PNM - Artículos de revistashttps://uvadoc.uva.es/handle/10324/363292022-08-13T04:01:02Z2022-08-13T04:01:02ZComparison of theoretical methods of the hydrogen storage capacities of nanoporous carbonsCabria Álvaro, Ivánhttps://uvadoc.uva.es/handle/10324/426462021-06-24T07:32:53Z2020-01-01T00:00:00ZThe hydrogen storage capacities of nanoporous carbons, simulated as graphene slit-shaped pores, have been calculated using simple theoretical methods that do not involve computationally expensive calculations. The theoretical methods calculate the storage of hydrogen molecules on a solid porous material by using the Equation Of State, EOS, of the hydrogen gas and the interaction potential energy of H2 with the surfaces of the pores of the material. Calculations have been carried out using the same interaction potential energy and empirical EOS. The interaction potential energy is obtained from calculations of H2 on graphene, using a DFT-based method that includes the dispersion interactions. The storage capacities have been calculated as a function of pressure in the range 0.1–25 MPa, of pore width in the range 4.7–20 Å and at 80.15 and 298.15 K. The storage capacities obtained with the methods are compared and the advantages and limitations of the methods are discussed, as well as the storage capacities predicted by the methods for wide pores. These simple theoretical methods are useful to design novel materials for hydrogen storage.
2020-01-01T00:00:00ZMagnetostatic dipolar anisotropy energy and anisotropy constants in arrays of ferromagnetic nanowires as a function of their radius and interwall distanceCabria Álvaro, IvánPrida, Victor Manuel de lahttps://uvadoc.uva.es/handle/10324/426452022-03-29T12:13:39Z2020-01-01T00:00:00ZMagnetostatic dipolar anisotropy energy and the total dipolar anisotropy constant, ${K}_{{total}}$, in periodic arrays of ferromagnetic nanowires have been calculated as a function of the nanowire radius, the interwall distance of the nanowires in the arrays and the geometry of the array (square or hexagonal), by using a realistic atomistic model and the Ewald method. The simulated nanowires have a radius size up to 175 Å that corresponds to 31 500 atoms, and the simulated nanowire arrays have interwall distances between 35 and 3000 Å. The dependence of total magnetostatic dipolar anisotropy constant on the nanowire radius, their interwall distance and the type of array symmetry has been analyzed. The total dipolar anisotropy constant, which is the sum of the intrananowire dipolar anisotropy constant, ${K}_{{intra}}$, due to the dipolar interactions inside an isolated nanowire and the main responsible of the shape anisotropy, and of the internanowire dipolar anisotropy constant, ${K}_{{inter}}$, due to the magnetostatic dipolar interactions among nanowires in the array, have been calculated and compared with the magnetocrystalline anisotropy constant for three nanowire compositions and their crystalline structures. The simulations of the nanowire arrays with large interwall distances have been used to calculate the intrananowire anisotropy constant, ${K}_{{intra}}$, and to analyze the competition between the intrananowire, internanowire and magnetocrystalline anisotropies. According to some magnetic theories, the ratio $| {K}_{{inter}}/{K}_{{intra}}| $ equals to the areal filling fraction of a nanowire array. Present calculations indicate that the equation for the areal filling fraction matches perfectly for any interwall distance and radius of Ni and Co nanowire arrays. This first equation is used to write a general equation that relates the radius and interwall distance of nanowire arrays with the intrananowire, internanowire and magnetocrystalline anisotropies. This general equation allows to design the geometry of nanowire arrays with the desired orientation of the easy magnetization axis.
2020-01-01T00:00:00ZAnalysis of the Symmetry Properties of Large Periodic Magnetic Systems, to Reduce the Computation Time of the Calculation of the Magnetostatic Dipolar EnergyCabria Álvaro, Ivánhttps://uvadoc.uva.es/handle/10324/426362021-06-24T07:32:46Z2018-01-01T00:00:00ZThe computational effort to calculate the magnetostatic dipolar energy, MDE, of a periodic cell of N magnetic moments is an O(N 2 ) task. Compared with the calculation of the Exchange and Zeeman energy terms, this is the most computationally expensive part of the atomistic simulations of the magnetic properties of large periodic magnetic systems. To reduce the computational effort, the traditional Ewald method to calculate the MDE of periodic magnetic systems has been analyzed. The detailed analysis reveals that, for certain types of periodic systems, there are many matrix elements of the Ewald method identical to another elements, due to symmetry properties of the periodic systems. Computation timing experiments of the MDE of large systems, such as Ni fcc nanowires up to 31500 magnetic moments in the periodic cell, have been carried out and they show that the number of matrix elements that should be calculated is approximately equal to N, instead of N 2 /2 if these symmetries are used, and that the computation time decreases in an important amount. The time complexity of the analysis of the symmetries is O(N 3 ), which increases the time complexity of the traditional Ewald method and is in contrast with the computation timing experiments. This is explained by the fact that the MDE is a very small energy and therefore, the usual required precision of the calculation of the MDE is so high, about 10 -6 eV/cell, that the calculations of large periodic magnetic systems are very expensive and the use of the symmetries reduces, in practical terms, the computation time of the MDE in a significant amount, in spite of the increase of the time complexity.
