BAND DISPERSION IN THE RECURSION METHOD

The advantages of the supercell model in employing the recursion method are discussed in comparison with the cluster model. A transformation for changing complex Bloch-sum seed states to real seed states in recursion calculations is presented and band dispersion in the recursion method is extracted with use of the Lanczos algorithm. The method is illustrated by the band structure of GaAs in the empirical tight-binding parametrized model. In the supercell model, the treatment of boundary conditions is discussed for various seed-state choices. The method is useful in applying tight-binding techniques to systems with substantial deviations from periodicity.

Gound State Properties of Nuclei in (295)118 alpha-Decay Chain

The properties of nuclei belonging to the alpha-decay chain of superheavy element (295)118 have been studied in the framework of axially deformed relativistic mean field (RMF) theory with the parameter set of NL-Z2 in the blocked BCS approximation. Some ground state properties such as binding energies, deformations, and alpha-decay energies Q(alpha) have been obtained and agree well with those from finite-range droplet model (FRDM). The single-particle spectra of nuclei in (295)118 alpha-decay chain show that the shell gaps present obviously nucleon number dependence. The root-mean-square (rms) radii of proton, neutron and matter distributions change slowly from (283)112 to (295)118 but dramatically from (279)110 to (283)112, which may be due to the subshell closure at Z = 110 in (279)110. The alpha-decay half-lives in (295)118 decay chain are evaluated by employing the cluster model and the generalized liquid drop model (GLDM), and the overall agreement is found when they are compared with the known experimental data. The alpha-decay lifetimes obtained from the cluster model are slightly larger than those of GLDM ones. Finally, we predict the alpha-decay half-lives of Z = 118, 116, 114, 112 isotopes using the cluster model and GLDM, which also indicate these two models can corroborate each other in studies on superheavy nuclei. The results from GLDM are always lower than those obtained from the cluster model.

Theoretical Analysis and New Formulae for Half-Lives of Proton Emission

The proton radioactivity half-lives of spherical proton emitters are calculated by the cluster model with the contribution of a centrifugal potential barrier considered separately. The results are compared with the experimental data and other theoretical data, and good agreement is found for most nuclei. In addition, two formulae are proposed for the proton decay half-life of spherical proton emitters through the least squares fit to the experimental data available, and could reproduce the experimental half-lives successfully.

alpha-decay half-lives of superheavy nuclei and general predictions

The generalized liquid drop model (GLDM) and the cluster model have been employed to calculate the alpha-decay half-lives of superheavy nuclei (SHN) using the experimental alpha-decay Q values. The results of the cluster model are slightly poorer than those from the GLDM if experimental Q values are used. The prediction powers of these two models with theoretical Q values from Audi et al. (Q(Audi)) and Muntian et al. (Q(M)) have been tested to find that the cluster model with Q(Audi) and Q(M) could provide reliable results for Z > 112 but the GLDM with Q(Audi) for Z <= 112. The half-lives of some still unknown nuclei are predicted by these two models and these results may be useful for future experimental assignment and identification.

Half-Lives of Superheavy Nuclei in Z=113 Alpha Decay Chain

The generalized liquid drop model (GLDM), including the proximity effects and centrifugal potential, and the cluster model with Cosh potential are used to study the half-lives of some Z=113 isotopes and their alpha-decay products.The experimental half-lives of (284)113, (283)113, (282)113and their alpha-decay products are well reproduced by the two models when zero angular momenta transfer is assumed. For (278)113 and its alpha-decay products, both the GLDM andthe cluster model could provide satisfactory results if we assume the alpha particle carry five units of angular momenta, which indicates that possible non zero angular momenta transfer and need further experimental measurements with high precision. Finally, we show that half-lives of alpha-decay are quite sensitive to the angular momentum transfers, and a formula could be used to describe the correlation between alpha-decay half-life and angular momentum transfer successfully.

