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The challenges imposed by climate change require the continued improvement and identification of materials for the development of green technologies. Point defect engineering is a promising technology for producing green hydrogen by taking advantage of catalytic hydrogen evolution reactions. In this work, we investigate the role of anionic and cationic vacancy point defects, as well as the nature of the active sites, in the catalytic activation of Cu2WS4 single layers. The stability of the pristine and defective structures of Cu2WS4 has been thoroughly investigated using density-functional theory calculations. A deep analysis of the formation enthalpy indicates that the Cu vacancy is the chemically most favorable vacancy. However, the calculated adsorption energy indicates that the presence of such vacancies slightly enhances the hydrogen evolution reaction. In contrast, the formation of an S vacancy considerably magnifies the same reaction in Cu2WS4 single layers.

The electronic structure of nitrobenzene (C6H5NO2) has been studied by means of the complete active space self-consistent field (CASSCF) and multi-state second-order perturbation (MS-CASPT2) methods. To this end, an active space of 20 electrons distributed in 17 orbitals has been selected to construct the reference wave function. In this work, we have calculated the vertical excitation energies and the energy barrier for the dissociation of the molecule on the ground state into phenyl and nitrogen dioxide. After applying the corresponding vibrational corrections to the electronic energies, it is demonstrated that the MS-CASPT2//CASSCF values obtained in this work yield an excellent agreement between calculated and experimental data. In addition, other active spaces of lower size have been applied to the system in order to check the active space dependence in the results. UNIVERSIDAD DE MALAGA

Heterometallic lanthanide [LnLn′] coordination complexes that are accessible thermodynamically are very scarce because the metals of this series have very similar chemical behaviour. Trinuclear systems of this category have not been reported. A coordination chemistry scaffold has been shown to produce molecules of type [LnLn′Ln] of high purity, i.e. exhibiting high metal distribution ability, based on their differences in ionic radius. Through a detailed analysis of density functional theory (DFT) based calculations, we discern the energy contributions that lead to the unparalleled chemical selectivity of this molecular system. Some of the previously reported examples are compared here with the newly prepared member of this exotic list, [Er2Pr(LA)2(LB)2(py)(H2O)2](NO3) (1) (H2LA and H2LB are two β-diketone ligands). A magnetic analysis extracted from magnetization and calorimetry determinations identifies the necessary attributes for it to act as an addressable, conditional multiqubit spin-based quantum gate. Complementary ab initio calculations confirm the feasibility of these complexes as composite quantum gates, since they present well-isolated ground states with highly anisotropic and distinct g-tensors. The electronic structure of 1 has also been analyzed by EPR. Pulsed experiments have allowed the establishment of the quantum coherence of the transitions within the relevant spin states, as well as the feasibility of a coherent control of these states via nutation experiments., The authors thank the European Union's Horizon 2020 research and innovation programmes QUANTERA project SUMO and FET-OPEN grant 862893 FATMOLS (GA and OR) the Spanish AEI for grants PGC2018-098630-B-I00 (GA and DA), PID2020-118329RB-I00 (OR), PID2020-112806RB-I00 (DGR and CB) and the Severo Ochoa Excellence Accreditation 2020–2023 CEX2019-000925-S (DGR and CB), the ICIQ foundation (DGR and CB) and the Generalitat de Catalunya for a Beatriu de Pinós fellowship (2019-BP-00015, DR), an FI grant (also financed by AGAUR and the European Social Fund, DGR) an FI-SDUR Grant (2020-FISDU-00492, DM), the CERCA program and the ICREA Academia 2018 Prize (GA), as well as the National EPR Facility at the University of Manchester for support with the EPR measurements. This work used resources of the ALBA synchrotron through access to beamline 13-XALOC.

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.

Most of the archetypal aromatic compounds present high symmetry and have degenerate highest-occupied molecular orbitals. These orbitals can be fully occupied resulting in a closed-shell structure or can be same-spin half-filled. This closed-shell or same-spin half-filled electronic structure provides an extra stabilisation and it is the origin of several rules of aromaticity such as the Hückel 4N+2 rule, the lowest-lying triplet excited state 4N Baird rule, the 4N rule in Möbius-type conformation, the 4N+2 Wade-Mingos rule in closo boranes or the 2(N+1)2 Hirsch rule in spherical aromatic species. Combinations of some of these rules of aromaticity can be found in some particular species. Examples of these combinations will be discussed and the validity of some of these rules will be assessed. Moreover, it is possible to establish connections with some of these rules of (anti)aromaticity and, therefore, they can be partially generalised, which represents a step forward to a future unified theory of aromaticity Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) (Project No. CTQ2017-85341-P) and the DIUE of the Generalitat de Catalunya (Project No. 2017SGR39 and ICREA Academia 2014 to M.S.) is acknowledged. The EU under the FEDER grant UNGI10-4E-801 has also funded this research

Nonlinear optical properties (NLOPs) play a major role in photonics, electro-optics and optoelectronics, and other fields of modern optics. The design of new NLO molecules and materials has benefited from the development of computational tools to analyze the relationship between the electronic structure of molecules and their optical response. In this paper, we present a new means to analyze the response property through the partition of NLOPs in terms of orbital contributions (PNOC). This tool can be used to obtain a real-space representation of the NLOPs, providing a powerful visualization aid to connect the magnitude of the optical property with some parts of the molecule. Unlike other methods to analyze NLOPs, the PNOC decomposes the optical property into orbitals of the unperturbed system, furnishing this method with the ability to assess the performance of single- and multi-determinant electronic structure methods. PNOC can be also used to design small basis sets for an accurate description of large systems, saving a substantial amount of computer time for the calculation of optical properties This research has been funded by Spanish MINECO/FEDER Projects CTQ2014-52525-P and EUIN2017-88605, the Catalan DIUE 2014SGR931 and IKERBASQUE

In this work, using DFT + U formalism, we investigate the effect of order–disorder in the A-site occupation by La3+ and Sr2+ on the stability of the ferromagnetic order in La2/3Sr1/3MnO3 with R 3 - c symmetry. To date, a detailed theoretical discussion of such phenomenon, using a combination of different representations of the electronic structure, is still missing in the Literature. We employed structural models consisting of 120 atom supercells constructed according to the precise stoichiometry of the compound. Two configurations, describing randomized and ordered occupation of the La3+/Sr2+ ions, were evaluated. We demonstrate that the ferromagnetic arrangement of La2/3Sr1/3MnO3 with randomly distributed La3+ and Sr2+ ions is more stable. In such configuration we find that the Mn3+ and Mn4+ ions are not distinguished, favoring the double-exchange mechanism, enhanced by the higher degree of covalence in the Mn O bonds near the Fermi level between the spin-up Mn-eg orbitals and the O-p orbitals.