2018-01-01T00:00:00ZNeutron Brillouin scattering and ab initio simulation study of the collective dynamics of liquid silverGuarini, EleonoraDe Francesco, AlessioBafile, UbaldoLaloni, AlessioGonzález del Río, BeatrizGonzález Fernández, David JoséGonzález Tesedo, Luis EnriqueBarocchi, FabrizioFormisano, Ferdinandohttps://uvadoc.uva.es/handle/10324/426292021-06-24T07:32:38Z2020-01-01T00:00:00ZWe present a thorough investigation of the collective dynamics of liquid Ag combining neutron Brillouin scattering and ab initio molecular dynamics (AIMD) determinations of the dynamic structure factor S(Q,ω). The main scope of this work is not only to provide experimental results for some important dynamical properties of this liquid metal in the wave-vector range 4<Q<16nm−1, but also to inquire about the scarce detectability of shear waves apparently characterizing two elements of group IB, differently from other metals. In fact, as in the case of Au, a transverse-like dynamics is not deducible from the experimental S(Q,ω) of Ag, despite the indisputable quality of the neutron data collected on the BRISP spectrometer at the Institut Laue Langevin in Grenoble. However, the significant agreement between experiment and AIMD calculations allowed for an in-depth study of the simulated S(Q,ω) in a Q range overlapping and extending the experimental one. A multimode analysis, already proven very successful in the description of various dynamical properties of fluid systems, is shown to be extremely effective also to analyze the intermediate scattering function predicted by AIMD at the various Q values, and eventually enables a reliable determination of both longitudinal and transverse branches in the dispersion curve of this liquid. Throughout the paper we highlight the importance of referring to theoretically well-founded models for S(Q,ω) and of imposing physical constraints in a fit-based analysis: These ensure that the used models obey fundamental properties of the dynamic structure factor.
2020-01-01T00:00:00ZDepth-dependent dynamics of liquid metal surfaces with first principles simulationsG. del Río, BeatrizGonzález Tesedo, Luis Enriquehttps://uvadoc.uva.es/handle/10324/426282021-06-24T07:32:40Z2020-01-01T00:00:00ZLiquid metal surfaces have gained increased interest over the last decade due to new applications in synthesis of 2D materials, catalysis, or fusion reactors. Static properties such as the reflectivity and density profile have been determined, both experimentally and computationally, for numerous liquid metals and alloys. However, the characterization of the dynamic properties has remained a challenging task and only one experimental study by Reichert et al. has evaluated the depth-dependence of different dynamic properties in the liquid indium (l-In) surface. In this paper, we present an ab inito molecular dynamics study of the collective dynamic properties of this same system at different depths, obtaining very good agreement with the experimental data. In addition, we are able to compute the properties much closer to the surface than experimentally attainable, and have discovered that at these shallower depths, the properties drastically differ from those deeper in the slab. Therefore, this study sheds light into the behavior of dynamic properties at the atomic interface and highlights the ability of ab initio molecular dynamics to study such unknown dynamic behavior of liquid metals surfaces at depths not yet attainable experimentally but of crucial importance for liquid surface physics.
