Andres Aguado

• University of Valladolid

ZnMg alloys of certain compositions in the Zn-rich side of the phase diagram are particularly efficient, and widely used, as anticorrosive coatings, but a sound understanding of the physico-chemical properties behind such quality is still far from being achieved. The present work focuses on the first stage of the corrosion process, namely the initi...

The accurate description of the potential energy landscape of moderate-sized nanoparticles is a formidable task, but of paramount importance if one aims to characterize, in a realistic way, their physical and chemical properties. We present here a Neural Network potential able to predict structures of pure and mixed nanoparticles with an error in e...

We 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 wit...

Metallic bonding in clusters can be roughly described employing empirical potentials such as Gupta potential[1], thus obtaining a simple description of the cluster’s geometric properties. However, these empirical potentials are largely inaccurate when comparing with firstprinciples calculations, the latter leading to more meaningful results at high...

First-principles calculations are reported for small Zn-Mg nanoalloys to get insight into relevant properties which can be important for an improved efficiency of Zn-Mg coatings against corrosion [1]. Noticeable charge transfer effects, promoted by the electronegativity difference between Zn and Mg, are found in Zn-Mg nanoalloys. This works in favo...

Metallic nanoalloys are fascinating objects of study both for fundamental and applied sciences. The rich diversity of structural and chemical ordering arrangements, coupled with the synergistic enhancement of some properties induced by alloying, result in a wide variety of possible applications in catalysis, magnetic recording, optical devices, etc...

Putative global minimum structures for neutral CdN and singly charged CdN⁺ and CdN⁻ 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-opt...

We report the results of a conjoint experimental/theoretical effort to assess the structures of free-standing zinc clusters with up to 73 atoms. Experiment provides photoemission spectra for ZnN⁻ cluster anions, to be used as fingerprints in structural assessment, as well as mass spectra for both anion and cation clusters. Theory provides both a de...

Zinc-magnesium alloys have better coating properties than both pure compounds. The reason for their improved corrosion protection have been studied at the density functional level of theory. Putative global minima were isolated for 20 atoms Zn-Mg nanoalloys spanning the whole composition range from the pure Mg to the pure Zn. The calculations revea...

Nanoparticles of gallium deposited on a sapphire substrate, which are now shown to remain stable in a state of solid/liquid coexistence across a temperature window wider than 600 K, may prove useful for studying the properties of solid/liquid interfaces and in plasmonic or catalytic applications.

A systematic theoretical study of the doping of Zn17 nanoparticle with 3d transition-metal (TM) impurities is presented. Calculations are conducted within the density functional framework as implemented in the VASP code at the generalized gradient approximation for the exchange and correlation effects. The ground state of Zn17 has been recently sho...

FeCo nanostructures are very interesting for storage media, sensing, and biomedical applications. To learn how an oxidizing environment may affect the physical and chemical properties of FeCo nanoparticles in the molecular limit, we investigated, by means of density functional theoretic calculations, the structural, electronic, and magnetic propert...

How many of the several attributes of the bulk metallic state persist in a nanoparticle containing a finite number of atoms of a metallic element? Do all those attributes emerge suddenly at a well-defined cluster size or do they rather evolve at different rates and in a broad size range? These fundamental questions have been addressed through a con...

We demonstrate, by means of fully unconstrained density functional theory calculations, that the cluster Zn17 endohedrally doped with a Cr impurity can be qualified as a magnetic superalkali cluster. We explain the origin of its high stability, its low vertical ionization potential and its high total spin magnetic moment which amounts to 6 μB, exac...

We report the global minimum (GM) structures, energetic and electronic properties of K clusters with up to 80 atoms, obtained by combining basin hopping unbiased optimizations (based on a many-body empirical potential) with subsequent reoptimization of several candidate structures employing a density functional theory method which accounts for van...

We report first-principles computer simulations of the melting-like transition in Al-n(Q) (n = 35-37, Q = +1,0, 1) clusters. Melting induces an abrupt modification of the electronic shell structure, whereby the HOMO-LUMO gap and chemical hardness sharply decrease, and the size dependence of electronic properties becomes smoother. This intimate coup...

