28 research outputs found
Magnetic excitations and electronic interactions in SrCuTeO: a spin-1/2 square lattice Heisenberg antiferromagnet
SrCuTeO presents an opportunity for exploring low-dimensional
magnetism on a square lattice of Cu ions. We employ ab initio
multi-reference configuration interaction calculations to unravel the Cu
electronic structure and to evaluate exchange interactions in SrCuTeO.
The latter results are validated by inelastic neutron scattering using linear
spin-wave theory and series-expansion corrections for quantum effects to
extract true coupling parameters. Using this methodology, which is quite
general, we demonstrate that SrCuTeO is an almost realization of a
nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling
of 7.18(5) meV.Comment: 10 pages, 7 figure
Size-consistent self-consistent configuration interaction from a complete active space : Excited states
The self-consistent size consistent on a complete active space singly and doubly configuration interaction (SC)2CAS-SDCI method is applied to excited states. The (SC)2 correction is performed on a closed shell state, and the excited states are obtained by diagonalization of the dressed matrix. A theoretical justification of the transferability of the improvement concerning the dressing state to all roots of the matrix is presented. The method is tested by three tests on the spectrum of small [email protected] ; [email protected]
Multi-scale multireference configuration interaction calculations for large systems using localized orbitals: Partition in zones
A new multireference configuration interaction method using localised orbitals is proposed, in which
a molecular system is divided into regions of unequal importance. The advantage of dealing with
local orbitals, i.e., the possibility to neglect long range interaction is enhanced. Indeed, while in the
zone of the molecule where the important phenomena occur, the interaction cut off may be as small
as necessary to get relevant results, in the most part of the system it can be taken rather large, so that
results of good quality may be obtained at a lower cost. The method is tested on several systems. In
one of them, the definition of the various regions is not based on topological considerations, but on
the nature, Ï or Ï, of the localised orbitals, which puts in evidence the generality of the approac
Wave-function-based approach to quasiparticle bands: new insight into the electronic structure of c-ZnS
Ab initio wave-function-based methods are employed for the study of
quasiparticle energy bands of zinc-blende ZnS, with focus on the Zn 3d
"semicore" states. The relative energies of these states with respect to the
top of the S 3p valence bands appear to be poorly described as compared to
experimental values not only within the local density approximation (LDA), but
also when many-body corrections within the GW approximation are applied to the
LDA or LDA+U mean-field solutions [T. Miyake, P. Zhang, M. L. Cohen, and S. G.
Louie, Phys. Rev. B 74, 245213 (2006)]. In the present study, we show that for
the accurate description of the Zn 3d states a correlation treatment based on
wave function methods is needed. Our study rests on a local Hamiltonian
approach which rigorously describes the short-range polarization and charge
redistribution effects around an extra hole or electron placed into the valence
respective conduction bands of semiconductors and insulators. The method also
facilitates the computation of electron correlation effects beyond relaxation
and polarization. The electron correlation treatment is performed on finite
clusters cut off the infinite system. The formalism makes use of localized
Wannier functions and embedding potentials derived explicitly from prior
periodic Hartree-Fock calculations. The on-site and nearest-neighbor charge
relaxation lead to corrections of several eV to the Hartree-Fock band energies
and gap. Corrections due to long-range polarization are of the order of 1.0 eV.
The dispersion of the Hartree-Fock bands is only little affected by electron
correlations. We find the Zn 3d "semicore" states to lie about 9.0 eV below the
top of the S 3p valence bands, in very good agreement with values from
valence-band x-ray photoemission.Comment: 44 pages, 8 figures, submitted to Phys. Rev.
Theoretical study of the electronic spectrum of p-benzoquinone
The electronic excited states of p-benzoquinone have been studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The calculation of the singletâsinglet and singletâtriplet transition energies comprises 19 valence singlet excited states, 4 valence triplet states, and the singlet 3s,3p, and 3d members of the Rydberg series converging to the first four ionization limits. The computed vertical excitation energies are found to be in agreement with the available experimental data. Conclusive assignments to both valence and Rydberg states have been performed. The main features of the electronic spectrum correspond to the ÏÏâ 1â1Agâ1â1B1u and ÏÏâ 1â1Agâ3â1B1u transitions, computed to be at 5.15 and 7.08 eV, respectively. Assignments of the observed low-energy Rydberg bands have been proposed: An nâ3p transition for the sharp absorption located at ca. 7.4 eV and two nâ3d and Ïâ3s transitions for the broad band observed at ca. 7.8 eV. The lowest triplet state is computed to be an nÏââ3B1g state, in agreement with the experimental [email protected] ; [email protected] ; [email protected]
Theoretical research program to predict the properties of molecules and clusters containing transition metal atoms
The primary focus of this research has been the theoretical study of transition metal (TM) chemistry. A major goal of this work is to provide reliable information about the interaction of H atoms with iron metal. This information is needed to understand the effect of H atoms on the processes of embrittlement and crack propagation in iron. The method in the iron hydrogen studies is the cluster method in which the bulk metal is modelled by a finite number of iron atoms. There are several difficulties in the application of this approach to the hydrogen iron system. First the nature of TM-TM and TM-H bonding for even diatomic molecules was not well understood when these studies were started. Secondly relatively large iron clusters are needed to provide reasonable results