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In the near future, Exascale systems will need to bridge three technology gaps to achieve high performance while remaining under tight power constraints: energy efficiency and thermal control; extreme computation efficiency via HW acceleration and new arithmetic; methods and tools for seamless integration of reconfigurable accelerators in heterogeneous HPC multi-node platforms. TEXTAROSSA addresses these gaps through a co-design approach to heterogeneous HPC solutions, supported by the integration and extension of HW and SW IPs, programming models, and tools derived from European research. This work is supported by the TEXTAROSSA project (G.A. n. 956831), as part of the EuroHPC initiative. Peer Reviewed Article signat per 56 autors/es: Giovanni Agosta (a) , Marco Aldinucci (f) , Carlos Alvarez (h) , Roberto Ammendola (n) , Yasir Arfat (f) , Olivier Beaumont (g) , Massimo Bernaschi (c) , Andrea Biagioni (j) , Tommaso Boccali (l) , Berenger Bramas (g) , Carlo Brandolese (a) , Barbara Cantalupo (f) , Mauro Carrozzo (c) , Daniele Cattaneo (a) , Alessandro Celestini (c) , Massimo Celino (b) , Iacopo Colonnelli (f) , Paolo Cretaro (j) , Pasqua D’Ambra (d) , Marco Danelutto (e) , Roberto Esposito (f) , Lionel Eyraud-Dubois (g) , Antonio Filgueras (h) , William Fornaciari (a) , Ottorino Frezza (j) , Andrea Galimberti (a) , Francesco Giacomini (k) , Brice Goglin (g) , Daniele Gregori (o) , Abdou Guermouche (g) , Francesco Iannone (b) , Michal Kulczewski (i) , Francesca Lo Cicero (j) , Alessandro Lonardo (j) , Alberto R. Martinelli (f) , Michele Martinelli (j) , Xavier Martorell (h) , Giuseppe Massari (a) , Simone Montangero (m), Gianluca Mittone (f) , Raymond Namyst (g) , Ariel Oleksiak (i) , Paolo Palazzari (b) , Pier Stanislao Paolucci (j) , Federico Reghenzani (a)∗ , Cristian Rossi (j) , Sergio Saponara (e) , Francesco Simula (j) , Federico Terraneo (a) , Samuel Thibault (g) , Massimo Torquati (e) , Matteo Turisini (j) , Piero Vicini (j) , Miquel Vidal (h) , Davide Zoni (a) , Giuseppe Zummo (b) // (a) Politecnico di Milano, Dipartimento di Elettronica, Informazione e Bioingegneria, Italy; (b) ENEA, Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile, Italy; (c) Istituto per le Applicazioni del Calcolo (IAC), Consiglio Nazionale delle Ricerche, Rome, Italy; (d) Istituto per le Applicazioni del Calcolo (IAC), Consiglio Nazionale delle Ricerche, Naples, Italy; (e) University of Pisa, Italy; (f) University of Torino, Italy; (g) Institut national de recherche en informatique et en automatique (Inria), France; (h) Barcelona Supercomputing Center, Spain; (i) Poznan Supercomputing and Networking Center, Poland; (j) Istituto Nazionale di Fisica Nucleare, Sezione di Roma, Italy; (k) Istituto Nazionale di Fisica Nucleare, CNAF, Italy; (l) Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Italy; (m) University of Padova and Istituto Nazionale di Fisica Nucleare Sezione di Padova, Italy; (n) Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tor Vergata, Italy; (o) E4 Computer Engineering Spa, Italy