We report the putative Global Minimum (GM) structures and electronic properties of Ga(N)(+), Ga(N) and Ga(N)(-) clusters with N = 13-37 atoms, obtained from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ an exchange-correlation functional which accounts for van der Waals di...

The dipole moments of Na n clusters in the size range 10 < n < 20, recently measured at very low temperature (20 K), are much smaller than predicted by standard density functional methods. On the other hand, the calculated static dipole polarizabilities in that range of sizes deviate non-systematically from the measured ones, depending on the emplo...

We report the global minimum (GM) structures and electronic properties of Csn clusters with up to 80 atoms, obtained employing a density functional theory method which accounts for van der Waals dispersion interactions (vdW-DFT). The GM structures of Csn are found to differ markedly from those of lighter alkali clusters like Nan. Three main physica...

We locate the putative global minimum structures of Na(x)Cs(55-x) and Li(x)Cs(55-x) nanoalloys through combined empirical potential and density functional theory calculations, and compare them to the structures of 55-atom Li-Na and Na-K nanoalloys obtained in a recent paper [A. Aguado and J. M. López, J. Chem. Phys. 133, 094302 (2010)]. Alkali nano...

The electric dipole moments of small Nan clusters have been recently measured [ J. Bowlan, A. Liang and W. A. de Heer Phys. Rev. Lett. 106 043401 (2011)] and found to be essentially zero, a distinguishing feature of metallicity. Previous theoretical calculations fail in describing this fundamental nanoscale property. In this paper, we resolve the d...

Current experimental techniques for measuring the heat capacity of size-selected and isolated atomic clusters are based on dissociation and differ in the critical step of deducing internal energy changes from the dissociation measurements. The heat capacity of Na41+ has been measured by two different techniques with uneven results: one suggests a g...

We have performed an extensive computational search for the global minimum (GM) structures of both neutral and anionic sodium clusters with up to 80 atoms. The theoretical framework combines basin hopping unbiased optimizations based on a Gupta empirical potential (EP) and subsequent reoptimization of many candidate structures at the density functi...

Cross sections for chemisorption of N2 onto Al44(+/-) cluster ions have been measured as a function of relative kinetic energy and the temperature of the metal cluster. There is a kinetic energy threshold for chemisorption, indicating that it is an activated process. The threshold energies are around 3.5 eV when the clusters are in their solid phas...

The structure of 55-atom Li-Na and Na-K nanoalloys is determined through combined empirical potential (EP) and density functional theory (DFT) calculations. The potential energy surface generated by the EP model is extensively sampled by using the basin hopping technique, and a wide diversity of structural motifs is reoptimized at the DFT level. A...

Recent developments allow heat capacities to be measured for size-selected clusters isolated in the gas phase. For clusters with tens to hundreds of atoms, the heat capacities determined as a function of temperature usually have a single peak attributed to a melting transition. The melting temperatures and latent heats show large size-dependent flu...

Photoelectron spectra of cold aluminum cluster anions Al(n)(-) have been measured in the size range n=13-75 and are compared to the results of density functional theory calculations. Good agreement between the measured spectra and the calculated density of states is obtained for most sizes, which gives strong evidence that the correct structures ha...

Oxygen-doped sodium cluster anions Na(n)O(2) (-) with n=41-148 have been studied by low temperature photoelectron spectroscopy and density functional theory (DFT), with a particular emphasis on those sizes where a spherical electron shell closing is expected. The experimental spectra are in good agreement with the electronic density of states of th...

Heat capacities have been measured as a function of temperature for aluminum cluster anions with 35–70 atoms. Melting temperatures and latent heats are determined from peaks in the heat capacities; cohesive energies are obtained for solid clusters from the latent heats and dissociation energies determined for liquid clusters. The melting temperatur...

Putative global minima of neutral (Aln) and singly charged (Aln+ and Aln−) aluminum clusters with n = 13–34 have been located from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ the generalized gradient approximation of Perdew, Burke, and Ernzerhof to describe exchange-corr...

