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This theoretical work presents the electronic and magnetic properties of Cr doped SrFe12O19 using first-principles calculations based on density functional theory. An improvement in the magnetic properties of SrFe12O19 is predicted when doped with the Cr3+ ion. It was found that the magnetic Cr3+ ion preferentially replaces Fe3+ ion at 2a, 4f1, and 12k sites. The calculation demonstrates that if Cr ion occupies the 4f1 site, a total magnetization of 52.920 emu/g is obtained. On the other hand, SrFe11.5Cr0.5O19 behaves as a semiconductor when Cr occupies the 2a and 12k sites while behaves like half-metal when Cr occupies the 4f1 site. The higher magnetization, identified for the SrFe11.5Cr0.5O19 compound, provides unique qualities in the applications of permanent magnets and/or related devices like spintronics and magnetic semiconductors.

En cierta forma, la mecánica cuántica da solución a todos los problemas de la química, lo único que hay que hacer es resolver las ecuaciones de Schrödinger para las moléculas de interés. Desafortunadamente, el costo computacional de resolver estas ecuaciones crece exponencialmente con el número de electrones y para más de ~100 electrones resulta imposible resolverlas con precisión química (~2 kcal/mol). Las ecuaciones de Kohn-Sham (KS) de la teoría del funcional de la densidad (density functional theory, DFT) permiten reformular las ecuaciones de Schrödinger usando la densidad de probabilidad electrónica como la variable central sin necesidad de calcular las funciones de onda de Schrödinger. El costo de resolver las ecuaciones de Kohn-Sham solo crece como N3, donde N es el número de electrones, lo que ha llevado a la inmensa popularidad de la DFT en química. A pesar de esta popularidad, incluso las aproximaciones más sofisticadas de las KS-DFT llevan a errores que limitan el uso de métodos basados exclusivamente en la densidad electrónica. En este artículo se discute cómo pueden desarrollarse nuevos métodos que escalen linealmente con N usando densidades de fragmentos como las variables principales en lugar de densidades totales, así como la forma en que estos métodos proveen una respuesta atractiva a la pregunta del subtítulo: ¿cuál es la forma de los átomos en las moléculas?. In some sense, quantum mechanics solves all the problems in chemistry: The only thing one has to do is solve the Schrödinger equation for the molecules of interest. Unfortunately, the computational cost of solving this equation grows exponentially with the number of electrons and for more than ~100 electrons, it is impossible to solve it with chemical accuracy (~ 2 kcal/mol). The Kohn-Sham (KS) equations of density functional theory (DFT) allow us to reformulate the Schrödinger equation using the electronic probability density as the central variable without having to calculate the Schrödinger wave functions. The cost of solving the Kohn-Sham equations grows only as N3, where N is the number of electrons, which has led to the immense popularity of DFT in chemistry. Despite this popularity, even the most sophisticated approximations in KS-DFT result in errors that limit the use of methods based exclusively on the electronic density. By using fragment densities (as opposed to total densities) as the main variables, we discuss here how new methods can be developed that scale linearly with N while providing an appealing answer to the subtitle of the article: What is the shape of atoms in molecules?.

We present detailed studies of the high-field magnetoresistance of the layered organic metal κ-(BETS)2Mn- [N(CN)2]3 under a pressure slightly above the insulator-metal transition. The experimental data are analyzed in terms of the Fermi surface properties and compared with the results of first-principles band structure calculations. The calculated size and shape of the in-plane Fermi surface are in very good agreement with those derived from Shubnikov-de Haas oscillations as well as the classical angle-dependent magnetoresistance oscillations. A comparison of the experimentally obtained effective cyclotron masses with the calculated band masses reveals electron correlations significantly dependent on the electron momentum. The momentum- or band-dependent mobility is also reflected in the behavior of the classical magnetoresistance anisotropy in a magnetic field parallel to layers. Other characteristics of the conducting system related to interlayer charge transfer and scattering mechanisms are discussed based on the experimental data. Besides the known high-field effects associated with the Fermi surface geometry, new pronounced features have been found in the angle-dependent magnetoresistance, which might be caused by coupling of the metallic charge transport to a magnetic instability in proximity to the metal-insulator phase boundary., We are grateful to N.D. Kushch for providing the high-quality crystals for our studies and to P.D. Grigoriev for numerous useful discussions. The work was supported by the German Research Foundation (DFG) via the Grant No. KA 1652/4-1. The high-field measurements were done under support of the LNCMI-CNRS, member of the European Magnetic Field Laboratory (EMFL). V.N.Z. acknowledges the support from RFBR Grant No. 18-02-00280. Work in Spain was supported by the Spanish Ministerio de Economa y Competitividad (Grants No. FIS2015-64886-C5-4-P and No. CTQ2015-64579-C3-3-P) and Generalitat de Catalunya (2017SGR1506, 2017SGR1289, and XRQTC). E.C. acknowledges support from the Severo Ochoa Centers of Excellence Program under Grant No. SEV-2015-0496. P.A. acknowledges support from the Maria de Maeztu Units of Excellence Program under Grant No. MDM-2017-0767.

