Geometric parameters of binary ͑1:1͒ PdZn and PtZn alloys with CuAu-L1 0 structure were calculated with a density functional method. Based on the total energies, the alloys are predicted to feature equal formation energies. Calculated surface energies of PdZn and PtZn alloys show that ͑111͒ and ͑100͒ surfaces exposing stoichiometric layers are more stable than ͑001͒ and ͑110͒ surfaces comprising alternating Pd ͑Pt͒ and Zn layers. The surface energy values of alloys lie between the surface energies of the individual components, but they differ from their composition weighted averages. Compared with the pure metals, the valence d-band widths and the Pd or Pt partial densities of states at the Fermi level are dramatically reduced in PdZn and PtZn alloys. The local valence d-band density of states of Pd and Pt in the alloys resemble that of metallic Cu, suggesting that a similar catalytic performance of these systems can be related to this similarity in the local electronic structures.

Adsorption complexes of palladium atoms on F s , F s ϩ , F s 2ϩ , and O 2Ϫ centers of MgO͑001͒ surface have been investigated with a gradient-corrected ͑Becke-Perdew͒ density functional method applied to embedded cluster models. This study presents the first application of a self-consistent hybrid quantum mechanical/molecular mechanical embedding approach where the defect-induced distortions are treated variationally and the environment is allowed to react on perturbations of a reference configuration describing the regular surface. The cluster models are embedded in an elastic polarizable environment which is described at the atomistic level using a shell model treatment of ionic polarizabilities. The frontier region that separates the quantum mechanical cluster and the classical environment is represented by pseudopotential centers without basis functions. Accounting in this way for the relaxation of the electronic structure of the adsorption complex results in energy corrections of 1.9 and 5.3 eV for electron affinities of the charged defects F s ϩ and F s 2ϩ , respectively, as compared to models with a bulk-terminated geometry. The relaxation increases the stability of the adsorption complex Pd/F s by 0.4 eV and decreases the stability of the complex Pd/F s 2ϩ by 1.0 eV, but it only weakly affects the binding energy of Pd/F s ϩ . The calculations provide no indication that the metal species is oxidized, not even for the most electron deficient complex Pd/F s 2ϩ . The binding energy of the complex Pd/O 2Ϫ is calculated at Ϫ1.4 eV, that of the complex Pd/F s 2ϩ at Ϫ1.3 eV. The complexes Pd/F s and Pd/F s ϩ exhibit notably higher binding energies, Ϫ2.5 and Ϫ4.0 eV, respectively; in these complexes, a covalent polar adsorption bond is formed, accompanied by donation of electronic density to the Pd 5s orbital.

Using a combination of local density functional theory and cluster exact diagonalization based dynamical mean field theory, we calculate many body electronic structures of several Mott insulating oxides including undoped high Tc materials. The dispersions of the lowest occupied electronic states are associated with the Zhang-Rice singlets in cuprates and with doublets, triplets, quadruplets and quintets in more general cases. Our results agree with angle resolved photoemission experiments including the decrease of the spectral weight of the Zhang-Rice band as it approaches k=0.Quasiparticle excitations in insulating transition metal oxides (TMOs) such as classical Mott-Hubbard systems or undoped high temperature superconductors (HTSCs) have been puzzling electronic structure theorists for many years [1,2]. While photoemission experiments in these materials show [3] the existence of the d-states located both right below the Fermi energy and at much higher binding energies (typically ∼10 eV), it is difficult to understand this genuine many-body redistribution of the spectral weight using calculations based on a static mean field theory [4,5], such as the density functional theory (DFT) in its local density approximation (LDA) [6]. Modern approaches, such as LDA+U [7], can differentiate between charge-transfer and Mott-Hubbard natures of these systems [8], but still have difficulties in recovering insulating behavior of the paramagnetic (PM) state and tackling more complicated many-body features such as Zhang-Rice (ZR) singlet of HTSCs [3,9]. Only most recent developments based on a combination of local density approximation (LDA) and dynamical mean field theory (DMFT) [10] have started to address those issues [11,12,13].In the present work, using a novel implementation of LDA plus cluster exact diagonalization based DMFT we demonstrate how to obtain accurate spectra of transition metal oxides and, in particular, describe full momentum dependent low-energy excitations associated in those systems with antiferromagnetic (AFM) Kondo-like coupling between a spin of oxygen hole injected by photoemission process and a local magnetic moment of the transition metal ion. These narrow energy bands are composed from the well known Zhang-Rice singlet states in cuprates [9], which have recently renewed their attention in connection with the disappearance of their spectral weight as the wave vector approaches the Brillouin Zone (BZ) center, and the observed high energy kink entitled as "waterfall" feature [14]. Zhang-Rice doublets have been discussed in NiO [15], and their further generalizations to triplets (CoO), quadruplets (FeO) and quintets (MnO) all naturally emerge from our LDA+DMFT calculations. We find that the ZR states exhibit a similar behavior in all systems including the loss of their spectral weight at the Γ point, which can be understood as the lack of hybridization between transition metal d states and neighboring oxygen p states, the effect most pronounced in HTSCs. There is a generally good agreement between o...

ABSTRACT:We present a novel procedure for treating the exchange-correlation contributions in the Kohn-Sham procedure. The approach proposed is fully variational and closely related to the so-called "fitting functions" method for the Coulomb Hartree problem; in fact, the method consistently uses this auxiliary representation of the electron density to determine the exchange-correlation contributions. The exchange-correlation potential and its matrix elements in a basis set of localized (atomic) orbitals can be evaluated by reusing the three-center Coulomb integrals involving fitting functions, while the computational cost of the remaining numerical integration is significantly reduced and scales only linearly with the size of the auxiliary basis. We tested the approach extensively for a large set of atoms and small molecules as well as for transition-metal carbonyls and clusters, by comparing total energies, atomization energies, structure parameters, and vibrational frequencies at the local density approximation and generalized gradient approximation levels of theory. The method requires a sufficiently flexible auxiliary basis set. We propose a minimal extension of the conventional auxiliary basis set, which yields essentially the same accuracy for the quantities just mentioned as the standard approach. The new method allows one to achieve substantial savings compared with a fully numerical integration of the exchange-correlation contributions.