Perovskite-like materials which include magnetic elements have relevance due to the technological perspectives in the spintronics industry. In this work, the magnetic, structural and electronic properties of the Ba₂CoMoO₆ double perovskite are investigated. Calculations are carried out through the Full-Potential Linear Augmented Plane Wave method within the framework of the Density Functional Theory with exchange and correlation effects in the Generalized Gradient and Local Density approximations, including spin polarization. From the minimization of energy as a function of volume using 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, and the band structure, showing that this compound evidences a conductive character for a spin channel and insulation for the other, and presents an integer value for the effective magnetic moment (3.0 μB), which allows it to be classified as a half-metallic material. The effects of pressure and temperature on thermophysical properties such as specific heat, Debye temperature, coefficient of thermal expansion and the Grüneisen parameter were calculated and analyzed from the state equation of the system. Obtained results reveal that, in the low-temperature regime, the specific heat at constant volume and pressure presents an analogous behavior to each other, with a tendency to the limit of Dulong-Petit typical of the structures of cubic perovskite-type, showing a value of 246.3 J/mol.K at constant volume and slightly higher values at constant pressure. The dependence of the thermal expansion coefficient, the temperature of Debye and the Grüneisen parameter with the increase in temperature are discussed in relation to other perovskite-like materials., Los materiales de tipo perovskita que incluyen elementos magnéticos tienen relevancia debido a las perspectivas tecnológicas en la industria de la espintrónica. En este trabajo se efectúa un estudio exhaustivo de las propiedades magnéticas, estructurales y electrónicas de la perovskita doble Ba₂CoMoO₆. Los cálculos se realizan a través del método de ondas planas aumentadas y linealizadas dentro del marco de la teoría del funcional de la densidad con efectos de intercambio y correlación en las aproximaciones del gradiente generalizado y de densidad local, incluyendo polarización de espín. A partir de la minimización de la energía en función del volumen, utilizando la ecuación de estado de Murnaghan, se obtuvieron el parámetro de red de equilibrio y las propiedades cohesivas de este compuesto. El estudio de la estructura electrónica se basó en el análisis de la densidad electrónica de estados y la estructura de bandas, mostrando que este compuesto evidencia un carácter conductor para un canal de espín y aislante para el otro, presentando un valor entero para el momento magnético efectivo (3.0 μB), que permite clasificarlo como un material espín-metálico. Los efectos de la presión y la temperatura sobre las propiedades termofísicas, como el calor específico, la temperatura de Debye, el coeficiente de expansión térmica y el parámetro Grüneisen, se calcularon y analizaron a partir de la ecuación de estado del sistema. Los resultados obtenidos revelan que, en el régimen de baja temperatura, el calor específico a volumen y presión constantes presenta un comportamiento análogo entre sí, con una tendencia al límite de Dulong-Petit típico de las estructuras de tipo perovskita cúbica, mostrando un valor de 246.3 J/mol.K a volumen constante y valores ligeramente más altos a presión constante. La dependencia del coeficiente de expansión térmica, la temperatura de Debye y el parámetro Grüneisen con el aumento de temperatura se discute en relación con otros materiales de tipo perovskita., Instituto de Física La Plata, Grupo de Estudio de Materiales y Dispositivos Electrónicos

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2019, Tutor: Jordi Cirera Fernández, Metal-organic cages (MOCs) containing four Fe(II) metal centres ([Fe4]) represent a novel class of molecules that have attracted a lot of interest due to, among others, its efficiency in encapsulating greenhouse gases such as SF6. On the top of that, the [Fe4] molecules exhibit spin-crossover (SCO) behaviour, which can be tuned as a function of the guest molecule encapsulated in the system. Due to their size, conventional electronic structure calculations are not suited to study the origin of such interaction and its effect on the SCO behaviour and different computational methodologies need to be developed in order to study the nature of the host-guest interaction. First, by performing DFT calculations, the Fe-N bond will be modelled using a Morse potential. This data will be adjusted simultaneously against an energy scan along the totally symmetric normal mode in order to generate an ab initio force field for the molecule. Also, a series of electronic structure calculations will be carried out at CASSCF/NEVPT2 level to extract the relevant Angular Overlap Model parameters for a very simple molecule, [Fe(NH3)6]2+, representative of the Fe(II) environment in [Fe4] MOCs; necessary to construct the force field. In order to see if the results of [Fe(NH3)6]2+ can be extrapolated to more complex systems like the [Fe4] cages, the ammonia results will be compared with systems of increasing complexity yet closely related to the actual Fe environment in the [Fe4] system: [Fe(py)6]2+ and [Fe(C6H6N2)3]2+. Once validated, then the complete force field for the [Fe4] system will be assembled and tested. Finally, LFMM simulations will be run in the NVE, NVT or NPT/NσT ensemble to study the interaction between the [Fe4] system and the SF6 molecule in a large enough system so the crystal packing effects can be properly modelled

The electronic structure of AgCuO2, and more specifically the possible charge delocalization and its implications for the transport properties, has been the object of debate. Here the problem is faced by means of first-principles density functional theory calculations of the electron and phonon band structures as well as molecular dynamics simulations for different temperatures. It is found that both Cu and Ag exhibit noninteger oxidation states, in agreement with previous spectroscopic studies. The robust CuO2 chains impose a relatively short contact distance to the silver atoms, which are forced to partially use their dz2 orbitals to build a band. This band is partially emptied through overlap with a band of the CuO2 chain, which should be empty if copper were in a Cu3+ oxidation state. In that way, although structural correlations could roughly be consistent with an Ag+Cu3+O2 formulation, the appropriate oxidation states for the silver and copper atoms become Ag(1+δ)+ and Cu(3−δ)+, and as a consequence, the stoichiometric material should be metallic. The study of the electronic structure suggests that Ag atoms form relatively stable chains that can easily slide despite the linear coordination with oxygen atoms of the CuO2 chains. Phonon dispersion calculations and molecular dynamics simulations confirm the stability of the structure although pointing out that sliding of the silver chains is an easy motion that does not lead to substantial modifications of the electronic structure around the Fermi level and, thus, should not alter the good conductivity of the system. However, this sliding of the silver atoms from the equilibrium position explains the observed large thermal factors., This work was supported by MINECO (Spain) through Grant Nos. FIS2015-64886-C5-4-P, RTI2018-097753-B-I00, CTQ2015-64579-C3-3-P, and MAT2015-65192-R, Generalitat de Catalunya (2017SGR1506, 2017SGR1289, and XRQTC), and FONDECYT (Chile) through Project No. 11107163. E.C., N.C.-P., and A.G. acknowledge support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant No. SEV-2015-0496, and P.A. is supported from the Maria de Maeztu Units of Excellence Program under Grant No. MDM-2017-0767.