To achieve high performance and high energy efficiency on near-future exascale computing systems, three key technology gaps needs to be bridged. These gaps include: energy efficiency and thermal control; extreme computation efficiency via HW acceleration and new arithmetics; methods and tools for seamless integration of reconfigurable accelerators in heterogeneous HPC multi-node platforms. TEXTAROSSA aims at tackling this gap through a co-design approach to heterogeneous HPC solutions, supported by the integration and extension of HW and SW IPs, programming models and tools derived from European research. This work is supported by the TEXTAROSSA project G.A. n.956831, as part of the EuroHPC initiative. Peer Reviewed Article signat per 51 autors/es: Giovanni Agosta, Daniele Cattaneo, William Fornaciari, Andrea Galimberti, Giuseppe Massari, Federico Reghenzani, Federico Terraneo, Davide Zoni, Carlo Brandolese (DEIB – Politecnico di Milano, Italy, name.surname@polimi.it) | Massimo Celino, Francesco Iannone, Paolo Palazzari, Giuseppe Zummo (ENEA, Italy, name.surname@enea.it) | Massimo Bernaschi, Pasqua D’Ambra (Istituto per le Applicazioni del Calcolo (IAC) - CNR, Italy, name.surname@cnr.it) | Sergio Saponara, Marco Danelutto, Massimo Torquati (University of Pisa, Italy, name.surname@unipi.it) | Marco Aldinucci, Yasir Arfat, Barbara Cantalupo, Iacopo Colonnelli, Roberto Esposito, Alberto R. Martinelli, Gianluca Mittone (University of Torino, Italy, name.surname@unito.it) | Olivier Beaumont, Berenger Bramas, Lionel Eyraud-Dubois, Brice Goglin, Abdou Guermouche, Raymond Namyst, Samuel Thibault (Inria - France, name.surname@inria.fr) | Antonio Filgueras, Miquel Vidal, Carlos Alvarez, Xavier Martorell (BSC - Spain, name.surname@bsc.es) | Ariel Oleksiak, Michal Kulczewski (PSNC, Poland, ariel@man.poznan.pl, kulka@man.poznan.pl) | Alessandro Lonardo, Piero Vicini, Francesca Lo Cicero, Francesco Simula, Andrea Biagioni, Paolo Cretaro, Ottorino Frezza, Pier Stanislao Paolucci, Matteo Turisini (INFN Sezione di Roma - Italy, name.surname@roma1.infn.it) | Francesco Giacomini (INFN CNAF - Italy, name.surname@cnaf.infn.it) | Tommaso Boccali (INFN Sezione di Pisa - Italy, name.surname@pi.infn.it) | Simone Montangero (University of Padova and INFN Sezione di Padova - Italy, name.surname@pd.infn.it) | Roberto Ammendola (INFN Sezione di Roma Tor Vergata - Italy, name.surname@roma2.infn.it)

The thermodynamic stability of hydroxylated graphane, that is, fully sp3 graphene derivatives coordinated with -H and -OH groups, has been recently demonstrated by ab initio calculations. Within the density functional theory approach, we investigate the electronic property modifications of graphane by progressive hydroxylation, that is, by progressively substituting -H with -OH groups. When 50% of graphane is hydroxylated, the energy bandgap reaches its largest value of 6.68 eV. The electronic affinity of 0.8 eV for graphane can widely change in the 0.28-1.60 eV range depending on the geometric configuration. Hydroxylated graphane has two interfaces with vacuum, hence its electron affinity can be different on each interface with the formation of an intrinsic dipole perpendicular to the monolayer. We envisage the possibility of using hydroxylated graphane allotropes with tunable electronic affinity to serve as interfacial layers in 2D material-based heterojunctions.

International audience; The principles of Findability, Accessibility, Interoperability, and Reusability (FAIR) havebeen put forward to guide optimal sharing of data. The potential for industrial and social innovation is vast. Domain-specific metadata standards are crucial in this context, but are widely missing in the energy sector. This report provides a collaborative response from the low carbon energy research community for addressing the necessity of advancing FAIR metadata standards. We review and test existing metadata practices in the domain based on a series of community workshops. We reflect the perspectives of energy data stakeholders. The outcome is reported in terms of challenges and elicits recommendations for advancing FAIR metadata standards in the energy domain across a broad spectrum of stakeholders.

The surface of nanowires is a source of interest mainly for electrical prospects. Thus, different surface chemical treatments were carried out to develop recipes to control the surface effect. In this work, we succeed in shifting and tuning the semiconductivity of a Si nanowire-based device from n- to p-type. This was accomplished by generating a hole transport layer at the surface by using an electrochemical reaction-based nonequilibrium position to enhance the impact of the surface charge transfer. This was completed by applying different annealing pulses at low temperature (below 400 °C) to reserve the hydrogen bonds at the surface. After each annealing pulse, the surface was characterized by XPS, Kelvin probe measurements, and conductivity measured by FET based on a single Si NW. The mechanism and conclusion were supported experimentally and theoretically. To this end, this strategy has been demonstrated as an essential tool which could pave a new road for regulating semiconductivity and for other low-dimensional nanomaterials., M.B. is thankful for the MAOF Grant from the Council for Higher Education in Israel for new faculty members. A.S. is appreciative of the institutional scholarships for Ph.D. students that they received from Ben-Gurion University of the Negev. W.W. is thankful for the fundamental research funds from the Central Universities (JC2002). L.Z. is thankful for the support of a postdoctoral fellowship from the Jacob Blaustein Center for Scientific Cooperation, the National Natural Science Foundation of China (61904134), and the National Natural Science Foundation of Shaanxi Province (2019JQ-291). The computing resources and the related technical support used for this work have been provided by CRESCO/ENEAGRID High Performance Computing infrastructure and its staff, along with additional support from S. Migliori and S. Giusepponi. E.N.E.A. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 824158 (EoCoE-II). J.A. acknowledges funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO coordinated project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya.