Caloric curves for sodium clusters with N=139 and 147 atoms show a fine structure near the solid-to-liquid transition. Neither of the two sizes exhibit surface melting. For N=139, diffusion of the surface vacancies is observed, which is not possible in the closed-shell N=147 cluster. A few kelvin above the peak in the heat capacity, N=139 is comple...

Dissociation energies have been determined for Al(n)(+) clusters (n=25-83) using a new experimental approach that takes into account the latent heat of melting. According to the arguments presented here, the cohesive energies of the solidlike clusters are made up of contributions from the dissociation energies of the liquidlike clusters and the lat...

Heat capacities have been measured for Al(n-1)Cu(-) clusters (n=49-62) and compared with results for pure Al(n) (+) clusters. Al(n-1)Cu(-) and Al(n) (+) have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transi...

Melting-point depression by soluble impurities is an entropy-driven phenomenon. Studying partially oxidized free sodium nanoparticles, we found an additional mechanism, which is caused by insoluble impurities. Oxidization of sodium clusters with 135-192 atoms by a single oxygen molecule causes a melting-point depression of 17+/-6 K; additional oxyg...

In an article [A. Aguado and P. A. Madden, J. Chem. Phys. 119, 7471 (2003)] published in this journal, Ewald summation expressions were derived for the energy, interatomic forces, pressure tensor, electrostatic field, and electrostatic field gradients in simulation system composed of molecules with charges, induced dipoles, and quadrupoles. In this...

This chapter provides an overview of the computer simulation of the solid–liquid phase transition in alkali metal nanoparticles. The orbital-free molecular dynamics (MD) method allows performing reliable large-scale simulations of alkali clusters. The computational advantages (mostly linear scaling) and disadvantages (use of local pseudopotentials,...

Putative global minima of sodium clusters with up to 380 atoms have been located for two model interatomic potentials in order to identify the structures responsible for the size-dependence of the thermodynamic properties in experiments. Structures based upon the Mackay icosahedra predominate for both potentials, and the magic numbers for the Mur...

We investigate the possible influence of a global cluster rotation on the melting-like transition of Na-{30}, Na-{55} and Na-{147}, by means of orbital-free density-functional molecular dynamics simulations. The results reveal that very large angular momenta are needed to significantly alter the results of simulations performed on nonrotating clust...

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Many modeling problems in materials science involve finite temperature simulations with a realistic representation of the interatomic interactions. These problems often necessitate the use of large simulation cells or long run times, which puts them outside the range of direct ab initio simulation. In ionic systems, it is possible to introduce phys...

The meltinglike transition of Na30 is studied by orbital-free density-functional molecular dynamics simulations. The potential energy surface of Na30 is sampled by simulated annealing and regular quenchings performed along the dynamical trajectories. Both the ground-state structure and low-energy structural excitations are found to exhibit substant...

We present plausible candidates for the global minimum structures of Al(N)(+) (N = 46-62) cluster ions, determined by pseudopotential density functional theory static calculations under the spin-polarized generalized gradient approximation. Our calculations provide a first important step toward the rationalization of recent calorimetric experiments...

A set of molecular dynamics simulations at constant total energy has been performed in order to investigate the melting-like transition of the impurity-doped cluster K1Na54. An orbital-free density-functional-theory technique is employed to obtain the forces on atoms in an efficient way. The total simulation time is approximately 1.5ns, which is re...

An orbital-free density-functional-theory molecular dynamics technique is applied to investigate the minimum-energy structure and meltinglike transition of Cs55, Li13Na32Cs42, and Li55Cs42 nanoparticles. Icosahedral packing is found to be optimal for homogeneous Cs55, as expected. Heterogeneous particles show a complete segregation of Cs atoms to t...

The meltinglike transition in unsupported icosahedral Na(N)() clusters, with N = 135-147, has been studied by isokinetic molecular dynamics simulations based on an orbital-free version of density functional theory. A maximum in the melting temperature, T(m), is obtained for Na141, while the latent heat, deltaE, and entropy of melting, deltaS, are m...