We report anomalies observed in photoelectron spectroscopy measurements performed on α-(BEDT-TTF)2I3 crystals. In particular, above its metal-insulator transition temperature (T = 135 K), we observe the lack of a sharp Fermi edge in contradiction with the metallic transport properties exhibited by this quasi-bidimensional organic material. We interpret these unusual results as a signature of a one-dimensional electronic behavior confirmed by DFT band structure calculations. Using photoelectron spectroscopy we probe a Luttinger liquid with a large correlation parameter (α > 1) that we interpret to be caused by the chain-like electronic structure of α-(BEDT-TTF)2I3 surface doped by iodine defects. These new surface effects are inaccessible by bulk sensitive measurements of electronic transport techniques., We acknowledge A. Santander-Syro, M. Gabay and P. Auban- Senzier for fruitful discussions. Work in Spain was supported by the Spanish Ministerio de Econom a y Competitividad (Grants FIS2015-64886-C5-4-P and CTQ2015-64579-C3-3-P) and Generalitat de Catalunya (2017SGR1506, 2017SGR1289 and XRQTC). E.C. acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2015-0496. P.A. acknowledges support from the Maria de Maeztu Units of Excellence Program under Grant MDM-2017-0767.

Producción Científica, Putative global minimum structures for neutral CdN and singly charged Cd+ N and Cd− N clusters in the small size regime up to N = 21 atoms are reported. A global optimization approach based on the basin hopping method and a Gupta potential fitted to cluster properties is employed to generate a diverse databank of trial structures, which are then re-optimized at the densityfunctional level of theory. Novel, previously unreported, structures are found for many sizes. Our results successfully reproduce and interpret the size-dependent stabilities known from mass spectrometry, and strongly suggest that experiments aimed at determining the relative stabilities of neutral cadmium clusters are really measuring cation stabilities. We provide an in-depth analysis of electronic structure and use it to explain the gradual emergence of metallic-like behaviour as the cluster size increases., Junta de Castilla y León (Project VA124G18), Ministerio de Ciencia, Innovación y Universidades (Project PGC2018-093745- B-100)

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)

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 Sr 2 HoNbO 6 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 Sr 2 HoNbO 6 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. Resumen 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 Sr 2 HoNbO 6 . 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 Sr 2 HoNbO 6 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.

Despite B and Al belong to the same group 13 elements, the B6²- cluster prefers the planar D2h geometry, whereas Al6²- favours the Oh structure. In this work, we analyse the origin of the relative stability of the D2h and Oh forms in these clusters by means of an energy decomposition analysis based on the turn-upside-down approach. Our results show that what causes the different trend observed is the orbital interaction term, which combined with the electrostatic component do (Al6²- and Ga6²-) or do not (B6²-) compensate the higher Pauli repulsion of the Oh form. Analysing the orbital interaction term in more detail, we find that the preference of B6²- for the planar D2h form has to be attributed to two particular molecular orbital interactions. Our results are in line with a dominant delocalisation force in Al clusters and the preference for more localised bonding in B metal clusters. For mixed clusters, we have found that those with more than two B atoms prefer the planar structure for the same reasons as for B6²- This work has been supported by the Ministerio de Economía y Competitividad (MINECO) of Spain (Project CTQ2014-54306-P) and the Generalitat de Catalunya (project 2014SGR931, Xarxa de Referència en Química Teòrica i Computacional, ICREA Academia 2014 prize for M.S., and grant No. 2014FI_B 00429 to O.E.B.). The EU under the FEDER grant UNGI10-4E-801 (European Fund for Regional Development)has also funded this research. J. P. thanks the National Research School Combination - Catalysis (NRSCC), and The Netherlands Organization for Scientific Research (NWO/CW and NWO/NCF)

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.

Esta tesis doctoral recoge el resultado de una investigación teórica y computacional dirigida a establecer una conexión entre la solución de la ecuación de Schrödinger electrónica en sistemas cristalinos y el enlace químico. Se ha utilizado el cristal de Cs2UO2Cl4 con el doble objetivo de avanzar en el entendimiento de su comportamiento bajo presión hidrostática, resolviendo ciertas controversias en relación con la regla de Badger, y de conocer los detalles computacionales y metodológicos de los cálculos de primeros principios basados en la aproximación de la teoría del funcional de la densidad electrónica (DFT) en el estado sólido. El núcleo central de la tesis es el análisis y aplicación del formalismo de la presión química (CP) en moléculas (H2 , CO2 , S8 ) y sólidos exhibiendo todo tipo de interacciones químicas: covalentes (grafeno, diamante), iónicas (haluros e hidruros alcalinas), metálicas (elementos del grupo 15, metales alcalinos), de van der Waals (CO2 , S8 , grafito) y enlaces de hidrógeno (hielo-I). A partir de la distribución de la energía DFT en los puntos de una rejilla en la que se divide la celda unidad cristalina es posible construir mapas 1D, 2D y 3D de presiones químicas. Estos mapas sirven para identificar con claridad las regiones responsables de las atracciones y repulsiones que dan lugar a la cohesión en todos los sistemas cristalinos explorados. Enlaces, pares solitarios, dominios de interacciones dispersivas o de repulsiones estéricas pueden ser discutidos en términos de las contribuciones energéticas propias de la DFT. El formalismo DFT-CP ilustra además la validez del modelo interpretativo de las estructuras cristalinas de compuestos inorgánicos en el que se entiende que es la subred metálica (Na, Ca, BaSn) la que induce la posición final de los aniones en sus respectivos cristales (NaCl o NaF, CaO o CaF2 , BaSnO3 ). This doctoral thesis is the result of a theoretical and computational research aimed at establishing a connection between the solution of the Schrödinger electronic equation in crystalline systems and the chemical bond. The crystal of Cs2UO2Cl4 has been used with the double objective to advance in the understanding of its behavior under hydrostatic pressure solving certain controversies in relation to the rule of Badger and to know the computational and methodological details of first principles calculations based on the density functional theory approximation (DFT) in the solid state. The central nucleus of the thesis is the analysis and application of the formalism of chemical pressure (CP) in molecules (H2 , CO2 , S8 ) and solids exhibiting all kinds of chemical interactions: covalent (graphene, diamond), ionic (alkali halides and hydrides), metals (group 1 and group 15 elements), van der Waals (solid CO2 and S8 , graphite) and hydrogen bonds (ice-I). From the distribution of the DFT energy at the points of a grid in which the crystal unit cell is divided, it is possible to construct 1D, 2D and 3D maps of chemical pressures. These maps serve to clearly identify the regions responsible for attractions and repulsions that give rise to cohesion in all explored crystalline systems. Bonds, lone pairs, domains of dispersive interactions or steric repulsions can be discussed in terms of the DFT's own energy contributions. The DFT-CP formalism also illustrates the validity of the interpretive model of the crystal structure of inorganic compounds in which it is understood that the metallic sub-lattice (Na, Ca, BaSn, etc.) induces the final position of the anions in their respective crystals (NaCl or NaF, CaO or CaF2 , BaSnO3 , etc.).