Producción Científica, Nickel and silver are metals with interesting properties of technological relevance: nickel is a well known ferromagnet and silver has antibacterial properties. Both exist in the face centered cubic phase but are immiscible. In the context of alloys at the nanoscale, one can play with the size to ne tune a desired property, or to achieve new properties and functionalities that do not exist at the macroscopic regime. In this work, we explore how the subtle interaction between Ni and Ag triggers the chemical order, the electronic structure, and the magnetic properties of a AgNi nanoalloy of 55 atoms, a size that can accommodate core/shell con gurations with sizable parts. Calculations are conducted within the density functional theory at the generalized gradient approximation for exchange and correlation. We determine, in the whole composition range, the chemical order, absolute and relative stabilities by means of binding energy, excess energy and second energy di erence, as well as total and part-projected spin-polarized electronic densities of states and local charge and spin magnetic moments distribution. Ni-core/Ag-shell structures are particularly stable, but contrary to what one would expect by simply extrapolating the properties of the pure Ag and Ni clusters or of pure fcc bulks, we nd unexpected behaviors along the composition range, such as quenched magnetic moments in Ni, total magnetic moments essentially contributed in some cases by Ag, or electronic charge transfer that changes its sign depending on the stoichiometry. These behaviors lead to magnetic transitions as a function of the composition, and di er, in some cases, from those of the smaller 13-atoms AgNi nanoalloys of the same symmetry with which we compare, a further demonstration of the complex nature of nanostructures. The above trends are robust against ionization and electron capture., Ministerio de Economía, Industria y Competitividad y Fondo Europeo de Desarrollo Regional (FEDER) (project FIS2014-59279-P ), Consejo Nacional de Ciencia y Tecnologia (CONACyT) (project n. 237882)