The meltinglike transition in unsupported Na(N) clusters (N = 55, 92, 147, 181, 189, 215, 249, 271, 281 and 299) is studied by first-principles isokinetic molecular dynamics simulations. The irregular size dependence of the melting temperatures Tm observed in the calorimetry experiments of Schmidt et al. [Nature (London) 393, 238 (1998)] is quantit...

We provide a plausible resolution of a long-standing controversy relevant to the geophysics community, namely, that the experimental slope of the melting curve Tm(P) of MgO at low pressures is about 3 times smaller than that obtained from computer simulation of the melting of the normal rock-salt-structured crystal. With increasing temperature at z...

A theoretical analysis of the equilibrium geometry and thermal behavior of the ternary Li13Na30Cs12 alkali nanoalloy is presented. The calculations are based on the orbital-free approach to density functional theory and the classical Newtonian equations to deal with the electronic and atomic subsystems, respectively. An onion-like polyicosahedral s...

Equilibrium geometries and the meltinglike transition of Na13Cs42 and Li13Na42 are studied by means of orbital-free density-functional-theory molecular dynamics simulations. A polyicosahedral structure is found to be energetically favored for Na13Cs42 , with a core shell formed by Na atoms and complete segregation of Cs atoms to the cluster surface...

Large scale computer simulations (those involving explicit consideration of a large number of atoms and/or very long simulation times) are needed in order to get a proper understanding of many material properties (phase transitions, transport properties, etc.). Given the computational cost associated with ab initio electronic structure codes, compu...

The melting-like transition in Li1Na54 and Cs1Na54 clusters is studied by using constant-energy molecular dynamics simulations. An orbital-free density functional theory technique is used, which scales linearly with system size, allowing efficient investigation of thermal behavior in these medium-size clusters. The range of temperatures covered by...

In a recent paper [J. Chem. Phys. 118, 2308 (2003)], Pyper has concluded that the ab initio Perturbed Ion (AIPI) model [Comput. Phys. Commun. 103, 287 (1997)] does not provide useful insights into the physical origins of short-range repulsive forces in ionic crystals. In this comment, I show that Pyper's conclusion is wrong.

The structural, elastic, and high-pressure properties of tin have been studied by pseudopotential density functional theory static calculations. Both local density (LDA) and generalized gradient (GGA) approximations are used to model exchange-correlation effects. Only LDA structural and elastic results are found to be in good agreement with experim...

Transferable potential models of interatomic interactions in CaO, SrO, and BaO are obtained by fitting the forces and stress tensor given by the aspherical ion model (AIM) [J. Chem. Phys. 108, 10209 (1998)] to those obtained from the ab initio molecular dynamics code CASTEP on condensed phase ion configurations generated at different values of pres...

An aspherical ion model (AIM) description of interatomic interactions in alkaline earth oxides MgO, CaO, SrO, and BaO has been obtained. The parameters were determined by matching the forces predicted by the models to those calculated from an ab initio dynamics calculation in finite temperature simulations. The models were shown to be transferable...

Oxide potentials which transfer well between different materials have to account explicitly for many-body contributions to the interaction potentials between the ions. These include dipole and quadrupole polarization effects and the compression and deformation of an oxide ion by its immediate coordination environment. Such complex potentials necess...

In a recent paper [J. Chem. Phys. 118, 2308 (2003)], Pyper has concluded that the ab initio Perturbed Ion (AIPI) model [Comput. Phys. Commun. 103, 287 (1997)] does not provide useful insights into the physical origins of short-range repulsive forces in ionic crystals. In this comment, I show that Pyper's conclusion is wrong.

The structure and surface tension of the liquid–vapor interface of pure LiCl and KCl molten salts and of LiCl-KCl binary mixtures of different concentrations have been studied by using molecular dynamics simulations and a slab geometry. For the surface tension and its temperature and composition dependences, good agreement with experimental determi...

An embedded cluster analysis of the lattice distortions induced by As3+, Sb3+, and Bi3+ substitutional impurities in crystalline KCl is presented. Active clusters containing more than 100 atoms that are embedded in a quantum representation of the crystalline environment are used. Charge compensation is achieved by introducing two cation vacancies i...

