Fe 16 N 2, have been performed with use of the LMTO-ASA method in the frame of local spin density functional formalism.
TL;DR: The surface band structure of the Au(111) surface along high-symmetry lines in the surface Brillouin zone, with the top layer occupying the fcc, hcp, and bridge sites is calculated and it is found that the surface electronic structure is almost independent of the position of the top layers.
Abstract: On the reconstructed Au(111) surface, atoms on the surface layer occupy both the hcp and the fcc sites. Using first-principles calculations to obtain the surface energies of the system with the top Au layer occupying the fcc, hcp, top, and bridge sites, we found that the hcp site is only 1 mRy per surface atom higher in energy than the fcc site. The complex Au(111) reconstruction is then discussed with use of a two-dimensional Frenkel-Kontorowa model. We calculated the surface band structure of the Au(111) surface along high-symmetry lines in the surface Brillouin zone, with the top layer occupying the fcc, hcp, and bridge sites. We found that the surface electronic structure is almost independent of the position of the top layer.