En el presente trabajo se utilizó el código Wien2k, en el marco de la teoría del funcional de la densidad de Kohn-Sham, aplicando el método de ondas planas aumentadas y linealizadas (full-potential linearized augmented plane wave, FP- LAPW) y adoptando la aproximación de gradiente generalizado (GGA) de Perdew, Burke y Ernzerhof para la energía de intercambio y correlación, así como la aproximación de densidad local (local density approximation, LDA) para el cálculo de la densidad de estados y la estructura de bandas de la perovskita doble Sr2HoNbO6. Para los cálculos se consideró el grupo Fmm (#225), experimentalmente obtenido a partir de mediciones de difracción de rayos X y del método de refinamiento de Rietveld. El parámetro de red experimental fue de 8.018 Å, el cual concuerda en un 99,2 % con las predicciones teóricas efectuadas a partir de la minimización de la energía mediante la ecuación de estado de Murnaghan. A partir de mediciones de susceptibilidad magnética en función de la temperatura y del ajuste con la ley de Curie, se obtuvo el valor del momento magnético efectivo 10,01 μB. Este valor es muy cercano del esperado teóricamente a partir de las reglas de Hund (10,60 μB). La brecha de energía determinada entre las bandas de valencia y de conducción fue de 3,3 eV, lo que revela el carácter aislante de la perovskita compleja Sr2HoNbO6 para la configuración de espín hacia arriba, en tanto que se observó el carácter semiconductor para la polarización de espín hacia abajo, con una brecha de energía de 0,77 eV. Las propiedades termodinámicas se calcularon a partir de la ecuación de estado usando el modelo cuasi-armónico de Debye. Un comportamiento del calor específico, con CV≈CP, se encontró a temperaturas inferiores a T = 500 K, con valores del límite de Dulong-Petit que doblaban los que se han reportado para materiales del tipo de la perovskita. In this work we used the Wien2k code, within the framework of the Kohn-Sham Density Functional Theory (DFT), applying the Full-Potential Linearized Augmented Plane Wave method (FP-LAPW) and adopting the Generalized Gradient approximation (GGA) for the exchange-correlation energy due to Perdew, Burke, and Ernzerhof, as well as the Local Density approximation (LDA) for the calculation of the Density of States and band structure of the Sr2HoNbO6 double perovskite. For calculations, we considered the Fmm (#225) space group, which was experimentally obtained from X-ray diffraction measurements and Rietveld refinement. The experimental lattice parameter was 8.018 Å, which is 99.2% in agreement with the theoretical prediction from the minimization of energy through the Murnaghan state equation. From the measurements of magnetic susceptibility as a function of temperature and the adjustment with the Curie law, we obtained a value for the effective magnetic moment of 10.01 μB, which is close to the theoretical expected from Hund’s Rule (10.60 μB). An energy gap of 3.3 eV between the valence band and the conduction band revealed the insulator character of the Sr2HoNbO6 complex perovskite for the spin up configuration, but a semiconductor feature was observed for the spin down polarization, with an energy gap of 0.77 eV. The thermodynamic properties were calculated from the state equation by using the Debye quasiharmonic model. A specific heat behavior of CV≈CP was found at temperatures below T = 500 K, with Dulong-Petit limit values doubling those reported for perovskite materials.

Los hidratos clatratos de dióxido de carbono son compuestos de inclusión que se han propuesto como posibles materiales moleculares en la lucha contra problemas ambientales relacionados con la captura y el almacenamiento de gases de efecto invernadero. A pesar del creciente interés en tales hidratos y sus aplicaciones tecnológicas, el nivel de comprensión molecular de su formación y caracterı́sticas aún está lejos de ser completo. El modelado de las interacciones es una tarea costosa computacionalmente, sin embargo es esencial para determinar de manera confiable las propiedades moleculares. Este trabajo tiene como objetivo estudiar las interacciones jaula-huésped dominadas por enlaces de hidrógeno y fuerzas van der Waals en un clatrato con estructura sI y CO 2 encapsulado. Se investigaron sistemáticamente diferentes funcionales de densidad por medio de cálculos periódicos ab initio en el marco de la Teorı́a del Funcional Densidad para lo que se realizó una evaluación comparativa del rendimiento de los funcionales PBE, revPBE, PW86PBE, PW91, BLYP, opt88B y optB86b, considerando correcciones de dispersión locales tales como el modelo de Momento Dipolar de Intercambio (XDM) y el modelo semiempı́rico de Grimme (DFT-D2), ası́ como funcionales no locales tipo optB88vdW y optB86bvdW. Las interacciones se estudiaron mediante la estimación de las energı́as de unión, de cohesión y saturación para diferentes configuraciones del sistema. Los resultados obtenidos señalan que las interacciones de dispersión son importantes en la estabilización de las energı́as jaula-huésped y la encapsulación del CO2 dentro de las jaulas es energéticamente favorable. Ademas se concluye que los funcionales PW86PBE y PBE con dispersión XDM son capaces de predecir la estructura de equilibrio y dar razón de las interacciones del sistema. Abstract: Clathrate hydrates of CO 2 are inclusion compounds that have been proposed as possible molecular materials in the fight against environmental problems related with greenhouse gases capture and storage. Despite the increasing interest in such hydrates and their technological applications, the molecular level understanding of their formation and characteristics is still far from complete. Modeling of interactions is a challenging and computational demanding task, however it is essential to reliably determine the molecular properties. The objective of this work is to study the cage-host interactions dominated by hydrogen bonds and van der Waals forces in a sI clathrate with encapsulated CO 2 . Different density functionals were systematically investigated by periodic ab initio calculations within the framework of the Functional Density Theory. A benchmarking the performance of PBE, revPBE, PW86PBE, PW91, BLYP, opt88B and optB86b functionals was carried out, this work considers local dispersion corrections such as the Exchange-hole Dipole Moment (XDM) and the Grimme semi-empirical model (DFT-D2), as well as the non-local functional optB88vdW and optB86bvdW. The interactions were studied by estimating the binding, cohesion and saturation energies for different system configurations. The results indicate that the dispersion interactions are important in the stabilization of the cage-host energies, that encapsulation of the CO2 in the cages is energetically favorable. We may conclude that functionals PW86PBE and PBE with XDM correction are able to predict the equilibrium structure and give a descripcion for the interactions of the system. Maestría