An embedded cluster analysis of the lattice distortions induced by As³⁺, Sb³⁺, and Bi³⁺ substitutional impurities in crystalline KCl is presented. Active clusters containing more than 100 atoms that are embedded in a quantum representation of the crystalline environment are used. Charge compensation is achieved by introducing two cation vacancies i...

A refinement of the parameters appearing in the Aspherical Ion Model (AIM) potential is presented. The strategy involves fitting to the ab initio forces and stresses as obtained from the Density Functional Theory code CASTEP at ion configurations obtained from molecular dynamics runs at T=1500 K and P=0 GPa on a small system. The resulting potentia...

Selected as Editor's Choice, the present issue features a theoretical study of the lattice distortions around a Pb2+ substitutional impurity in NaI, RbI and CsF crystals by means of an embedded cluster analysis. The work has been performed by Andrés Aguado, formerly at the University of Valladolid (Spain) where he received a special mention from th...

Molecular dynamics simulations are used to calculate the surface tension γ and study the structural properties of the liquid–vapor interface of the simple molten salt KI. The focus of the present paper is the effect on the calculated surface tension of different terms in the interionic potential and of the way that long-ranged interactions are trea...

An examination of the finite system size effects on the calculated interfacial properties of the molten salt KI is described, as an extension of the work described in the preceding paper [J. Chem. Phys. 115, 8603 (2001)]. It is shown that, for system sizes beyond ∼ 1000 ions, the finite size effects are accounted for by the predictions of capillary...

The local lattice distortions around a Tl+-dimer substitutional impurity in NaI and KI have been investigated by using a mixed ab initio/parametrized methodology. One important conclusion of the work is that an explicit consideration of these distortions up to at least the first four coordination shells of ions around the impurity is needed in orde...

The melting-like transition in disordered sodium clusters Na92 and Na142 is studied by performing density functional constant-energy molecular dynamics simulations. The orbital-free version of the density functional formalism is used. In Na142 the atoms are distributed in two distinct shells (surface and inner shells) and this cluster melts in two...

A theoretical analysis of the lattice distortions induced by nonisovalent Ge2+, Sn2+, and Pb2+ substitutional impurities in crystalline NaCl, and of the off-center equilibrium position adopted by those impurities in their ground ns2 electronic configuration is presented. The calculations are based in the cluster approach, and involve large active c...

We present ab initio perturbed ion calculations on the structures and relative stabilities of doubly charged [(NaCl)_m(Na)_2]2+ cluster ions. The obtained stabilities show excellent agreement with experimental abundances obtained from mass spectra. Those enhanced stabilities are found to be a consequence of highly compact structures that can be bui...

The meltinglike transition in potassium clusters KN, with N=20, 55, 92, and 142, is studied by using an orbital-free density-functional constant-energy molecular dynamics simulation method, and compared to previous theoretical results on the meltinglike transition in sodium clusters of the same sizes. Melting in potassium and sodium clusters procee...

A computational study of the doping of alkali halide crystals (AX: A = Na, K; X = Cl, Br) by ns2 cations (Ga+, In+ and Tl+) is presented. Active clusters of increasing size (from 33 to 177 ions) are considered in order to deal with the large scale distortions induced by the substitutional impurities. Those clusters are embedded in accurate quantum...

The emergence of CsCl bulk structure in (CsCl)nCs+ cluster ions is investigated using a mixed quantum-mechanical/semiempirical theoretical approach. We find that rhombic dodecahedral fragments (with bulk CsCl symmetry) are more stable than rock-salt fragments after the completion of the fifth rhombic dodecahedral atomic shell. From this size (n=184...

The structures and relative stabilities of doubly-charged nonstoichiometric (CaO)$_n$Ca$^{2+}$ (n=1--29) cluster ions and of neutral stoichiometric (MgO)$_n$ and (CaO)$_n$ (n=3,6,9,12,15,18) clusters are studied through {\em ab initio} Perturbed Ion plus polarization calculations. The large coordination-dependent polarizabilities of oxide anions fa...