2020-01-01T00:00:00ZBorophene vs. graphene interfaces: Tuning the electric double layer in ionic liquidsGómez González, VíctorOtero Mato, J. ManuelMontes Campos, HadriánGarcía Andrade, XabierGarcía Fuente, AmadorVega Hierro, AndrésCarrete, JesúsCabeza, OscarGallego, Luis J.Varela, Luis M.https://uvadoc.uva.es/handle/10324/426272021-06-24T07:32:45Z2020-01-01T00:00:00ZIn this work we perform molecular dynamics simulations of mixtures of a prototypical protic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), with lithium tetrafluoroborate (LiBF4), confined between two borophene walls of three different surface charges, −1, 0 and +1 e/nm2, where e is the elementary charge. The properties of the system are analyzed by means of ionic density profiles, angular orientations of [BMIM]+ cations close to the wall and vibrational densities of states for the salt cations close to the walls. The lateral structure of the first layer close to the surface is also studied on one hand, calculating Minkowski parameters and the Shannon entropy of the patterns of the 2D density maps of the anions placed there and, on the other hand, computing the 2D-Fourier transform of the positions of these anions. Our results are compared with those obtained previously for the same mixtures confined between two graphene walls. Although similarities exist between both cases, interesting differences are observed in the lateral structure that the ionic liquid adopts near borophene interfaces due to their strong anisotropy. In particular, we have observed that borophene induces more markedly ordered 2D patterns in the innermost layer of the ionic liquid electric double layer, specially when they are charged. It is this feature that makes borophene a potential candidate for battery electrode applications with possibilities beyond those of graphene.
2020-01-01T00:00:00ZStructural and Electronic Rearrangements in Fe2S2, Fe3S4, and Fe4S4 Atomic Clusters under the Attack of NO, CO, and O2Amitouche, FadilaSaad, FaridaTazibt, SlimaneBouarab, SaidVega Hierro, Andréshttps://uvadoc.uva.es/handle/10324/426252021-06-24T07:32:31Z2019-01-01T00:00:00ZWe report results, based on density functional theory–generalized gradient approximation calculations, that shed light on how NO, CO, and O2 interact with Fe2S2, Fe3S4, and Fe4S4 clusters and how they modify their structural and electronic properties. The interest in these small iron sulfide clusters comes from the fact that they are at the protein cores and that elucidating fundamental aspects of their interaction with those light molecules which are known to modify their functionality may help in understanding complex behaviors in biological systems. CO and NO are found to bind molecularly, leading to moderate relaxations in the clusters, but nevertheless to changes in the spin-polarized electronic structure and related properties. In contrast, dissociative chemisorption of O2 is much more stable than molecular adsorption, giving rise to significant structural distortions, particularly in Fe4S4 that splits into two Fe2S2 subclusters. As a consequence, oxygen tends to strongly reduce the spin polarization in Fe and to weaken the Fe–Fe interaction inducing antiparallel couplings that, in the case of Fe4S4, clearly arise from indirect Fe–Fe exchange coupling mediated by O. The three molecules (particularly CO) enhance the stability of the iron–sulfur clusters. This increase is noticeably more pronounced for Fe2S2 than for the other iron–sulfur clusters of different compositions, a result that correlates with the fact that in recent experiments of CO reaction with FemSm (m = 1–4), the Fe2S2CO product results as a prominent one.
2019-01-01T00:00:00ZTuning the Magnetic Moment of Small Late 3d-Transition-Metal Oxide Clusters by Selectively Mixing the Transition-Metal ConstituentsAguilera del Toro, Rodrigo HumbertoTorres, MaríaAguilera Granja, Juan FaustinoVega Hierro, Andréshttps://uvadoc.uva.es/handle/10324/426232021-08-18T09:50:12Z2020-01-01T00:00:00ZTransition-metal oxide nanoparticles are relevant for many applications in different areas where their superparamagnetic behavior and low blocking temperature are required. However, they have low magnetic moments, which does not favor their being turned into active actuators. Here, we report a systematical study, within the framework of the density functional theory, of the possibility of promoting a high-spin state in small late-transition-metal oxide nanoparticles through alloying. We investigated all possible nanoalloys An−xBxOm (A, B = Fe, Co, Ni; n = 2, 3, 4; 0≤x≤n) with different oxidation rates, m, up to saturation. We found that the higher the concentration of Fe, the higher the absolute stability of the oxidized nanoalloy, while the higher the Ni content, the less prone to oxidation. We demonstrate that combining the stronger tendency of Co and Ni toward parallel couplings with the larger spin polarization of Fe is particularly beneficial for certain nanoalloys in order to achieve a high total magnetic moment, and its robustness against oxidation. In particular, at high oxidation rates we found that certain FeCo oxidized nanoalloys outperform both their pure counterparts, and that alloying even promotes the reentrance of magnetism in certain cases at a critical oxygen rate, close to saturation, at which the pure oxidized counterparts exhibit quenched magnetic moments
2020-01-01T00:00:00ZFirst principles determination of some static and dynamic properties of the liquid 3d transition metals near meltingG. del Río, BeatrizPascual, CarlosRodríguez, O.González Tesedo, Luis EnriqueGonzález Fernández, David Joséhttps://uvadoc.uva.es/handle/10324/426192021-06-24T07:32:36Z2020-01-01T00:00:00ZWe report an ab initio molecular dynamics simulation study of several static and dynamic properties of the liquid 3d transition metals. The calculated static structure factors show qualitative agreement with the available experimental data, and its second peak displays an asymmetric shape which suggests a signi1cant local icosahedral short-range order. The dynamical structure reveals propagating density 2uctuations whose dispersión relation has been evaluated; moreover, its long wavelength limit is compatible with their respective experimental sound velocity. Results are reported for the longitudinal and transverse current spectral functions as well as for the respective dispersion relations. We also analyze the possible appearance of transverse-like low-energy excitations in the calculated dynamic structure factors. Several transport coeWcients have been evaluated and
compared with the available experimental data.