Perovskite-like materials, which include magnetic elements, have relevance because their technological perspectives of applications in the spintronics industry. In this work, the magnetic, structural, electronic and thermodynamic properties of the Ba2TiMnO6 of the perovskite-like manganite are investigated. Calculations are carried out through the Full-Potential Linear Augmented Plane Wave method (FP-LAPW) within the framework of the Density Functional Theory (DFT) with exchange and correlation effects in the Generalized Gradient (GGA) and Local Density (LDA) approximations, including spin polarization. From the minimization of energy as a function of volume using the Murnaghan’s state equation the equilibrium lattice parameter and cohesive properties of this compound were obtained. The study of the electronic structure was based in the analysis of the electronic density of states (DOS), and the band structure, showing that this compound evidences an effective magnetic moment of 3.0 µB. The pressure and temperature dependence of specific heat, entropy, thermal expansion coefficient, Debye temperature and Grüneisen parameter were calculated by DFT from the state equation using the quasi-harmonic model of Debye. A specific heat behavior CV≈CP was found at temperatures below T = 400 K, with Dulong-Petit limit values, which are quite higher than those, reported for simple perovskites. Resumen Los materiales de tipo perovskita que incluyen elementos magnéticos son relevantes debido a sus perspectivas de aplicabilidad tecnológica en la industria de la espintrónica. En este trabajo fueron investigadas las propiedades magnéticas, estructurales, electrónicas y termodinámicas de la manganita de tipo perovskita Ba2TiMnO6. Los cálculos fueron realizados a través del método del potencial de ondas planas aumentadas y linealizadas (FP-LAPW), en el marco de la Teoría del Funcional Densidad (DFT), con efectos de intercambio y correlación en las aproximaciones de gradiente generalizado (GGA) y de densidad local (LDA), incluyendo polarización de espín. A partir de la minimización de la energía en función del volumen, usando la ecuación de estado de Murnaghan se obtuvieron los parámetros de equilibrio de la red las propiedades de cohesión de este compuesto. El estudio de la estructura electrónica se basó en el análisis de la densidad de estados (DOS) y la estructura de bandas, mostrando que este compuesto evidencia un momento magnético efectivo de 3.0 µB. la dependencia con la temperatura y la presión del calor específico, la entropía, el coeficiente de expansión térmica, la temperatura de Debye y el parámetro de Grüneisen fueron calculados mediante DFT a partir de la ecuación de estado, usando el modelo cuasi-armónico de Debye. Se encontró que el calor específico CV≈CP para temperaturas por debajo de T = 400 K, con un límite de Dulong-Petit relativamente mayor que el reportado para perovskitas simples.

Triarylmethyls (TAMs) are a class of long-lived purely organic radicals discovered at the beginning of the twentieth century. The chemical versatility and high stability of TAMs have lead to their application in many fields of science and technology. All compounds of this class are composed of three aryl rings bonded to a central carbon atom, where their unpaired electron mainly resides. Due to the π-conjugated electronic nature of this molecular structure, the possibility arises of controlling the unpaired electron localization (i.e. spin localization) by the torsion angles of the three aryl rings. By using density functional theory calculations (DFT) we have carefully investigated this phenomenon for a wide range of TAMs and probed how it is influenced by other important parameters such as chemical functionalization and temperature. Our results demonstrate that a single general spin versus structure relation is followed for all of our studied TAMs confirming that having a predictable structure-dependent spin localisation is an intrinsic feature of these radicals. Considering that spin localisation in TAMs is linked to many other important properties (e.g. magnetic interactions, optical absorption bands, magnetoresistance phenomena), the fact that manipulation of aryl ring twist angles could lead to molecular level control over such features presents enormous potential for future scientific and technological applications.

