1,720,971 research outputs found
Direct-space analysis of the electronic structure of the YBa2Cu3O6 and YBa2Cu3O7 crystals
No full textThe electronic structure of the YBa2Cu3O6 and YBa2Cu3O7 crystals is investigated by means of fully periodic Restricted Hartree-Fock LCAO pseudopotential calculations. In order to give a detailed description of the bonding and atomic properties of the materials studied we selected the Quantum Theory of Atoms in Molecules approach, and complementary information is obtained from the analysis of the Electron Localisation Function in specific cell regions. The main features of the YBa2Cu3O6 and YBa2Cu3O7 crystals electron densities are detailed and compared with available experimental data. As the superconducting transition requires oxidation of the YBa2Cu3O6 crystal, we focus on the electronic rearrangements related to the inclusion of oxygen. Cu-O planes, probably responsible for the superconducting transition, exhibit peculiar electronic properties
Application of valence-bond techniques to the study of weakly bound complexes. The potential energy surface of the Ne-CH4 system
No full textWe present a comprehensive survey of the Molecular Orbital-Valence Bond ( MO-VB) method, a theoretical scheme developed within the framework of the Valence Bond theory to deal with weakly bound intermolecular complexes. According to the MO-VB, the wavefunction of the system is expressed as a truncated non-orthogonal Configuration Interaction expansion, which is size extensive and a priori free of basis set superposition error. We report on the recent developments of the method, which extend the range of application of the MO-VB to intermolecular complexes with a quite large number of correlated electrons, showing that VB-based methods are nowadays a valid alternative to Molecular Orbital approaches also in this field. The MO-VB has been applied to study extensively the Ne-CH4 complex, and compared with the more standard MP4 and CCSD( T) results. We determined two analytical Potential Energy Surfaces ( PES) for this system, computed at MO-VB and MP4 level, which represent the first ones coming entirely from ab initio computations. The features of our potentials are discussed, and compared to the single analytical potential which includes the anisotropy available in the literature, determined about twenty years ago by Udo Buck and co-workers using a semiempirical approach [ U. Buck, A. Kolhase, D. Secrest, T. Phillips, G. Scoles and F. Grein, Mol. Phys., 1985, 55, 1233]. The differences among the three PES are quite relevant, and are due to play a relevant role in the theoretical simulations of the dynamical properties of the Ne-CH
Solubility of Metal Atoms in Helium Droplets: Exploring the Effect of the Well Depth Using the Coinage Metals Cu and Ag
No full textWe report a theoretical investigation of the solution properties of Cu and Ag atoms dissolved in He clusters. Employing our recent ab initio ground state pair potential for Me-He (Me = Ag, Cu), we simulated the species Me@He(n) (n = 2-100) by means of diffusion Monte Carlo (DMC) obtaining exact information on their energetics and the structural properties. In particular, we investigated the sensitivity of structural details on the well depth of the two interaction potentials. Whereas Ag structures the first He solvation layer similarly, to some extent, to a positive ion such as Na(+), Cu appears to require the onset of a second solvation shell for a similar dense structure to be formed despite an interaction well of 28.4 mu hartree. An additional signature of the different solution behavior between Ag and Cu appears also in the dependence of the energy required to evaporate a single He atom on the size of the MeHe(n) clusters. The absorption spectrum for the (2)P <- (2)S excitation of the metals was also simulated employing the semi-classical Lax approximation to further characterize Me@He(n) (n = 2-100) using novel accurate interaction potentials between He and the lowest (2)P state of Ag and Cu in conjunction with the Diatomic-in-Molecules approach. The results indicated that Ag exciplexes should not form via a direct vertical excitation into an attractive region of the excited manifolds and that there is an interesting dependence of the shape of the Cu excitation bands on the local structure of the first,solvation shell
Germanium K edge in GeO2 polymorphs. Correlation between local coordination and electronic structure of germanium
No full textThe electronic properties of two room Temperature persistent phases of germanium dioxide have been studied by means of
experimental and theoretical techniques. We collected the Ge-K edge XANES spectra of these materials at the GILDA beamline of
ESRF. The Density Of States of the two crystal phases, obtained from fully periodic Hartree-Fock and Density Functional calculations,
is taken as the reference term to rationalise and assign the manifolds of the XANES spectra. Although this scheme requires a number of
severe approximations, we obtained a good overall agreement between experiment and theory. The topological analysis of the
theoretical electron density distribution in the crystals gave further information regarding the electronic properties of germanium
dioxide
A new MO-VB approach for the determination of intermolecular forces. Theory and application to the He2, He-CH4 and He-H2O systems
No full textWe present the improvement of a previously developed strategy for the evaluation of intermolecular forces.
The approach defines a variational VB (valence bond) wave function, consisting of single and double excitations
from the SCF-MI (self-consistent field for molecular interactions) determinant. The central idea of the method
is the determination of optimal virtual orbitals, to contract the virtual space spanned by all singly and doubly
excited localized configurations, by means of an iterative optimization procedure. The performance of the
strategy is tested by comparison with results where the full virtual space is considered, and the entire approach
is also compared with more conventional quantum chemical methods. Test calculations on three weakly
interacting complexes, namely, He2, He-CH4, and He-H2O, are presented. Whatever the system studied, we
found an overall agreement between VB, MP4, and CCSD(T) results. The VB well depths estimates are
somewhat larger than MP4 and CCSD(T) ones
Potential energy surface, bound states, and rotational inelastic cross sections of the He-CH4 system: A theoretical investigation
No full textWe determined two potential energy surfaces (PES) for the He-CH4 system by means of MP4 and Valence Bond (VB) calculations. The MP4 potential is similar to the one commonly adopted for this system [U. Buck, K. H. Kohl, A. Kolhase, M. Faubel, and U. Staemmler, Mol. Phys. 55, 1255 (1985)], while the VB PES is slightly more attractive. To evaluate the reliability of these potentials, we investigated the scattering properties by performing close coupling calculations, and concluded that: (i) the available experimental data do not permit the ranking among the PES considered; (ii) some theoretical predictions differ considerably from the experimental data, and these discrepancies cannot entirely be ascribed to the inaccuracy of the ab initio calculations; (iii) the scattering properties at low energy might discriminate between the MP4 and VB potentials
Predicting atomic dopant solvation in helium clusters: The MgHen case
No full textWe present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg-doped helium clusters MgHen with n=2-50. Three high-level [MP4, CCSD(T), and CCSDT] MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in He-n. Moreover, we found interesting size effects for the behavior of the Mg impurity. As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited P-1 Mg atom and the He moiety has been approximated using the diatomics-in-molecules method and the two excited (1)Pi and (1)Sigma MgHe potentials. The shape of the simulated MgHe50 spectra shows a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed. To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact density-functional theory has been adapted to the case of doped He-n and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviors. (C) 2005 American Institute of Physics
The electronic structure of nitrilimine: absence of the carbenic form
No full textThe electronic structure of nitrilimine HCNNH is shown to essentially be propargylic by CASSCF and Spin-Coupled (modern VB) calculations; in contrast to a recent claim, the carbenic resonance form is absent
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