Comunicación científica In recent years, there has been a renewed interest in the study of open-shell polycyclic aromatic hydrocarbons (PAHs). Among them, biradicals are characterized by a four states energetic model for the low lying electronic states S0, S1, S2, T1. Such a distribution is related to the attractive electronic, magnetic and optical properties which make biradicals promising candidates for a wide range of applications in organic electronics, such as nonlinear optics, molecular spintronics, energy storage and organic photovoltaic devices sensitized by singlet fission. Despite all the promising properties predicted for open-shell PAHs, a common drawback still needs to be overcomed: the high reactivity of radicals, which implies that most open-shell species tend to be too short-lived for practical aplications and even for characterization [1]. In this regard, Raman spectroscopy has shown to be a suitable tool for studying this kind of molecular systems, able to provide valuable information that could be used in the development of new synthetic approaches [2]. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech.

We report experimental and theoretical results of crystal structure, morphology, magnetic and electric features, and electronic structure for the Sr2DyNbO6 ceramic compound. Samples were produced by the solid-state reaction recipe. X-ray diffraction experiments show that the material crystallizes in a monoclinic structure, P21/n space group. SEM images exhibit a granular submicrometric surface. Temperature curves of magnetic susceptibility reveal a paramagnetic response. Curie law fitting permitted to obtain a magnetic moment 10.28 (B . Polarization as a function of electric fields shows a hysteretic feature with 264.28 relative dielectric constant at 300 K. DFT calculations of electronic structure suggest the semiconductor character of this material, energy gap 3.21 eV for the spin-up polarization and 0.26 eV for spin-down. The calculated effective magnetic moment was 10.0 (B , which is strongly in accordance with the measured value. The theoretical cell parameters obtained from the Murnaghan state-equation are 98.5% in agreement with the experimental result. Resumen Se reportan resultados experimentales y teóricos de estructura cristalina, morfología, carácter magnético y eléctrico, y estructura electrónica para el material cerámico Sr2DyNbO6. Las muestras fueron producidas mediante reacción sólida. Resultados de difracción de rayos X muestran que el material cristaliza en una estructura monoclínica, grupo espacial P21/n . Imágenes de MEB muestran superficies granulares submicrométricas. La susceptibilidad en función de la temperatura revela una respuesta paramagnética con momento magnético 10.28 (B . La histéresis de polarización en función del campo eléctrico a 300K permitió obtener una constante dieléctrica de 264.28. Cálculos de estructura electrónica por DFT sugieren que el material es semiconductor con brecha de energía 3.21 eV para la orientación espín arriba y 0.26 eV para espín abajo. El momento magnético calculado es 10.0 (B , muy próximo del valor medido. Los parámetros de red obtenidos mediante la ecuación de Murnaghan están de acuerdo con los resultados experimentales en un 98.5%.

We study the effect that a statically screened Coulomb potential represented by a Debye-Hückel-Yukawa potential has in the electronic structure of the simplest molecule H2+ within the Born-Oppenheimer approximation. The method of solution is based on a two-center partial-wave expansion expressed in confocal elliptic coordinates using B-spline polynomials. General algorithms for the computation of energies, wave functions, and dipole and nonadiabatic radial matrix elements are given in detail. As it occurs in atoms, screening in simple molecules shifts the energies of bound states upwards so that, as screening increases, every bound state eventually crosses the upper ionization threshold at a critical screening value. The loss of long-range Coulomb interactions has its effect in the structure of wave functions, and consequently in the dipole and nonadiabatic matrix elements at intermediate and long internuclear distances, which determine the dynamics in external electromagnetic fields and collisional processes. Other issues related to a practical solution of the arbitrary sign problem, as well as the assignment of angular and radial nodes to the variational eigenfunctions, and the appearance of molecular shape resonances and Borromean states in H2+ as screening increases, are also addressed in this work, Work supported by the Advanced Grant of the European Research Council XCHEM 290853, the European COST Action XLIC CM1204, and the MINECO Project No. FIS2013-42002-R. A.F.O.-L. acknowledges financial support from EACEA through an Erasmus Mundus scholarship. J.L.S.-V. acknowledges financial support from Vicerrectoria de investigacion at Universidad de Antioquia (Contract No. E01538 and Estrategia de Sostenibilidad 2016-2017) and from Departamento Administrativo de Ciencia, Tecnologia e Innovacion (COLCIENCIAS, Colombia) under Contract No.111565842968

We carried out a theoretical study on the structural stability, elastic, electronic and thermodynamic properties of the Sc x Ga 1− x P compound ( x = 0, 0.25, 0.50, 0.75 and 1) in the ZnS and NaCl crystallographic phases. For this purpose, we performed ab initio calculations using the DFT within LDA and GGA approximations. To solve the Kohn-Sham equations, we implemented the accurate full-potential linearized augmented plane wave method. From the results obtained, it can be noted that for the 0≤ x ≤0.30 range, the most stable structure of Sc x Ga 1− x P is the ZnS phase, and for the 0.30 x ≤1 interval, the most stable structure is the NaCl phase. The structural results also show a phase transition from the ZnS to NaCl at a pressure of ∼ 2.84 GPa for a Sc concentration value of 25%. Electronic band structure analysis shows that in the ZnS phase, for a 25% of Sc concentration, Sc x Ga 1− x P is a direct semiconductor, and from 50% to 100% concentrations in the NaCl phase, the compound exhibits a metallic behavior. Calculated phase diagrams predict Sc x Ga 1− x P to be stable as homogeneous alloy phases at high temperature for both ZnS and NaCl phases.

A first-principles DFT study of the electronic structure of the two-chain molecular conductor NMP-TCNQ is reported. It is shown that the charge transfer occurring in this salt is not 1 but 2/3, finally settling the debate concerning the real charge transfer in this molecular metal. These calculations also lead to a simple rationalization of the three different regimes of 2kF and 4kF CDW instabilities occurring in the solid solutions (NMP)x(Phen)1−xTCNQ.