We have carried out first-principles total-energy calculations in order to study the electronic structure and thermodynamic properties of In 1-xBxP semiconducting alloys using the GGA and LDA formalisms within density functional theory (DFT) with a plane-wave ultrasoft pseudopotential scheme. We have also taken into account the correlation effects of the 3d-In orbitals within the LDA+U method to calculate the band-gap energy. We use special quasirandom structures to investigate the effect of the substituent concentration on structural parameter, band gap energy, mixing enthalpy and phase diagram of In1-xBxP alloys for x = 0, 0.25, 0.50, 0.75 and 1. It is found that the lattice parameters of the In 1-xBxP alloys decrease with B-concentration, showing a negative deviation from Vegard's law, while the bulk modulus increases with composition x, showing a large deviation from the linear concentration dependence (LCD). The calculated band structure presents a similar behavior for any B-composition using LDA, PBE or LDA+U approach. Our results predict that the band-gap shows a x-dependent nonlinear behavior. Calculated band gaps also shows a transition from (Γ→Γ)-direct to (Γ→Δ)- indirect at x = 0.611 and 0.566 for LDA and PBE functionals, respectively. Our calculations predict that the In1-xBxP alloy to be stable at unusual high temperature for both LDA and PBE potentials. © 2014 Elsevier B.V. All rights reserved. Hemos llevado a cabo cálculos de energía total de primeros principios para estudiar el Estructura electrónica y propiedades termodinámicas del semiconductor In 1-xBxP. aleaciones que utilizan los formalismos GGA y LDA dentro de la teoría funcional de la densidad (DFT) Con un esquema pseudopotencial ultrasoft de onda plana. También hemos tomado en cuenta los efectos de correlación de los orbitales 3d-In dentro del método LDA + U para calcular la energía de la brecha de banda. Usamos estructuras especiales cuasir aleatorias para investigar el efecto de la concentración de sustituyentes en el parámetro estructural, banda energía de separación, mezcla de entalpía y diagrama de fase de las aleaciones In1-xBxP para x = 0, 0.25, 0.50, 0.75 y 1. Se encuentra que los parámetros de red de las aleaciones In 1-xBxP disminuye con la concentración de B, mostrando una desviación negativa de la ley de Vegard, mientras que el módulo de volumen aumenta con la composición x, mostrando una gran desviación De la dependencia de la concentración lineal (LCD). La estructura de la banda calculada. presenta un comportamiento similar para cualquier composición B utilizando LDA, PBE o LDA + U enfoque. Nuestros resultados predicen que la brecha de banda muestra una no lineal dependiente de x comportamiento. Las brechas de banda calculadas también muestran una transición de (Γ → Γ) -directa a (Γ → Δ) - indirecto en x = 0.611 y 0.566 para las funciones LDA y PBE, respectivamente. Nuestros cálculos predicen que la aleación In1-xBxP se mantendrá estable en niveles inusuales Temperatura tanto para potenciales LDA como para PBE. © 2014 Elsevier B.V. Todos los derechos. reservado.

FeGa$_3$ is an unusual intermetallic semiconductor that presents intriguing magnetic responses to the tuning of its electronic properties. When doped with Ge, the system evolves from diamagnetic to paramagnetic to ferromagnetic ground states that are not well understood. In this work, we have performed a joint theoretical and experimental study of FeGa$_{3-x}$Ge$_x$ using Density Functional Theory and magnetic susceptibility measurements. For low Ge concentrations we observe the formation of localized moments on some Fe atoms and, as the dopant concentration increases, a more delocalized magnetic behavior emerges. The magnetic configuration strongly depends on the dopant distribution, leading even to the appearance of antiferromagnetic interactions in certain configurations., See "Note added to arXiv version" in text