Laser-driven proton acceleration, as produced during the interaction of a high-intensity (I > 1 × 1018 W/cm2), short pulse (

The solubilization mechanism of lipid membranes in the presence of Triton X-100 (TX-100) is investigated at molecular resolution using molecular dynamics (MD) simulations. Thanks to the large time and length scales accessible by the hybrid particle-field formulation of the models employed here, the complex process of membrane solubilization has been studied, with the goal of verifying the three stage model reported in the literature. DPPC lipid bilayers and vesicles have been studied at different concentrations of the TX-100 detergent employing coarse grained (CG) models. Systems up to ∼600.000 beads, corresponding to more than 2 millions heavy atoms, have been simulated. Moreover, in order to clarify several experimental pieces of evidence, both slow and fast detergent partition scenarios have been investigated. Flat and curved (vesicles) lipid bilayer surfaces, interacting with TX-100, have been considered to study the curvature effects on the detergent partition rate in the membrane. Shape and conformational changes of mixed DPPC/TX-100 vesicles, as a function of TX-100 content, have also been studied. In particular, high curvature surfaces, corresponding to a higher local TX-100 content, promote a membrane rupture. In flat lipid surfaces, on the time scale simulated the detergent partition is almost absent, following a different pathway of the solubilization membrane mechanism. © 2017 the Owner Societies.

Doping of MgH2 with transition metals and their oxides is well-known procedure to improve its hydrogen (de) sorption properties, namely to lower the temperature of desorption and to achieve the kinetics speedup. In order to assess the influence Ti and TiO2 doping has on H mobility and to characterize structurally and electronically observed differences, MgH2-Mg interface doped with both Ti and TiO2 have been studied using ab-initio interface molecular dynamics and bulk calculations. Results suggest different mechanisms of MgH2 structure destabilization. The presence of dopants significantly stabilize MgH2-Mg interface, which is confirmed by work of adhesion computation. Calculated formation energies show that interface system with doped TiO2 is more stable. In terms of H mobility, molecular dynamics simulations confirm that Ti doping is more effective than TiO2 in lowering the desorption temperature. The mobility of hydrogen atoms close to dopant is much higher in the case of Ti than in the case of TiO2. Electronic structure characterization reveals that oxygen atoms with high electron affinity forms more pronounced ionic bonding with Ti and the other neighbor Mg atoms. This in turn cause a shorter Ti-H bonds in first coordination than in the case of Ti doping and further reduction of H atoms mobility. This is in accordance with molecular dynamics predictions. (C) 2016 Elsevier B.V. All rights reserved.

In order to optimize the optoelectronic properties of novel solar cell architectures, such as the amorphous-crystalline interface in silicon heterojunction devices, we calculate and analyze the local microscopic structure at this interface and in bulk a-Si:H, in particular with respect to the impact of material inhomogeneities. The microscopic information is used to extract macroscopic material properties, and to identify localized defect states, which govern the recombination properties encoded in quantities such as capture cross sections used in the Shockley-Read-Hall theory. To this end, atomic configurations for a-Si:H and a-Si:H/c-Si interfaces are generated using molecular dynamics. Density functional theory calculations are then applied to these configurations in order to obtain the electronic wave functions. These are analyzed and characterized with respect to their localization and their contribution to the (local) density of states. GW calculations are performed for the a-Si:H configuration in order to obtain a quasi-particle corrected absorption spectrum. The results suggest that the quasi-particle corrections can be approximated through a scissors shift of the Kohn-Sham energies., 14 pages, 10 figures, to appear in Springer Lecture Notes in Computer Science, Proceedings of the JHPCS16 conference