We report two new single‐ion magnets (SIMs) of a family of oxydiacetate lanthanide complexes with D3 symmetry to test the predictive capabilities of complete active space ab initio methods (CASSCF and CASPT2) and the semiempirical radial effective charge (REC) model. Comparison of the theoretical predictions of the energy levels, wave functions and magnetic properties with detailed spectroscopic and magnetic characterisation is used to critically discuss the limitations of these theoretical approaches. The need for spectroscopic information for a reliable description of the properties of lanthanide SIMs is emphasised. SPINMOL ERC-CoG-647301 DECRESIM MAT2014-56143-R CTQ2014-52758-P CTQ2011-23861-C02-01 CTQ2015-64579-C3-1-P MDM-2015-0538 We report two new single‐ion magnets (SIMs) of a family of oxydiacetate lanthanide complexes with D3 symmetry to test the predictive capabilities of complete active space ab initio methods (CASSCF and CASPT2) and the semiempirical radial effective charge (REC) model. Comparison of the theoretical predictions of the energy levels, wave functions and magnetic properties with detailed spectroscopic and magnetic characterisation is used to critically discuss the limitations of these theoretical approaches. The need for spectroscopic information for a reliable description of the properties of lanthanide SIMs is emphasised.

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).

We report the synthesis, the structural characterization, the ferroelectric behavior and the electronic properties of complex perovskite Ba2ZrTiO6. Samples of Ba2ZrTiO6 were synthesized through the standard solid state reaction method. The crystalline structure was studied by means of X-ray diffraction experiments and Rietveld-like analysis. Results reveal that the material crystallizes in a rhomboidal structure, space group R-3 (#148), with cell parameter a=5.8038(7) Å. The ferroelectric response of material was established from curves of polarization as a function of applied electric field. Our results reveal that the double perovskite Ba2ZrTiO6 has a ferroelectric hysteretic behavior at room temperature. The studies of the electronic structure show that Ba2ZrTiO6 behaves as a nonmetallic material with gap energy 2.32 eV. The structural parameters obtained from energy minimization, through the Murnaghan equation state are 99.5% in agreement with the experimental data. En este trabajo reportamos la síntesis, la caracterización estructural, el comportamiento ferroeléctrico y las propiedades electrónicas de la perovskita compleja Ba2ZrTiO6. Las muestras fueron sintetizadas mediante el método estándar de reacción de estado sólido. La estructura cristalina se estudió a través de experimentos de difracción de rayos X y análisis de tipo Rietveld. Los resultados revelan que el material cristaliza en una estructura romboédrica, perteneciente al grupo espacial R-3 (#148), con un parámetro de red a= 5,8038(7) Å. La respuesta ferroeléctrica del material se estableció a partir de curvas de polarización en función del campo eléctrico aplicado. Nuestros resultados muestran que la perovskita Ba2ZrTiO6 evidencia un comportamiento histerético ferroeléctrico a temperatura ambiente. Los estudios de la estructura electrónica muestran que esta cerámica se comporta como un material no metálico con brecha de energía 2,32 eV. Los parámetros estructurales obtenidos a partir de la minimización de energía, a través de la ecuación de estado Murnaghan, son 99.5% acordes con los datos experimentales.

"A theoretical study for the adrenalin molecule was carried out to reproduce the electronic properties for the neutral, protonated and oxidized forms of adrenalin. The dihedral angle H-C-N-H and the three angles from the alcohols groups, –COH, were rotated to obtain the Potential Energy Surface (PES), in which the corresponding minimal energy was for the extended geometry, namely, MinGlob. This species was used as reference to derive the protonated and oxidized species. The predominant species at the physiological pH is the protonated form in which a proton interacts with the amino group imparting stability to the molecule. The proton addition reduces susceptibility of the molecule towards oxidation-reduction reactions of the amino group. However, the cathecol group from the ring of the protonated species becomes rather prone to react. The HOMO and LUMO iso-surfaces distribution are presented for each of the chemical adrenalin species and their electronic redistribution was obtained as shown by the chemical potential and the reactivity index for each species. The calculated global hardness leads to the following stability order: neutral > protonated > oxidized products, while with the chemical potential the opposite stability order is obtained."

The non-coherent rate equation approach to the electrical transport in a serial quantum dot system is presented. The charge density in each quantum dot is obtained using the transfer Hamiltonian formalism for the current expressions. The interactions between the quantum dots and between the quantum dots and the electrodes are introduced by transition rates and capacitive couplings. Within this framework analytical expressions for the current and the charge in each quantum dot are presented. The effects of the local potential are computed within the self-consistent field regime. Despite the simplicity of the model, well-known effects are satisfactorily explained and reproduced. We also show how this approach can be extended into a more general case.

We report structural analysis, magnetic ordering, ferroelectric behavior, and the electronic structure of SR2TIMNO6 complex perovskite. Samples were produced by the solid state reaction recipe. Crystallographic analysis was performed by Rietveld refinement of experimental X-ray diffraction patterns. Results show that this material crystallizes in a perovskite tetragonal structure which corresponds to the space group I4/mmm. We obtain a matching of 99% between structural properties predicted by the density functional theory and structural ordering, found from the Rietveld refinement method. From field cooling measurements of susceptibility as a function of temperature, we determine the occurrence of a magnetic ordering for a critical temperature of 44.8 K. From a fitting with the molecular field theory of ferrimagnetism, we establish that the ordering corresponds to a paramagnetic-ferrimagnetic transition, which is corroborated by the behavior of the inverse of susceptibility as a function of temperature. The Curie constant allowed for us to obtain an effective magnetic moment of 3.5 mB. The result of magnetization as a function of the applied field, measured at T = 40 K, shows a ferromagnetic-hysteretic behavior. The curve of polarization as a function of applied voltage evidences a ferroelectric character. It is shown experimentally that below 44.8 K this perovskite material evidences a multiferroic feature. Ab initio calculations of density of states and band structure for this manganite-like material were carried out by means of the density functional theory (DFT) and the linearized augmented plane waves method for both spin orientations. For calculations, the exchange and correlation potentials were included through the local density LDA+U approximation. The density of states study was carried out considering both up and down spin polarizations by the generalized gradient approximation (GGA). Results of total and partial density of states reveal the insulator character of this material with an effective magnetic moment of 3.3 mB. En este trabajo reportamos el análisis estructural, ordenamiento magnético, carácter ferroeléctrico y estructura electrónica de la perovskite compleja SR2TIMNO6. Las muestras fueron producidas mediante reacción de estado sólido. El análisis cristalográfico fue realizado a través de refinamiento Rietveld de los patrones experimentales de rayos x. Los resultados muestran que este material cristaliza en una perovskita tetragonal correspondiente al grupo espacial I4/mmm. Hemos obtenido un acuerdo del 99% entre las propiedades estructurales predichas por la teoría del funcional de la densidad y el ordenamiento estructural determinado mediante refinamiento Rietveld. A través de medidas de susceptibilidad en función de la temperatura encontramos ordenamiento magnético para una temperatura crítica de 44.8 K. Por medio del ajuste con la teoría molecular de ferrimagnetismo establecimos que el ordenamiento magnético se relaciona con una transición paramagnético-ferrimagnético, la cual se corrobora por el comportamiento del inverso de la susceptibilidad en función de la temperatura. La constante de Curie permitió determinar un momento magnético efectivo de 3.5 mB. Medidas de magnetización en función del campo aplicado a T = 40 K, muestra un comportamiento histerético. La curva de polarización en función del campo eléctrico evidencia una característica ferroeléctrica. Experimentalmente se demuestra que el material se comporta como un multiferróico. Cálculos Ab initio de la densidad de estados fueron realizados por medio de la teoría de funcional densidad y del método ondas planas aumentadas linealizadas para orientaciones de espín arriba y abajo. Los cálculos de intercambio y potencial de correlación fueron incluidos a través de la aproximación de densidad local LDA+U. Los resultados de la densidad de estados muestran la característica aislante de este material con un momento magnético efectivo de 3.3mB.