In this paper, aluminum tungstate Al2(WO4)3 powders were synthesized using the co-precipitation method at room temperature and then submitted to heat treatment processes at different temperatures (100, 200, 400, 800, and 1000 °C) for 2 h. The structure and morphology of the powders were characterized by means of X-ray diffraction (XRD), Rietveld refinement data, and field emission scanning electron microscopy (FE-SEM) images. Their optical properties were examined with ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data showed that Al2(WO4)3 powders heat treated at 1000 °C for 2 h have a orthorhombic structure with a space group (Pnca) without the presence of deleterious phases. FE-SEM images revealed that these powders are formed by the aggregation of several nanoparticles leading to the growth of microparticles with irregular morphologies and an agglomerated nature. UV–vis spectra indicated that optical band gap energy increased from 3.16 to 3.48 eV) as the processing temperature rose, which was in turn associated with a reduction in intermediary energy levels. First-principle calculations were performed in order to understand the behavior of the PL properties using density functional theory at the B3LYP calculation level on periodic model systems and indicate the presence of stable electronic excited states (singlet). The analyses of the band structures and density of states at both ground and first excited electronic states provide insight into the main features, based on structural and electronic order–disorder effects in octahedral [AlO6] clusters and tetrahedral [WO4] clusters, as constituent building units of this material. The Brazilian authors acknowledge the financial support of the Brazilian research financing institutions: CNPq (304531/2013-8), FAPESP (09/50303-4; 12/18597-0), and CAPES. The Spanish authors would like to thank the Spanish Government MAT2010-21088-C03 project and the Catalan Government PIGC project 2009-SGR-0674. J. Andrés acknowledges Generalitat Valenciana (Prometeo/2009/053 project), Ministerio de Ciencia e Innovación (CTQ-2012-36253-C03-01) Programa de Cooperación Científica con Iberoamerica (Brazil), Ministerio de Educación (PHB2009-0065-PC project).

Rational design of improved transition metal based materials mostly relies on their electronic structure descriptors, typically estimated by density functional theory and so unduly affected by self-interaction or static correlation errors. Here we show for all 30 transition metals that original or width-corrected d-band centers, and Hilbert transform highest peak descriptors are unaffected by self-interaction, while poor treatment of static correlation by hybrid functionals leads to an unbalanced description. Thus, descriptors have a general validity unbiased by a specific computational method.

Fully relativistic full-potential density functional calculations with an all-electron linearized augmented plane waves plus local orbitals method were carried out to perform a comparative study on the structural and electronic properties of the cubic oxideδ-Bi2O3phase, which is considered as one of the most promising materials in a variety of applications including fuel cells, sensors, and catalysts. Three different density functionals were used in our calculations, LDA, the GGA scheme in the parametrization of Perdew, Burke, and Ernzerhof (PBE96), and the hybrid scheme of Perdew-Wang B3PW91. The examined properties include lattice parameter, band structure and density of states, and charge density profiles. For this modification the three functionals reveal the characteristics of a metal and the existence of minigaps at high symmetry points of the band structure when spin-orbit coupling is taken into account. Density of states exhibits hybridization of Bi 6s and O 2p orbitals and the calculated charge density profiles exhibit the ionic character in the chemical bonding of this compound. The B3PW91 hybrid functional provided a better agreement with the experimental result for the lattice parameter, revealing the importance of Hartree-Fock exchange in this compound.

An extension of the spin density functional theory simultaneously accounting for dielectric mismatch between neighboring materials and nonparabolicity corrections originating from interactions between conduction and valence bands is presented. This method is employed to calculate ground state and addition energy spectra of homogeneous and multishell spherical quantum dots. Our calculations reveal that corrections become especially relevant when they come into play simultaneously in strong regimes of spatial confinement.

6 pages.-- PACS numbers: 73.21.-b, 73.22.-f, 71.15.Mb.-- Final full-text version of the paper available at: http://dx.doi.org/10.1103/PhysRevB.76.245302., Within density functional theory, we address the effect of an in-plane electric field [script E] on the ground state and the density dipole response of a many-electron quantum ring, which is also submitted to a perpendicular magnetic field B. Addition energies and density dipole spectra are discussed as a function of [script E]. For the two-electron case, an exact numerical calculation is performed, obtaining the spin-phase diagram in the [script E]-B plane and showing that transitions between singlet and triplet spin states can be induced by varying the fields. We also find that, in spite of the deformation that [script E] causes in the electronic density, the spin of the ground state of a given electron number ring is very robust, changing little as the strength of the electric field is reasonably increased., This work has been performed under Grants No. FIS2005-01414 and No. FIS2005-02796 from DGI (Spain), No. 2005SGR00343 from Generalitat de Catalunya, and Grant No. INFN07-30 from the Italian INFN-Spanish DGI agreement.