A density functional theory study of CO and atomic oxygen chemisorption on Pt(111)

Ab initio total energy calculations within a density functional theory framework have been performed for CO and atomic oxygen chemisorbed on the Pt(111) surface. Optimised geometries and chemisorption energies for CO and O on four high-symmetry sites, namely the top, bridge, fee hollow and hcp hollow sites, are presented, the coverage in all cases being 0.25 ML. The differences in CO adsorption energies between these sites are found to be small, suggesting that the potential energy surface for CO diffusion across Pt(111) is relatively flat. The 5 sigma and 2 pi molecular orbitals of CO are found to contribute to bonding with the metal. Some mixing of the 4 sigma and 1 pi molecular orbitals with metal states is also observed. For atomic oxygen, the most stable adsorption site is found to be the fee hollow site, followed in decreasing order of stability by the hcp hollow and bridge sites, with the top site being the least stable. The differences in chemisorption energies between sites for oxygen are larger than in the case of CO, suggesting a higher barrier to diffusion for atomic oxygen. The co-adsorption of CO and O has also been investigated. Calculated chemisorption energies for CO on an O/fcc-precovered surface show that of the available chemisorption sites, the top site at the oxygen atom's next-nearest neighbour surface metal atom is the most stable, with the other four sites calculated bring at least 0.29 eV less stable. The trend of CO site stability in the coadsorption system is explained in terms of a 'bonding competition' model. (C) 2000 Elsevier Science B.V. All rights reserved.

Ab initio theoretical comparative study of magnetic coupling in KNiF3 and K2NiF4s

The origin of magnetic coupling in KNiF3 and K2 NiF4 is studied by means of an ab initio cluster model approach. By a detailed study of the mapping between eigenstates of the exact nonrelativistic and spin model Hamiltonians it is possible to obtain the magnetic coupling constant J and to compare ab initio cluster-model values with those resulting from ab initio periodic Hartree-Fock calculations. This comparison shows that J is strongly determined by two-body interactions; this is a surprising and unexpected result. The importance of the ligands surrounding the basic metal-ligand-metal interacting unit is reexamined by using two different partitions and the constrained space orbital variation method of analysis. This decomposition enables us to show that this effect is basically environmental. Finally, dynamical electronic correlation effects have found to be critical in determining the final value of the magnetic coupling constant.

Local character of magnetic coupling in ionic solids

Magnetic interactions in ionic solids are studied using parameter-free methods designed to provide accurate energy differences associated with quantum states defining the Heisenberg constant J. For a series of ionic solids including KNiF3, K2NiF4, KCuF3, K2CuF4, and high- Tc parent compound La2CuO4, the J experimental value is quantitatively reproduced. This result has fundamental implications because J values have been calculated from a finite cluster model whereas experiments refer to infinite solids. The present study permits us to firmly establish that in these wide-gap insulators, J is determined from strongly local electronic interactions involving two magnetic centers only thus providing an ab initio support to commonly used model Hamiltonians.

Ab Initio study of magnetic interactions in the KCuF3 and K2CuF4 low-dimensional Systems

The ab initio cluster model approach has been used to study the electronic structure and magnetic coupling of KCuF3 and K2CuF4 in their various ordered polytype crystal forms. Due to a cooperative Jahn-Teller distortion these systems exhibit strong anisotropies. In particular, the magnetic properties strongly differ from those of isomorphic compounds. Hence, KCuF3 is a quasi-one-dimensional (1D) nearest neighbor Heisenberg antiferromagnet whereas K2CuF4 is the only ferromagnet among the K2MF4 series of compounds (M=Mn, Fe, Co, Ni, and Cu) behaving all as quasi-2D nearest neighbor Heisenberg systems. Different ab initio techniques are used to explore the magnetic coupling in these systems. All methods, including unrestricted Hartree-Fock, are able to explain the magnetic ordering. However, quantitative agreement with experiment is reached only when using a state-of-the-art configuration interaction approach. Finally, an analysis of the dependence of the magnetic coupling constant with respect to distortion parameters is presented.

Electronic structure and magnetic interactions of the spin-chain compounds Ca2CuO3 and Sr2CuO3

The results are presented of a combined periodic and cluster model approach to the electronic structure and magnetic interactions in the spin-chain compounds Ca2CuO3 and Sr2CuO3. An extended t-J model is presented that includes in-chain and interchain hopping and magnetic interaction processes with parameters extracted from ab initio calculations. For both compounds, the in-chain magnetic interaction is found to be around -240 meV, larger than in any of the other cuprates reported in the literature. The interchain magnetic coupling is found to be weakly antiferromagnetic, -1 meV. The effective in-chain hopping parameters are estimated to be ~650 meV for both compounds, whereas the value of the interchain hopping parameter is 30 meV for Sr2CuO3 and 40 meV for Ca2CuO3, in line with the larger interchain distance in the former compound. These effective parameters are shown to be consistent with expressions recently suggested for the Néel temperature and the magnetic moments, and with relations that emerge from the t-J model Hamiltonian. Next, we investigate the physical nature of the band gap. Periodic calculations indicate that an interpretation in terms of a charge-transfer insulator is the most appropriate one, in contrast to the suggestion of a covalent correlated insulator recently reported in the literature.