The intermetallic compound FeGa3 is a narrow-gap semiconductor with a measured gap between 0.2 and 0.6 eV. The presence of iron d states on the top of the valence band and on the bottom of the conduction band, together with its moderate electronic correlation (U/W ∼ 0.6), have led to the question of whether there is magnetic order in this compound. We have examined the possible presence of magnetism in FeGa3 as well as its electronic structure at high pressures, using the density functional theory (DFT) + U method with the intermediated double-counting scheme. We have found that for an optimized value of the Yukawa screening length λ, there is no magnetic moment on the iron ions (μ = 0), implying that FeGa3 is nonmagnetic. We have also found that around a pressure of 25 GPa a metal-insulator transition takes place.

Se realiza una investigación teórica mediante simulaciones químico-cuánticas en la estructura cristalina del semiconductor tipo perovskita Titanato de Plomo PbTiO3 en su fase cúbica, generando vacancias de átomos de oxígeno y F–centros. Los desplazamientos de los átomos vecinos, el incremento de las propiedades ópticas y eléctricas, las energías producto de la relajación de la red cristalina y otros nuevos efectos debido a la presencia de estos defectos se detallan y analizan. Además se presenta una comparación con estudios similares e importantes conclusiones de los resultados.

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.

We study the scaling behavior of the characteristic length of precursor magnetic nanostructures above the Curie temperature with the correlation length of quenched-in disorder. We found that the modulation length of the nanostructures $\ensuremath{\xi}$ follows the scaling law $\ensuremath{\xi}\ensuremath{\sim}\sqrt{\overline{D}}$, where $\overline{D}$ is the average size of the magnetized regions in the material. The scaling behavior of the average size of these regions, $\overline{D}$, with the correlation length of the disorder, $\ensuremath{\sigma}$, depends on the properties of the disorder. For Gaussian disorder, we find that $\overline{D}$ scales with the disorder correlation length as $\overline{D}\ensuremath{\sim}{\ensuremath{\sigma}}^{a∕2}$, where $a$ is the exponent of the leading term of the pair correlation function of the disorder in the limit $r\ensuremath{\rightarrow}0$, $\ensuremath{\Gamma}(r)\ensuremath{\approx}\ensuremath{\epsilon}[1\ensuremath{-}(1∕a){(r∕\ensuremath{\sigma})}^{a}]$. These results are quite general and applicable to other systems, e.g., ferroelectric precursors, independent of the nature of the long-range dipolar forces.

The computational approach to the Hirshfeld [Theor. Chim. Acta 44, 129 (1977)] atom in a molecule is critically investigated, and several difficulties are highlighted. It is shown that these difficulties are mitigated by an alternative, iterative version, of the Hirshfeld partitioning procedure. The iterative scheme ensures that the Hirshfeld definition represents a mathematically proper information entropy, allows the Hirshfeld approach to be used for charged molecules, eliminates arbitrariness in the choice of the promolecule, and increases the magnitudes of the charges. The resulting Hirshfeld-I charges correlate well with electrostatic potential derived atomic charges

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.

The atomic and electronic structure, formation energy, and the energy barriers for migration have been calculated for the neutral O vacancy point defect F center in cubic SrTiO3 employing various implementations of density functional theory DFT. Both bulk and TiO2-terminated 001 surface F centers have been considered. Supercells of different shapes containing up to 320 atoms have been employed. The limit of an isolated single oxygen vacancy in the bulk corresponds to a 270-atom supercell, in contrast to commonly used supercells containing 40–80 atoms. Calculations carried out with the hybrid B3PW functional show that the F center level approaches the conduction band bottom to within 0.5 eV, as the supercell size increases up to 320 atoms. The analysis of the electronic density maps indicates, however, that this remains a small-radius center with the two electrons left by the missing O ion being redistributed mainly between the vacancy and the 3d z2 atomic orbitals of the two nearest Ti ions. As for the dynamical properties, the calculated migration energy barrier in the low oxygen depletion regime is predicted to be 0.4 eV. In contrast, the surface F center exhibits a more delocalized character, which leads to significantly reduced ionization and migration energies. Results obtained are compared with available experimental data.

The observation of coherent tunnelling in Cu2+ - and Ag2+ -doped MgO and CaO:Cu2+ was a crucial discovery in the realm of the Jahn-Teller (JT) effect. The main reasons favoring this dynamic behavior are now clarified through ab initio calculations on Cu2+ - and Ag2+ -doped cubic oxides. Small JT distortions and an unexpected low anharmonicity of the eg JT mode are behind energy barriers smaller than 25 cm-1 derived through CASPT2 calculations for Cu2+ - and Ag2+ -doped MgO and CaO:Cu2+ . The low anharmonicity is shown to come from a strong vibrational coupling of MO610- units (M=Cu,Ag) to the host lattice. The average distance between the d9 impurity and ligands is found to vary significantly on passing from MgO to SrO following to a good extent the lattice parameter.

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.

The electronic structure and spectroscopic features of the optical spectra of oxygen vacancies in the bulk and on the 0001 surface of -Al2O3 have been studied by first-principles methods. The effect of oxygen vacancies on the crystalline structure has been determined by the appropriate atomic structure optimization carried out using a periodic model and density functional theory DFT calculations. Both, neutral F center and charged F+ center oxygen vacancies have been considered. Optical transitions arising from the excitations of the electron s trapped in the F+ and F centers, both on the surface and in the bulk of the material, have been studied using a suitable embedded cluster model approach. The cluster geometry, including the relaxation around the vacancy, has been taken from the periodic calculations. The transition energies and intensities have been obtained by applying the explicitly correlated multiconfigurational second-order perturbation theory method and also time-dependent DFT approaches. The present results are in good agreement with available experimental data for the bulk and provide a guide to interpret forthcoming experiments involving the spectroscopy of oxygen vacancies present in irradiated or defective -Al2O3 0001 surfaces.