An ab initio study of the doping of alkali halide crystals (AX: A = Li, Na, K, Rb; X = F, Cl, Br, I) by ns2 anions (Ag- and Cu-) is presented. Large active clusters with 179 ions embedded in the surrounding crystalline lattice are considered in order to describe properly the lattice relaxation induced by the introduction of substitutional impuritie...

The melting-like transition in sodium clusters Na_N, with N=55, 92, and 142 is studied by using constant-energy molecular dynamics simulations. An orbital-free version of the Car-Parrinello technique is used which scales linearly with system size allowing investigation of the thermal behaviour of large clusters. The ground state isomer of Na_142 (a...

The band gap energy as well as the possibility of cross luminescence processes in alkaline-earth dihalide crystals have been calculated using the ab initio Perturbed-Ion (PI) model. The gap is calculated in several ways: as a difference between one-electron energy eigenvalues and as a difference between total energies of appropriate electronic stat...

Ab initio perturbed ion cluster-in-the-lattice calculations of the impurity centers NaI:Tl+ and CsI:Tl+ are presented. We study several active clusters of increasing complexity and show that the lattice relaxation around the Tl+ impurity implies the concerted movement of several shells of neighbors. The results also reveal the importance of conside...

Ab initio calculations using the perturbed ion model, with correlation contributions included, are presented for nonstoichiometric (NaI)nNa+ and (CsI)nCs+ (n<~14) cluster ions. The ground state and several low-lying isomers are identified and described. Rocksalt ground states are common and appear at cluster sizes lower than in the corresponding ne...

The band gap of alkali halides, alkaline-earth oxides, Al2O3 and SiO2 crystals has been calculated using the Perturbed-Ion model supplemented with some assumptions for the treatment of excited states. The gap is calculated in several ways: as a difference between one-electron energy eigenvalues and as a difference between the total energies of appr...

The melting-like transitions of Na8 and Na20 are investigated by ab initio constant energy molecular dynamics simulations, using a variant of the Car-Parrinello method which employs an explicit electronic kinetic energy functional of the density, thus avoiding the use of one-particle orbitals. Several melting indicators are evaluated in order to de...

Ab initio perturbed ion plus polarization calculations are reported for doubly-charged nonstoichiometric (MgO)nMg2+ (n=1-29) cluster ions. We consider a large number of isomers with full relaxations of the geometries, and add the correlation correction to the Hartree-Fock energies for all cluster sizes. The polarization contribution is included at...

Ab initio Perturbed Ion cluster-in-the-lattice calculations of the impurity centers NaI:Tl+ and CsI:Tl+ are pressented. We study several active clusters of increasing complexity and show that the lattice relaxation around the Tl+ impurity implies the concerted movement of several shells of neighbors. The results also reveal the importance of consid...

The structural and bonding properties of small neutral alkali-halide clusters (AX)n, with n less than or equal to 10, A=Li, Na, K, Rb and X=F, Cl, Br, I, are studied using the ab initio Perturbed Ion (aiPI) model and a restricted structural relaxation criterion. A trend of competition between rock-salt and hexagonal ring-like isomers is found and d...

Ab initio Perturbed Ion (PI) calculations are reported for neutral stoichiometric (MgO)n clusters (n<14). An extensive number of isomer structures was identified and studied. For the isomers of (MgO)n (n<8) clusters, a full geometrical relaxation was considered. Correlation corrections were included for all cluster sizes using the Coulomb-Hartree-F...

A systematic theoretical study of stoichiometric clusters (NaI)n up to n=15 is performed using the ab initio Perturbed-Ion (PI) model. The structures obtained are compared to previous pair potential results, and observed differences between (NaI)n clusters and previous ab initio results for other alkali halide clusters are discussed. (NaI)n cluster...

The microwave spectrum of 2-chloro-1,1,1,2-tetrafluoroethane has been analyzed in the frequency region 12–52 GHz using waveguide Fourier transform and Stark modulation microwave spectrometers. The spectra of35Cl and37Cl isotopomers and of the two lowest excited states of the CF3torsional vibration of35Cl species have been measured up toJ= 58. Chlor...