2020-01-01T00:00:00ZFirst principles study of liquid uranium at temperatures up to 2050 KGonzález del Río, BeatrizGonzález Tesedo, Luis EnriqueGonzález Fernández, David Joséhttps://uvadoc.uva.es/handle/10324/426132021-06-24T07:32:34Z2020-01-01T00:00:00ZUranium compounds are used as fissile materials in nuclear reactors. In present day reactors the most used nuclear fuel is uranium dioxide, but in generation-IV reactors other compounds are also being considered, such as uranium carbide and uranium mononitride. Upon possible accidents where the coolant would not circulate or be lost the core of the reactor would reach very high temperatures, and therefore it is essential to understand the behaviour of the nuclear fuel under such conditions for proper risk assessment. We consider here molten metallic uranium at several temperatures ranging from 1455 to 2050 K. Even though metallic uranium is not a candidate for nuclear fuel it could nevertheless be produced due to the thermochemical instability of uranium nitride at high temperatures. We use first principles techniques to analyse the behaviour of this system and obtain basic structural and dynamic properties, as well as some thermodynamic and transport properties, including atomic diffusion and viscosity.
2020-01-01T00:00:00ZProperties of bulk liquid Pd and Pt and their free liquid surface studied with first principles techniquesGonzález del Río, BeatrizGonzález Tesedo, Luis EnriqueGonzález Fernández, David Joséhttps://uvadoc.uva.es/handle/10324/426092021-06-24T07:32:33Z2020-01-01T00:00:00ZWe have performed first principles computer simulations in order to study the structural and dynamic properties of bulk liquid Pd and Pt near their melting points. We find good agreement with the available experimental static structure and transport properties, and furthermore we provide more detailed information that is not available from experiments. Additional simulations have also been undertaken so as to study the free liquid surface of both liquid metals. The calculated longitudinal ionic density profile exhibits an oscillatory behavior whose properties have been analyzed. For both metals, the associated intrinsic surface structure factor presents a marked maximum related to surface layering
2020-01-01T00:00:00ZStructure and dynamics of the liquid 3d transition metals near melting. An ab initio studyGonzález del Río, BeatrizPascual, CarlosGonzález Tesedo, Luis EnriqueGonzález Fernández, David Joséhttps://uvadoc.uva.es/handle/10324/426062021-06-24T07:32:32Z2020-01-01T00:00:00ZThe static and dynamic properties of several bulk liquid 3d transition metals at thermodynamic conditions near their respective melting points have been evaluated by using ab initio molecular dynamics simulations. The calculated static structure factors show an asymmetric second peak followed by a more or less marked shoulder which points to a sizeable amount of icosahedral local order. Special attention is devoted to the analysis of the longitudinal and transverse current spectral functions and the corresponding dispersion of collective excitations. For some metals, we have found the existence of two branches of transverse collective excitations in the second pseudo-Brillouin zone. Finally, results are also reported for several transport coefficients.