The corrosion of structural materials used in fast nuclear reactor design is a current major problem. It is due to the use of liquid metal as a coolant candidate in the heat transfer system. The liquid metal as lead-bismuth eutectic was found to make high corrosion to structural material as steel. One of the solutions of this problem is to inject some inhibitor into liquid metal. In this current work we simulate the effect of nitrogen injection as inhibitor candidate. The simulation will predict the proper concentration of injected nitrogen and also observe the microscopic structure of the material before and after injection to know the ability of nitrogen as an inhibitor. The simulation follows the molecular dynamics method and for preliminary study we use iron material rather than steel. We also use lennard-jones potential for simplification of the study. It is from our simulation we see nitrogen shows better corrosion mitigation compare with oxygen as in our previous study. The effective inhibition can be achieved by injecting at least 0.056wt.% nitrogen. This amount seems to be able to reduce the corrosion level of iron till about 99.5% for high corrosion at temperature 750 °C. © Published under licence by IOP Publishing Ltd.

Arg, Lys and Asp amino acids are known to play a critical role in the adhesion of the RKLPDA engineered peptide on the (101) surface of the titania anatase phase. To understand their contribution to peptide adhesion, we have considered the relevant charge states due to protonation (Arg and Lys) or deprotonation (Asp) occurring in neutral water solution, and studied their adsorption on the (101) anatase TiO2 surface by ab initio total energy calculations based on density functional theory. The adsorption configurations on the hydrated surface are compared to those on the dry surface considering also the presence of the hydration shell around amino acid side-chains. This study explains how water molecules mediate the adsorption of charged amino acids showing that protonated amino acids are chemically adsorbed much more strongly than de-protonated Asp. Moreover it is shown that the polar screening of the hydration shell reduces the adsorption energy of the protonated amino acids to a small extent, thus evidencing that both Arg and Lys strongly adhere on the (101) anatase TiO2 surface in neutral water solution and that they play a major role in the adhesion of the RKLPDA peptide.

A multiscale scheme is proposed and validated for Triton X-100 (TX-100), which is a detergent widely employed in biology. The hybrid particle field formulation of the model allows simulations of large-scale systems. The coarse-grained (CG) model, accurately validated in a wide range of concentrations, shows a critical micelle concentration, shape transition in isotropic micellar phase, and appearance of hexagonal ordered phase in the experimental ranges reported in the literature. The fine resolution of the proposed CG model allows one to obtain, by a suitable reverse mapping procedure, atomistic models of micellar assemblies and of the hexagonal phase. In particular, atomistic models of the micelles give structures in good agreement with experimental pair distance distribution functions and hydrodynamic measurements. The picture emerging by detailed analysis of simulated systems is quite complex. Polydisperse mixtures of spherical-, oblate-, and prolate-shaped aggregates have been found. The shape and the micelle behavior are mainly dictated by the aggregation number (Nagg). Micelles with low Nagg values (∼40) are spherical, while those with high Nagg values (∼140 or larger) are characterized by prolate ellipsoidal shapes. For intermediate Nagg values (∼70), fluxional micelles alternating between oblate and prolate shapes are found. The proposed model opens the way to investigations of several mechanisms involving TX-100 assembly in protein and membrane biophysics. © 2015 American Chemical Society.

The low-density (LDA) to high-density (HDA) transformation in amorphous Ge at high pressure is studied by first-principles molecular dynamics simulations in the framework of density functional theory. Previous experiments are accurately reproduced, including the presence of a well-defined LDA-HDA transition above 8 GPa. The LDA-HDA density increase is found to be about 14%. Pair and bond-angle distributions are obtained in the 0-16 GPa pressure range and allowed us a detailed analysis of the transition. The local fourfold coordination is transformed in an average HDA sixfold coordination associated with different local geometries as confirmed by coordination number analysis and shape of the bond-angle distributions. © 2016 IOP Publishing Ltd.

The characterization of self-diffusion in MgO grain boundaries is a materials science problem of general interest, being relevant to the stability and reactivity of MgO layers in artificial nanostructures as well as to the understanding of mass transport and morphological evolution in polycrystalline metal oxides which are employed in many technological applications. In addition, atomic transport in MgO is a key factor to describe the rheology of the Earth’s lower mantle. In this work, we tackle the problem using a classical molecular dynamics model and finite-temperature simulations. To this purpose, we first design a stable grain boundary structure, which is meant to be representative of general internal interfaces in nanocrystalline MgO. The Mg and O self-diffusion coefficients along this grain boundary are then determined as a function of temperature by calculating the mean-square ionic displacement in the boundary region. Two different diffusion regimes at low and high temperature are identified, allowing to obtain the relevant activation enthalpies for migration from the temperature dependance of the diffusion coefficients. Our results prove that Mg diffusion along MgO grain boundaries is sufficiently fast to explain the recently reported development of MgO hollow structures during repeated hydrogen sorption cycles in Mg/MgO nanoparticles.

