46 research outputs found
Spin-state transition and spin-polaron physics in cobalt oxide perovskites: ab initio approach based on quantum chemical methods
A fully ab initio scheme based on quantum chemical wavefunction methods is
used to investigate the correlated multiorbital electronic structure of a
3d-metal compound, LaCoO3. The strong short-range electron correlations,
involving both Co and O orbitals, are treated by multireference techniques. The
use of effective parameters like the Hubbard U and interorbital U', J terms and
the problems associated with their explicit calculation are avoided with this
approach. We provide new insight into the spin-state transition at about 90 K
and the nature of charge carriers in the doped material. Our results indicate
the formation of a t4e2 high-spin state in LaCoO3 for T>90 K. Additionally, we
explain the paramagnetic phase in the low-temperature lightly doped compound
through the formation of Zhang-Rice-like O hole states and ferromagnetic
clusters
Ab initio wavefunction based methods for excited states in solids: correlation corrections to the band structure of ionic oxides
Ab initio wavefunction based methods are applied to the study of electron
correlation effects on the band structure of oxide systems. We choose MgO as a
prototype closed-shell ionic oxide. Our analysis is based on a local
Hamiltonian approach and performed on finite fragments cut from the infinite
solid. Localized Wannier functions and embedding potentials are obtained from
prior periodic Hartree-Fock (HF) calculations. We investigate the role of
various electron correlation effects in reducing the HF band gap and modifying
the band widths. On-site and nearest-neighbor charge relaxation as well as
long-range polarization effects are calculated. Whereas correlation effects are
essential for computing accurate band gaps, we found that they produce smaller
changes on the HF band widths, at least for this material. Surprisingly, a
broadening effect is obtained for the O 2p valence bands. The ab initio data
are in good agreement with the energy gap and band width derived from
thermoreflectance and x-ray photoemission experiments. The results show that
the wavefunction based approach applied here allows for well controlled
approximations and a transparent identification of the microscopic processes
which determine the electronic band structure
Adsorption of benzene on Si(100) from first principles
Adsorption of benzene on the Si(100) surface is studied from first
principles. We find that the most stable configuration is a
tetra--bonded structure characterized by one C-C double bond and four
C-Si bonds. A similar structure, obtained by rotating the benzene molecule by
90 degrees, lies slightly higher in energy. However, rather narrow wells on the
potential energy surface characterize these adsorption configurations. A
benzene molecule impinging on the Si surface is most likely to be adsorbed in
one of three different di--bonded, metastable structures, characterized
by two C-Si bonds, and eventually converts into the lowest-energy
configurations. These results are consistent with recent experiments.Comment: 4 pages, RevTex, 2 PostScript gzipped figure
Ab initio Green's function formalism for band structures
Using the Green's function formalism, an ab initio theory for band structures
of crystals is derived starting from the Hartree-Fock approximation. It is
based on the algebraic diagrammatic construction scheme for the self-energy
which is formulated for crystal orbitals (CO-ADC). In this approach, the poles
of the Green's function are determined by solving a suitable Hermitian
eigenvalue problem. The method is not only applicable to the outer valence and
conduction bands, it is also stable for inner valence bands where strong
electron correlations are effective. The key to the proposed scheme is to
evaluate the self-energy in terms of Wannier orbitals before transforming it to
a crystal momentum representation. Exploiting the fact that electron
correlations are mainly local, one can truncate the lattice summations by an
appropriate configuration selection scheme. This yields a flat configuration
space; i.e., its size scales only linearly with the number of atoms per unit
cell for large systems and, under certain conditions, the computational effort
to determine band structures also scales linearly. As a first application of
the new formalism, a lithium fluoride crystal has been chosen. A minimal basis
set description is studied, and a satisfactory agreement with previous
theoretical and experimental results for the fundamental band gap and the width
of the F 2p valence band complex is obtained.Comment: 20 pages, 3 figures, 1 table, RevTeX4, new section on lithium
fluorid
Water Chemisorption and Reconstruction of the MgO Surface
The observed reactivity of MgO with water is in apparent conflict with
theoretical calculations which show that molecular dissociation does not occur
on a perfect (001) surface. We have performed ab-initio total energy
calculations which show that a chemisorption reaction involving a
reconstruction to form a (111) hydroxyl surface is strongly preferred with
Delta E = -90.2kJ/mol. We conclude that protonation stabilizes the otherwise
unstable (111) surface and that this, not the bare (001), is the most stable
surface of MgO under ambient conditions.Comment: RevTeX, 4 pages, 1 Encapsulated Postscript Figur
Slabs of stabilized jellium: Quantum-size and self-compression effects
We examine thin films of two simple metals (aluminum and lithium) in the
stabilized jellium model, a modification of the regular jellium model in which
a constant potential is added inside the metal to stabilize the system for a
given background density. We investigate quantum-size effects on the surface
energy and the work function. For a given film thickness we also evaluate the
density yielding energy stability, which is found to be slightly higher than
the equilibrium density of the bulk system and to approach this value in the
limit of thick slabs. A comparison of our self-consistent calculations with the
predictions of the liquid-drop model shows the validity of this model.Comment: 7 pages, 6 figures, to appear in Phys. Rev.