Adsorption energy and spin state of first-row transition metals adsorbed on MgO(100)

Slab and cluster model spin-polarized calculations have been carried out to study various properties of isolated first-row transition metal atoms adsorbed on the anionic sites of the regular MgO(100) surface. The calculated adsorption energies follow the trend of the metal cohesive energies, indicating that the changes in the metal-support and metal-metal interactions along the series are dominated by atomic properties. In all cases, except for Ni at the generalized gradient approximation level, the number of unpaired electron is maintained as in the isolated metal atom. The energy required to change the atomic state from high to low spin has been computed using the PW91 and B3LYP density-functional-theory-based methods. PW91 fails to predict the proper ground state of V and Ni, but the results for the isolated and adsorbed atom are consistent within the method. B3LYP properly predicts the ground state of all first-row transition atom the high- to low-spin transition considered is comparable to experiment. In all cases, the interaction with the surface results in a reduced high- to low-spin transition energy.

Ground- and excited-state properties of M-center oxygen vacancy aggregates in the bulk and the surface of MgO

Aggregates of oxygen vacancies (F centers) represent a particular form of point defects in ionic crystals. In this study we have considered the combination of two oxygen vacancies, the M center, in the bulk and on the surface of MgO by means of cluster model calculations. Both neutral and charged forms of the defect M and M+ have been taken into account. The ground state of the M center is characterized by the presence of two doubly occupied impurity levels in the gap of the material; in M+ centers the highest level is singly occupied. For the ground-state properties we used a gradient corrected density functional theory approach. The dipole-allowed singlet-to-singlet and doublet-to-doublet electronic transitions have been determined by means of explicitly correlated multireference second-order perturbation theory calculations. These have been compared with optical transitions determined with the time-dependent density functional theory formalism. The results show that bulk M and M+ centers give rise to intense absorptions at about 4.4 and 4.0 eV, respectively. Another less intense transition at 1.3 eV has also been found for the M+ center. On the surface the transitions occur at 1.6 eV (M+) and 2 eV (M). The results are compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

Ionic-covalent transition in titanium oxides

The nature of the chemical bond in three titanium oxides of different crystal structure and different formal oxidation state has been studied by means of the ab initio cluster-model approach. The covalent and ionic contributions to the bond have been measured from different theoretical techniques. All the analysis is consistent with an increasing of covalence in the TiO, Ti2O3, and TiO2 series as expected from chemical intuition. Moreover, the use of the ab initio cluster-model approach combined with different theoretical techniques has permitted us to quantify the degree of ionic character, showing that while TiO can approximately be described as an ionic compound, TiO2 is better viewed as a rather covalent oxide.

Some Aspects of Domain Specific Conceptual Modeling for Simulation of Logistic Terminal Operations

When simulation modeling is used for performance improvement studies of complex systems such as transport terminals, domain specific conceptual modeling constructs could be used by modelers to create structured models. A two stage procedure which includes identification of the problem characteristics/cluster - ‘knowledge acquisition’ and identification of standard models for the problem cluster – ‘model abstraction’ was found to be effective in creating structured models when applied to certain logistic terminal systems. In this paper we discuss some methods and examples related the knowledge acquisition and model abstraction stages for the development of three different types of model categories of terminal systems

Propuesta para el desarrollo de un clúster logístico para un corredor logístico nacional e internacional competitivo en Colombia

Este trabajo es una ‘’Propuesta para el desarrollo de un cluster logístico para un Corredor Logístico Nacional e Internacional competitivo en Colombia’’; en esa medida, comienza por ser un recuento de información, cifras y datos, que desnuda el presente de Colombia, y reconoce las deficiencias existentes en materia de competitividad logística nacional frente al mundo, así como los riesgos que existen para la economía nacional al no tener la suficiente preparación para aprovechar las oportunidades comerciales del presente siglo, ni para enfrentar las amenazas que esto mismo trae consigo. Es por ello que adoptar un modelo de Clúster a la Logística Nacional adaptable a cualquier corredor logístico del país, será un frente de desarrollo potencial en la búsqueda de la competitividad logística colombiana, generando así mismo un beneficio colateral en materia de comercio exterior e internacional.