Magnesium hydride is a very promising material for solid-state hydrogen storage. However, some drawbacks have to be overcome to use it in real applications. The use of catalysts is a viable solution to lower the desorption temperature and increase the overall kinetics. An accurate model has been developed to study the mechanism of action of the catalyst and how it interacts with the interface MgH2-Mg, through which H atoms diffuse. The accurate evaluation of the work of adhesion and defect energy formation, versus the distance from the interface are linked to the atomic-scale structural distortion induced by the catalyst. Moreover, molecular dynamics simulations at several temperature provide a clear description of the desorption mechanism and an estimate of the desorption temperature. © 2015 Hydrogen Energy Publications, LLC.

Corrosion of structural materials in high temperature molten lead-bismuth eutectic is a major problem for design of PbBi cooled reactor. One technique to inhibit corrosion process is to inject oxygen into coolant. In this paper we study and focus on a way of inhibiting the corrosion of iron using molecular dynamics method. For the simulation results we concluded that effective corrosion inhibition of iron may be achieved by injection 0.0532 wt% to 0.1156 wt% oxygen into liquid lead-bismuth. At this oxygen concentration the structure of iron material will be maintained at about 70% in bcc crystal structure during interaction with liquid metal. © 2014 AIP Publishing LLC.

The structure of glassy GeS${}_{2}$ is studied in the framework of density functional theory, by using a fully self-consistent first-principles molecular dynamics (FPMD) scheme. A comparative analysis is performed with previous molecular dynamics data obtained within the Harris functional (HFMD) total energy approach. The calculated total neutron structure factor exhibits an unprecedented agreement with the experimental counterpart. In particular, the height of the first sharp diffraction peak (FSDP) improves considerably upon the HFMD results. Both the Ge and the S subnetworks are affected by a consistent number of miscoordinations, coexisting with the main tetrahedral structural motif. Glassy GeS${}_{2}$ features a short-range order quite similar to the one found in glassy GeSe${}_{2}$, a notable exception being the larger number of edge-sharing connections. An electronic structure localization analysis, based on the Wannier functions formalism, provides evidence of a more enhanced ionic character in glassy GeS${}_{2}$ when compared to glassy GeSe${}_{2}$.

The ideal tensile strength in the [0 0 1] direction of bcc crystal tungsten and its alloys (W–Re, W–Ta and W–V) has been investigated by using first-principles total energy method based on the density functional theory. Crystalline tungsten containing a single substitutional defect (concentration of defects about 2%) has been characterized in terms of structural and mechanical properties. The maximum tensile stress required to reach elastic instability under increasing load has been further computed.

Hydrogen desorption from hydride matrix is still an open field of research. Extensive, density functional theory based, ab-initio molecular dynamics simulations of MgH2-Mg interface catches the atomic level structural mechanism leading to hydrogen desorption. The numerical model estimates the desorption temperature for an interface with Fe catalyst in excellent agreement with experimental results. Formation energies and the analysis of the structural data reveal the role played by the catalysts to lower the desorption temperature. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The upper threshold of hydrogen adsorption in Li-doped and hydrogenated carbon nanotube densely packed arrays is calculated to check the ability of such systems to fulfill the target indicated by the United States Department of Energy (DOE). To this aim, model potential parameters have been obtained by Density Functional Theory and have been used to calculate the adsorption isotherms in honeycomb arrays containing up to seven tubes by means of Grand-Canonical Monte Carlo simulations. A hybrid model has been developed involving both atomistic potentials for short-range interactions and integrated potentials for hydrogen interacting with distant tubes. In the pressure range explored, it is shown that the hydrogen adsorption performances of Li-doped carbon nanotubes arranged in close packed honeycomb arrays, while being enhanced with respect to pristine carbon nanotubes, are still well below the DOE targets. © Springer Science+Business Media 2013.