A consistent extension of local spin density approximation (LSDA) to account for mass and dielectric mismatches in nanocrystals is presented. The extension accounting for variable effective mass is exact. Illustrative comparisons with available configuration interaction calculations show that the approach is also very reliable when it comes to account for dielectric mismatches. The modified LSDA is as fast and computationally low demanding as LSDA. Therefore, it is a tool suitable to study large particle systems in inhomogeneous media without much effort.

A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM = Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced by the structural rippling of this surface.

Both the intermolecular interaction energies and the geometries for $M\ensuremath{\cdots}$ thiophene, $M\ensuremath{\cdots}$ pyrrole, ${M}^{n+}\ensuremath{\cdots}$ thiophene, and ${M}^{n+}\ensuremath{\cdots}$ pyrrole (with $M=\mathrm{Li}$, Na, K, Ca, and Mg; and ${M}^{n+}={\mathrm{Li}}^{+},\phantom{\rule{0.3em}{0ex}}{\mathrm{Na}}^{+},\phantom{\rule{0.3em}{0ex}}{\mathrm{K}}^{+},\phantom{\rule{0.3em}{0ex}}{\mathrm{Ca}}^{2+}$, and ${\mathrm{Mg}}^{2+}$) have been estimated using four commonly used density functional theory (DFT) methods: B3LYP, B3PW91, PBE, and MPW1PW91. Results have been compared to those provided by HF, MP2, and MP4 conventional ab initio methods. The PBE and MPW1PW91 are the only DFT methods able to provide a reasonable description of the $M\ensuremath{\cdots}\ensuremath{\pi}$ complexes. Regarding ${M}^{n+}\ensuremath{\cdots}\ensuremath{\pi}$ complexes, the four DFT methods have been proven to be adequate in the prediction of these electrostatically stabilized systems, even though they tend to overestimate the interaction energies.

We explore the ability of the recently established quasilocal density functional theory for describing the isoscalar giant monopole resonance. Within this theory we use the scaling approach and perform constrained calculations for obtaining the cubic and inverse energy weighted moments (sum rules) of the RPA strength. The meaning of the sum rule approach in this case is discussed. Numerical calculations are carried out using Gogny forces and an excellent agreement is found with HF + RPA results previously reported in literature. The nuclear matter compression modulus predicted in our model lies in the range 210-230 MeV which agrees with earlier findings. The information provided by the sum rule approach in the case of nuclei near the neutron drip line is also discussed., 10 pages, LaTeX

Oxygen vacancies in metal oxides are known to determine their chemistry and physics. The properties of neutral oxygen vacancies in metal oxides of increasing complexity (MgO, CaO, alpha-Al2O3, and ZnO) have been studied using density functional theory. Vacancy formation energies, vacancy-vacancy interaction, and the barriers for vacancy migration are determined and rationalized in terms of the ionicity, the Madelung potential, and lattice relaxation. It is found that the Madelung potential controls the oxygen vacancy properties of highly ionic oxides whereas a more complex picture arises for covalent ZnO.

The electronic and magnetic structures of the LaMnO3 compound have been studied by means of periodic calculations within the framework of spin polarized hybrid density-functional theory. In order to quantify the role of approximations to electronic exchange and correlation three different hybrid functionals have been used which mix nonlocal Fock and local Dirac-Slater exchange. Periodic Hartree-Fock results are also reported for comparative purposes. The A-antiferromagnetic ground state is properly predicted by all methods including Hartree-Fock exchange. In general, the different hybrid methods provide a rather accurate description of the band gap and of the two magnetic coupling constants, strongly suggesting that the corresponding description of the electronic structure is also accurate. An important conclusion emerging from this study is that the nature of the occupied states near the Fermi level is intermediate between the Hartree-Fock and local density approximation descriptions with a comparable participation of both Mn and O states.