2020-01-01T00:00:00ZIncorporating charge transfer effects into a metallic empirical potential for accurate structure determination in (ZnMg)N nanoalloysÁlvarez Zapatero, PabloVega Hierro, AndrésAguado Rodríguez, Andréshttps://uvadoc.uva.es/handle/10324/426042022-01-21T12:53:28Z2020-01-01T00:00:00ZWe report the results of a combined empirical potential-Density Functional Theory (EP-DFT) study to assess the global minimum structures of free-standing zinc-magnesium nanoalloys of equiatomic composition and with up to 50 atoms. Within this approach, the approximate potential energy surface generated by an empirical potential is first sampled with unbiased basin hopping simulations, and then a selection of the isomers so identified is re-optimized at a first-principles DFT level. Bader charges calculated in a previous work [Corr. Sci. 124, 35 (2017)] revealed a significant transfer of electrons from Mg to Zn atoms in these nanoalloys; so the main novelty in the present work is the development of an improved EP, termed Coulomb-corrected-Gupta potential, which incorporates an explicit charge-transfer correction term onto a metallic Gupta potential description. The Coulomb correction has a many-body character and is feeded with parameterized values of the ab initio Bader charges. The potentials are fitted to a large training set containing DFT values of cluster energies and atomic forces, and the DFT results are used as benchmark data to assess the performance of Gupta and Coulomb-corrected-Gupta EP models. Quite surprisingly, the charge-transfer correction is found to represent only a 6% of the nanoalloy binding energies, yet this quantitatively small correction has a sizable benefitial effect on the predicted relative energies of homotops. Zn-Mg bulk alloys are used as sacrificial material in corrosion-protective coatings, and the long-term goal of our research is to disclose whether those corrosion-protected capabilities are enhanced at the nanoscale.
2020-01-01T00:00:00ZSimulations of volumetric hydrogen storage capacities of nanoporous carbons: Effect of dispersion interactions as a function of pressure, temperature and pore widthCabria Álvaro, Ivánhttps://uvadoc.uva.es/handle/10324/367442021-06-24T07:32:51Z2019-01-01T00:00:00ZSimulations of the hydrogen storage capacities of activated carbons require an accurate
treatment of the interaction of a hydrogen molecule physisorbed on the graphitic-like
surfaces of nanoporous carbons, which is dominated by the dispersion interactions.
These interactions are described accurately by high level quantum chemistry methods
such as the Coupled cluster method with single and double excitations and a non-iterative
correction for triple excitations (CCSD(T)), but those methods are computationally very
expensive for large systems and massive simulations. Density functional theory (DFT)
based methods that include dispersion interactions are less accurate, but computationally
less expensive. Calculations of the volumetric hydrogen storage capacities of nanoporous
carbons, simulated as benzene and graphene slit-shaped pores, have been carried out,
using a quantum-thermodynamic model of the physisorption of H2 on surfaces and the
interaction potential energy curves of H2 physisorbed on benzene and graphene obtained
using the CCSD(T) and second order Møller-Plesset (MP2) methods and the 14 most popular
DFT-based methods that include the dispersion interactions at different levels of
complexity. The effect of the dispersion interactions on the DFT-based volumetric capacities
as a function of the pressure, temperature and pore width is evaluated. The error of
the volumetric capacities obtained with the quantum-thermodynamic model and each
method is also calculated and analyzed.
2019-01-01T00:00:00ZMagnetostatic Dipolar Energy of Large Periodic Ni fcc Nanowires, Slabs and SpheresCabria Álvaro, Ivánhttps://uvadoc.uva.es/handle/10324/367342021-06-24T07:32:49Z2019-01-01T00:00:00ZThe computational effort to calculate the magnetostatic dipolar energy, MDE, of a periodic cell of N magnetic moments
is an O(N2) task. Compared with the calculation of the Exchange and Zeeman energy terms, this is the most
computationally expensive part of the atomistic simulations of the magnetic properties of large periodic magnetic
systems. Two strategies to reduce the computational effort have been studied: An analysis of the traditional Ewald
method to calculate the MDE of periodic systems and parallel calculations. The detailed analysis reveals that, for certain
types of periodic systems, there are many matrix elements of the Ewald method identical to another elements, due
to some symmetry properties of the periodic systems. Computation timing experiments of the MDE of large periodic
Ni fcc nanowires, slabs and spheres, up to 32000 magnetic moments in the periodic cell, have been carried out and
they show that the number of matrix elements that should be calculated is approximately equal to N, instead of N2/2,
if these symmetries are used, and that the computation time decreases in an important amount. The time complexity
of the analysis of the symmetries is O(N3), increasing the time complexity of the traditional Ewald method. MDE is
a very small energy and therefore, the usual required precision of the calculation of the MDE is so high, about 10−6
eV/cell, that the calculations of large periodic magnetic systems are very expensive and the use of the symmetries
reduces, in practical terms, the computation time of the MDE in a significant amount, in spite of the increase of the
time complexity. The second strategy consists on parallel calculations of the MDE without using the symmetries of
the periodic systems. The parallel calculations have been compared with serial calculations that use the symmetries.
2019-01-01T00:00:00Z