Molecular dynamics simulations of the meltinglike transition in Li13Na42 and Na13Cs42 clusters

Producción CientíficaEquilibrium 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. Li13Na42 adopts an amorphouslike structure, albeit with significant local polyicosahedral order, with the Na atoms preferentially occupying surface sites but with partial mixing of Li and Na species at the cluster core. Analysis of the thermal properties reveals that premelting effects are more important for heterogeneous than for homogeneous alkali clusters. The nature of these premelting effects is discussed in detail. For Na13Cs42, they involve isomerizations without significant atom diffusion; for Li13Na42, they also include partial melting of the surface formed by Na atoms. The mixing of Li and Na species is significantly enhanced above the melting temperature, while surface segregation of Cs in Na13Cs42 is maintained in the liquid state. From the study of these two clusters, we attemp to extract some general trends about the structural and thermal behaviors of heterogeneous alkali clusters

Structural and thermal behavior of compact core-shell nanoparticles: Core instabilities and dynamic contributions to surface thermal stability

Producción CientíficaAn 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 the cluster surface, and form perfect core-shell structures, that is, structures where each atomic species occupies and completes a different concentric atomic shell. For Li13Na32Cs42, the size mismatch between atomic species forming different shells leads to polyicosahedral packing. For Li55Cs42, however, the size mismatch is huge and perfect polyicosahedral ordering is frustrated, resulting in more complex structural behavior. The three clusters investigated share the same surface shell, formed by 42 Cs atoms, and comparison of their melting behaviors helps to rationalize the increased thermal stability of the cluster surface upon alloying. Cs55 melts homogeneously at approximately 85 K. Both Li13Na32Cs42 and Li55Cs42 show a substantial thermal stability, compared to Cs55 and other alloy compositions where a perfect core-shell structure does not appear. We demonstrate that an important contribution to this increased thermal stability in the nanoalloys comes from the large difference in the atomic masses of the constituent particles, which results in a poor coupling of atomic vibrations along the radial direction. We also give arguments to show that the meltinglike transition in these clusters is triggered by the thermal instability of interior rather than surface atoms. Segregation of Cs atoms to the cluster surface is fully maintained in the liquid state, so that core and surface shells form two inmiscible liquid layers

Anomalous Size Dependence in the Melting Temperatures of Free Sodium Clusters: An Explanation for the Calorimetry Experiments

Producción CientíficaThe meltinglike transition in unsupported NaN 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 quantitatively reproduced. We demonstrate that structural effects alone can explain all broad features of experimental observations. Specifically, maxima in Tm(N) correlate with high surface stability and with structural features such as a high compactness degree

Small sodium clusters that melt gradually: Melting mechanisms in Na30

Producción CientíficaThe 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 substantial polyicosahedral ordering. The most relevant feature of the potential energy landscape for the melting problem is the existence of many different structural isomers within an energy range of 1 meV/atom, resembling that of a glassy system (yet the structures have a high symmetry). The liquid phase is accessed gradually, with some isomerizations observed at a temperature as low as 30 K, while melting can be considered complete above approximately 200 K. The different dynamical mechanisms that allow the smooth opening of phase space available to the system as a function of temperature are identified and discussed. They can be classified in two different categories: (a) those that allow the exploration of isomers similar to the ground state, involving mainly surface isomerizations and surface melting, and leaving the structure of the cluster core unchanged; and (b) those associated with a more substantial structural change, more similar to the usual solid-solid phase transition in bulk phases; the structure of the cluster core changes only in this second type of transition. Mechanism (a) results in surface melting of the corresponding isomer upon heating; at that stage, mechanism (b) acts to transfer some excess energy from the surface to the core region, so that the surface melting is transiently avoided. Even in the fully developed liquid state, there are important differences from the bulk liquid due to the presence of the surface

Incorporating charge transfer effects into a metallic empirical potential for accurate structure determination in (ZnMg)N nanoalloys

Producción CientíficaWe 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.Junta de Castilla y León (Ref. VA124G18)Ministerio de Economía, Industria y Competitividad ((Project PGC2018-093745-B-I00

Atomic layering and related postmelting effects in small liquid metal clusters

Producción CientíficaThe specific heat of Na_25, as obtained by first-principles molecular dynamics simulations, shows interesting anomalies above the melting phase-change region: a) It steadily decreases with increasing temperature, an effect observed also in bulk liquid metals. This trend is explained in terms of the gradual conversion of vibrational modes into diffusion modes; b) on top of this decreasing trend, the specific heat shows broad undulations, induced by the temperature-dependent atomic layering in the liquid cluster. Extended liquid metal surfaces also show atomic layering, but it does not affect the bulk heat capacity. This effect is therefore genuine of finite atomic systems and should be expected to be quite general in metal clusters

Structure and bonding in small neutral alkali halide clusters

Producción CientíficaThe structural and bonding properties of small neutral alkali halide clusters, (AX)_n with n<10, A=Li^+,Na^+,K^+,Rb^+, and X=F^-,Cl^-,Br^-,I^-, are studied using the ab initio perturbed ion (PI) model and a restricted structural relaxation criterion. A trend of competition between rocksalt and hexagonal ringlike isomers is found and discussed in terms of the relative ionic sizes. The main conclusion is that an approximate value of r_C /r_A=0.5 (where r_C and r_A are the cationic and anionic radii) separates the hexagonal from the rocksalt structures. The classical electrostatic part of the total energy at the equilibrium geometry is enough to explain these trends. The magic numbers in the size range studied are n=4, 6, and 9, and these are universal since they occur for all alkali halides and do not depend on the specific ground-state geometry. Instead those numbers allow for the formation of compact clusters. Full geometrical relaxations are considered for (LiF)_n (n=3 – 7) and (AX)_3 clusters, and the effect of Coulomb correlation is studied in a few selected cases. These two effects preserve the general conclusions achieved thus far

Computational Characterisation of Structure and Metallicity in Small Neutral and Singly-Charged Cadmium Clusters

Producción CientíficaPutative 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

LinkedIn “Big Four”: Job Performance Validation in the ICT Sector.

Social networks websites, and specially the LinkedIn platform, have changed the landscape of recruitment and personnel selection to a unified organizational process. Thus, apart from using LinkedIn as a recruitment tool, professionals also use it to make evaluative inferences regarding the individual characteristics of the candidates (e.g., their personality). However, most of the research focused on LinkedIn has left aside the evidence about its validity for decision making in the work setting. In our study we analyze the criterion oriented validity of LinkedIn incumbents professional profiles (N = 615) in the information and communication technology (ICT) sector with some measures of job performance. The results show four major factors underlying LinkedIn profiles about professional experience, social capital, updating knowledge, and non-profesional information. These factors are significantly related to productivity, absenteeism, and the potential for professional development. These findings are discussed in light of their theoretical and practical implications.post-print322 K

Electronic effects on melting: Comparison of aluminum cluster anions and cations

Producción CientíficaHeat 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 temperatures, latent heats, and cohesive energies for the aluminum cluster anions are compared to previous measurements for the corresponding cations. Density functional theory calculations have been performed to identify the global minimum energy geometries for the cluster anions. The lowest energy geometries fall into four main families: distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and “disordered” roughly spherical structures. The comparison of the cohesive energies for the lowest energy geometries with the measured values allows us to interpret the size variation in the latent heats. Both geometric and electronic shell closings contribute to the variations in the cohesive energies (and latent heats), but structural changes appear to be mainly responsible for the large variations in the melting temperatures with cluster size. The significant charge dependence of the latent heats found for some cluster sizes indicates that the electronic structure can change substantially when the cluster melts