La síntesis y el estudio de complejos polinucleares de metales de transición constituyen un área de gran interés en la Química Inorgánica. La presencia de varios centros metálicos en una misma molécula puede modificar la reactividad que cabría esperar en compuestos metálicos. Por otra parte, estos sistemas polinucleares sirven como modelos a escala molecular para el estudio de ciertos procesos catalíticos como los que tienen lugar sobre superficies metálicas u óxidos metálicos.El estudio teórico de dos familias de compuestos metálicos bien diferenciadas constituye el núcleo de esta tesis. En ambos casos el objetivo principal es ayudar a racionalizar las características químicas de las especies analizadas. El tratamiento se basará en cálculos teóricos mediante la teoría del funcional de la densidad, que han demostrado ser apropiados a la hora de estudiar agregados metálicos. Una primera parte, consta del estudio de la nueva familia de compuestos cubánicos de titanio que han sido recientemente sintetizados por el grupo del profesor Mena de la universidad de Alcalá de Henares. Un estudio teórico de dichos compuestos en colaboración conjunta con el trabajo en el laboratorio se ha venido realizando. Fruto de esta colaboración, nuevas pautas de investigación se han tratado de proponer por ambas partes. Como ejemplo, el análisis teórico para la obtención de espectros de resonancia magnética nuclear por métodos ab initio, que se expone en el segundo capitulo de la tesis, es fruto de un esfuerzo por parte de nuestro grupo por entender y racionalizar los datos de RMN obtenidos en el laboratorio para la caracterización de los compuestos. Además, fruto de los excelentes resultados obtenidos en este estudio, los conocimientos han sido aplicados a otros campos de interés dentro del grupo. Un estudio de la viabilidad para el análisis teórico del espectro de los núcleos de 183W en compuestos de Keggin forma un apartado importante en esta memoria.Fruto de una estancia de seis meses en el grupo del doctor McGrady de la universidad de York surge un segundo apartado basado en el estudio de las importantes capacidades catalizadoras de los compuestos binucleares de molibdeno con azufres puente. La gran importancia industrial de procesos de hidrogenación catalizados por este tipo de compuestos ha despertado un gran interés, no obstante algunos aspectos del mecanismo de estas reacciones permanecen sin aclarar. El estudio teórico de esta reacciones puede proporcionar un poco de luz a estos aspectos. Dentro del estudio de los compuestos cubánicos de titanio, los resultados principales se refieren a los asuntos siguientes:- Descripción de las características estructurales y electrónicas de las especies implicadas. La disposición de los ligandos tridentados, (precubanos) de fórmula general [{TiCp*(-O)}3(3-CR)] y [{TiCp*}(-NH)}3(3-N)], para actuar como ligandos macrocíclicos donores de seis-electrones proporciona una ruta eficaz para la obtención de estructuras cúbicas con esqueleto [MTi3(3-Y)(-X)3]. Reacción del ligando con el fragmento metálico Es de esperar que fragmentos de casi todos los grupos metálicos sean capaces de interactuar con estos ligandos. Así se ha estudiado la formación de estructuras cubánicas a partir de fragmentos metálicos de los grupos 1,2, 4-6, 9, 13 y 14. Cambios en la naturaleza de la interacción, en el comportamiento del ligando, estructura molecular han sido analizados. Los resultados muestran las versatilidad de estos ligandos para actuar no sólo como donores de electrones si no también como posibles aceptores a través de los orbitales vacíos de los metales. Una mención especial en este apartado es necesaria para los isomeros acetilénicos y vilidénicos encontrados a partir de la deprotonación del compuesto [{TiCp*(-O)}3(3-CMe)] por la interacción con fragmentos metálicos del grupo 2. En este caso, el núcleo formado por los átomos de titanio sirve como base para la estabilización de estas especies.- Estudio de la estabilidad de los compuestos y de los procesos energéticos en que están implicados (energías de la reacción, procesos de la isomerización). Un estudio de las condiciones experimentales que conducen a la obtención de estructuras cúbicas será analizado. El desplazamiento de ligandos lábiles en fragmentos metálicos por parte de los ligandos tridentados para formar los correspondientes cubanos ha sido probado, o también la activación de los enlaces nitrógeno-protón del ligando por interacción con el fragmento metálico. Estos procesos vienen caracterizados por unas condiciones energéticas favorables, estas condiciones son analizadas y estudiadas por su utilidad a la hora de proponer nuevas vías de síntesis en el laboratorio. Además, algunos compuestos presentarán evidencias del tener más que un isómero, o de experimentar un comportamiento dinámico en solución. Estas situaciones también se han descrito. Como resultado ha sido descrito el comportamiento de estas estructuras como ligandos mono- di- y tridentados, y adicionalmente como ligandos neutros o aniónicos. A su vez, respecto de la interacción con el fragmento metálico, un comportamiento como ligandos simples, que solo estabilizan el metal, o dobles, estabilizando metal y su respectivo contraión, han sido descritos.- La influencia del solvente también se ha evaluado debido a la importancia que tiene su presencia en la obtención o no de ciertos compuestos. La termoquímica relacionada con la presencia del disolvente ha sido analizada desde el punto de vista teórico, debido a la imposibilidad de explicar ciertos procesos a partir de estudios en fase gas. Un modelo basado en un medio continuo de constante dieléctrica conocida se utiliza para demostrar los efectos del solvente. Para una misma familia de compuestos, solamente en algunos casos la interacción con los ligandos precubánicos conducen a la formación de los correspondientes cubanos, o en condiciones muy específicas. Por ejemplo, ha sido estudiado que los efectos del solvente son un factor decisivo en la obtención de los cubanos a partir de las sales de metales alcalinos y alcalino térreos. Este efecto permite explicar como la energía de red de las sales es fuertemente debilitada por la presencia del solvente, como factor principal en la obtención de las especies, por encima de la energía de interacción con el ligando. - Descripción de algunas características químicas importantes. La contribución de los espectros de resonancia magnética nuclear resulta decisiva para la caracterización de los compuestos. Una descripción en detalle de las particularidades en el cálculo de los desplazamientos químicos para espectros de RMN es revisado en un apartado puramente teórico, especialmente para los métodos DFT. La activación del enlace C-H del carbono apical en el ligando [{TiCp*(-O)}3(3-CH)] durante la formación de estructuras diméricas fue seguida mediante espectroscopia RMN de 13C. Los grandes cambios experimentados en los desplazamientos químicos entre las especies protonadas y no protonadas han sido descritos y racionalizados perfectamente mediante cálculos teóricos. Los resultados obtenidos corroboran las suposiciones experimentales y proporcionan una visión muy clara de los procesos electrónicos que conducen a estos resultados. El carácter predictivo de estos estudios para con otras especies es claro, no obstante cuantitativamente hablando se encuentran dificultades a la hora de atribuir todos los desplazamientos.Fruto de los excelentes resultados obtenidos en la descripción de los valores de RMN para los compuestos cubánicos, estos estudios se han ampliado a otros campos de interés para nuestro grupo de trabajo. Una sección, en parte diferente a la estructura global de la tesis, se ha dedicado ha mostrar estos primeros pasos a la hora de examinar las capacidades de los métodos DFT para computar los desplazamientos en núcleos de 183W en compuestos de Keggin. Nos encontramos ante unas mayores dificultades a la hora de reproducir los espectros experimentales, y gran parte de nuestros esfuerzos se han centrado en tratar de buscar la forma de trabajo más adecuada en términos de precisión y de coste computacional. La investigación y el interés continúan abiertos en un campo que necesita nuevas revisiones, sobre todo a la hora de racionalizar las posibles fuentes de error.Las importantes características catalíticas de los sulfuros de molibdeno son objeto de estudio en la segunda parte de la tesis. El estudio teórico intentará aclarar ciertos aspectos confusos en la mecanistica asociada con las reacciones de estos compuestos. Estos complejos dinucleares de molibdeno con ligandos puente de azufre son una de las familias que posee una gran flexibilidad estructural y electrónica. Parte de los posibles isómeros del compuesto tipo [(CpMo)-S2]2 han sido estudiados, prestando gran interés al intercambio electrónico metal ligando con el cambio de estructura. Ejemplos de estructuras cerradas (a-b) y abiertas (c-d) para el complejo La capacidad de los complejos del sulfuro de activar el hidrógeno molecular y de catalizar la reducción del substrato por el hidrógeno es particularmente relevante en la química. En especial los procesos relacionados con la activación de hidrógeno y posterior reducción de un sustrato ligado a los azufre puente son analizados; y en particular aquellos relacionados con la estructura electrónica del catalizador y del substrato. En estos estudios se pone de manifiesto la importancia catalizadora del metal, sirviendo como puente para protones en la reducción. A su vez se muestra como la naturaleza del sustrato resulta decisiva a la hora de observar una eliminación de hidrógeno molecular o una obtención del substrato reducido.Tarragona, 2004 J. Gracia, The overall framework of this report aims to summarize the work we have done over the past four years. Conceptually, this thesis is divided into three parts. The nucleus is based on a joint project with prof. Mena, of the University of Alcalá, whose group is involved in the synthesis and characterization of new aza- and oxo- titanium cubanes. These families of compounds are basically obtained from preorganized titanium tridentate ligands which can incorporate almost all the metals of the periodic table and yield the corresponding heterometallic cubanes. In this respect, the theoretical study is based on and adapted from the progress in the laboratory work. Practically every section in this part starts by describing the experimental data available for the specific system studied when we began the analysis. Then we go on to describe the theoretical study and attempt to involve the reader in the problems discussed. As a consequence of the advance in the experimental work, NMR spectra are used to characterize the new compounds. This leads us to initiate studies in the computational simulation of NMR chemical shifts. The most theoretical part of this report is an assimilation of these studies. The excellent results obtained for the cubane compounds enabled us to extend the knowledge acquired to other interesting fields and our group has started a study of the NMR shielding tensors in polyoxometalate compounds. The third part of the thesis focuses on some molybdenum-sulphide clusters and their activity as catalysts.Chapters 3 and 4 focuse on the electronic structure of the new titanium cubane compounds recently synthesized by Mena et al. We work parallel to an experimental group to compare and contrast the theoretical and experimental data, and so establish the base chemistry of these new compounds. In this sense, the various reaction energies were studied, and the compounds and their possible isomers were characterised by means of DFT methods. Particular emphasis is paid to the properties of incomplete Ti3 core cubane tridentate ligands (precubanes) as bases for obtaining cubanes. The electronic structure, bonding energies, electron transfer processes and reactivity in the titanium Ti3 core of the metal clusters are studied.The reaction of the azatitanium compound with some titanium derivatives leads to nitrocubane formation (see Scheme 1.3). This reaction forms a cubane compound by adding the fourth vertex to a structure that becomes a precubane. Scheme 1.3. Reaction of with Chapter 5 summarizes the work carried out during a six-month stay in Dr. John E. McGrady's group, which uses density functional theory to explore problems of structure, bonding and reactivity in inorganic compounds. The main focus of the research is metal-metal bonding in cluster compounds. Transition metals have a prominent position in catalysis, both in industrial and biological contexts, and the ultimate goal is to understand the part played by the metal and its ligand environment in facilitating a particular chemical transformation. Chapter 5 studies the electronic structure of the core catalysts and the mechanism of their, unresolved, catalytic activity. The various possible isomers with the generic formula were studied, and the energy of the electron transfer between the metallic centres and the sulphide ligands, so important in the catalytic process, was investigated in depth. Subsequently, the mechanism of the hydrogen activation reaction promoted by these bi-molybdenum compounds (Equation 1.1) was studied, and the nature of the different reaction pathways was elucidated.[(CpMo(-S))2(S2CH2)] promotes the hydrogenation of alkynes to cis alkenes, allenes to alkenes, and ketenes to aldehydes and the hydrogenolysis of carbon disulphide to hydrogen sulphide and thioformaldehide. The mechanisms of hydrogen interaction with these complexes have not been established. It is apparent that the sulphide ligands play a role in hydrogen coordination, and this ligand reactivity was a dominant characteristic throughout the study of these dinuclear molybdenum systems. Equation 1.1