ResumenSe presenta una revisión de los métodos que se han desarrollado dentro de la Química Teórica para estudiar a la materia en diversas escalas de espacio y tiempo. La trayectoria de un conocido químico teórico se utiliza para mostrar cómo, en 4 décadas, los desarrollos de teorías, programas (algoritmos) y de la tecnología computacional nos permiten en la actualidad el estudio confiable de la estructura electrónica de sistemas cuyos tamaños los hacían prohibitivos hace 40 años. Se señala y enfatiza que la dificultad central en el estudio teórico de la Química reside en que, en todas los escalas, se trata de un problema de muchos cuerpos cuya solución y comprensión siguen siendo un reto científico no resuelto y fascinante.AbstractA review of the methods developed within Theoretical Chemistry to study matter in many spatial and temporal scales is presented. The scientific career of a well known theoretical chemist is used to show how the astonishing advances that have been achieved in four decades in the development of theories, codes (algorithms) and computer technology allow nowadays the reliable study of the electronic structure of systems whose sizes were unmanageable forty years ago. It is emphasized that the central difficulty in studying Chemistry from a theoretical perspective is that, in every scale, one deals with a many-body problem whose solution and comprehension are still an unsolved and fascinating scientific challenge.

The electronic structure of an isolated oxygen vacancy in SrTiO3 has been investigated with a variety of ab initio quantum mechanical approaches. In particular we compared pure density functional theory ~DFT! approaches with the Hartree-Fock method, and with hybrid methods where the exchange term is treated in a mixed way. Both local cluster models and periodic calculations with large supercells containing up to 80 atoms have been performed. Both diamagnetic ~singlet state! and paramagnetic ~triplet state! solutions have been considered. We found that the formation of an O vacancy is accompanied by the transfer of two electrons to the 3d(z 2 ) orbitals of the two Ti atoms along the Ti-Vac-Ti axis. The two electrons are spin coupled and the ground state is diamagnetic. New states associated with the defect center appear in the gap just below the conduction band edge. The formation energy computed with respect to an isolated oxygen atom in the triplet state is 9.4 eV.

The performance of density-functional theory to solve the exact, nonrelativistic, many-electron problem for magnetic systems has been explored in a new implementation imposing space and spin symmetry constraints, as in ab initio wave function theory. Calculations on selected systems representative of organic diradicals, molecular magnets and antiferromagnetic solids carried out with and without these constraints lead to contradictory results, which provide numerical illustration on this usually obviated problem. It is concluded that the present exchange-correlation functionals provide reasonable numerical results although for the wrong physical reasons, thus evidencing the need for continued search for more accurate expressions.

CuF2 is known to be an antiferromagnetic compound with a weak ferromagnetism due to the anisotropy of its monoclinic unit cell ~Dzialoshinsky-Moriya mechanism!. We investigate the magnetic ordering of this compound by means of ab initio periodic unrestricted Hartree-Fock calculations and by cluster calculations which employ state-of-the-art configuration interaction expansions and modern density functional theory techniques. The combined use of periodic and cluster models permits us to firmly establish that the antiferromagnetic order arises from the coupling of one-dimensional subunits which themselves exhibit a very small ferromagnetic coupling between Cu neighbor cations. This magnetic order could be anticipated from the close correspondence between CuF2 and rutile crystal structures. @S0163-1829~99!15301-3#

The interface of MgO/Ag(001) has been studied with density functional theory applied to slabs. We have found that regular MgO films show a small adhesion to the silver substrate, the binding can be increased in off-stoichiometric regimes, either by the presence of O vacancies at the oxide film or by a small excess of O atoms at the interface between the ceramic to the metal. By means of theoretical methods, the scanning tunneling microscopy signatures of these films is also analyzed in some detail. For defect free deposits containing 1 or 2 ML and at low voltages, tunnelling takes place from the surface Ag substrate, and at large positive voltages Mg atoms are imaged. If defects, oxygen vacancies, are present on the surface of the oxide they introduce much easier channels for tunnelling resulting in big protrusions and controlling the shape of the image, the extra O stored at the interface can also be detected for very thin films.

The electronic structure of the molecular solid Ni(tmdt)2, the only well characterized neutral molecular metal to date, has been studied by means of first-principles density functional calculations. It is shown that these calculations correctly describe the metallic vs semiconducting behavior of molecular conductors of this type. The origin of the band overlap leading to the metallic character and the associated Fermi surfaces has been studied.