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.

Thomas-Fermi theory is developed to evaluate nuclear matrix elements averaged on the energy shell, on the basis of independent particle Hamiltonians. One- and two-body matrix elements are compared with the quantal results and it is demonstrated that the semiclassical matrix elements, as function of energy, well pass through the average of the scattered quantum values. For the one-body matrix elements it is shown how the Thomas-Fermi approach can be projected on good parity and also on good angular momentum. For the two-body case the pairing matrix elements are considered explicitly., Comment: 15 pages, REVTeX, 6 ps figures; changed content

The role of the bridging ligand on the effective Heisenberg coupling parameters is analyzed in detail. This analysis strongly suggests that the ligand-to-metal charge transfer excitations are responsible for a large part of the final value of the magnetic coupling constant. This permits to suggest a new variant of the Difference Dedicated Configuration Interaction (DDCI) method, presently one of the most accurate and reliable for the evaluation of magnetic effective interactions. This new method treats the bridging ligand orbitals mediating the interaction at the same level than the magnetic orbitals and preserves the high quality of the DDCI results while being much less computationally demanding. The numerical accuracy of the new approach is illustrated on various systems with one or two magnetic electrons per magnetic center. The fact that accurate results can be obtained using a rather reduced configuration interaction space opens the possibility to study more complex systems with many magnetic centers and/or many electrons per center., Comment: 7 pages, 4 figures