The long-term objective of the European Fusion Development Agreement (EFDA) fusion materials programme is to develop structural and armor materials in combination with the necessary production and fabrication technologies for reactor concepts beyond the International Thermonuclear Experimental Reactor. The programmatic roadmap is structured into four engineering research lines which comprise fabrication process development, structural material development, armor material optimization, and irradiation performance testing, which are complemented by a fundamental research programme on "Materials Science and Modeling." This paper presents the current research status of the EFDA experimental and testing investigations, and gives a detailed overview of the latest results on materials research, fabrication, joining, high heat flux testing, plasticity studies, modeling, and validation experiments. (C) 2013 Elsevier B.V. All rights reserved.

Hydrogen desorption from hydride matrix is still an open field of research. By means of accurate first-principle molecular dynamics (MD) simulations an Mg–MgH2 interface is selected, studied and characterized. Electronic structure calculations are used to determine the equilibrium properties and the behavior of the surfaces in terms of structural deformations and total energy considerations. Furthermore, extensive ab-initio molecular dynamics simulations are performed at several temperatures to characterize the desorption process at the interface. The numerical model successfully reproduces the experimental desorption temperature for the hydride.

International audience; There is not a wide consensus on the role played by the icosahedral short range order on the stability of undercooled simple metals. The scenario is even less clear for undercooled metals under external pressure. Classical molecular dynamics simulations are performed to explain experimental results recently obtained on liquid and undercooled liquid copper under pressure. The atomic configurations are characterized by a common neighbor analysis to reveal the icosahedral short range order and its relation with external pressure. External pressure increases the probability to find atomic bonds with icosahedral symmetry both in the liquid and in the undercooled copper.

We performed a comparative study of the adsorption of an in vitro selected peptide on two different carbon surfaces: a flat graphene and a curved (0,15) nanotube. The sequence was selected from recent experiments, as the one giving the highest carbon affinity for carbon nanotubes. Rigid docking of the molecule on the two surfaces by a genetic algorithm was followed by molecular dynamics simulations with empirical force fields (OPLS-AA) in water at finite temperature. The total free energies of folding and adhesion and the quality of surface binding were determined, based on a combination of solvation energy, formation of hydrogen bonds, and amount of the apolar (hydrophobic) contact surface between peptide and carbon surface. For both cases, we find a strong adhesion energy and large nonpolar contact surface. Isoleucines and tryptophans are the most strongly bound residues to the two carbon surfaces, the latter one largely dominating. In the case of the carbon nanotube, the peptide shows several competing stable structures, corresponding to different possible molecular foldings, and a propensity to enhance the intramolecular stability by forming new hydrogen bonds. In both systems, different arrangements of the histidine and tryptophan residues enable a better adaptation to the carbon surfaces. These findings suggest that the experimentally observed surface specificity of the peptide on nanotubes may depend on its capability to support multiple strongly bound configurations.

We describe two examples of application focusing on first-principles molecular dynamics as an effective tool to unravel the atomic-scale structure of condensed-matter systems. The first application is on disordered network-forming materials and the second is on silicon-doped fullerenes. We show that an accurate modelling of interatomic forces based on density functional theory, when combined with an account of the temperature evolution, is an unavoidable prerequisite for analyzing and interpreting experimental results on a quantitative basis. In the case of disordered systems, we describe the basic structural features of amorphous GeSe4 and highlight the predominant chemical order in this system. The effect of adding or removing an electron charge on the stability of Si-doped fullerenes is exemplified by considering the finite temperature evolution of heterofullerenes. © (2008) Trans Tech Publications, Switzerland.

In this letter, the strain field below uncapped Ge islands of a different shape on a Si(001) substrate is estimated by molecular dynamics simulations at a realistic scale. Comparison to the Fourier transform maps of transmission electron micrographs, recently reported in literature, shows a very good agreement. We point out that the complex deformation in silicon, just below the edges of the Ge islands, is far from being uniaxial. The stress distribution generated by such a strain determines the range of interdot repulsion. (C) 2002 American Institute of Physics.

Tight binding molecular dynamics is used to predict the structure and the total energy of the most relevant intrinsic point defects in C54 and C49 TiSi2. The comparison between the relative formation energies of point defects of the two phases in contact with a Si substrate suggests that the metastable C49 form has a higher concentration of point defects. In particular, we point out that Si vacancies and (010) stacking faults should be quite common in the C49 structure. This issue could be important in explaining the kinetic advantage of the latter phase in film growth by solid state reaction.

The total energy of equilibrated pyramids layer by layer films and flat prepyramids in the epitaxial growth of Ge on Si (001) was investigated. Molecular dynamics (MD) simulations were applied to equilibrate the samples at atomic scale by the efficient code. It was shown that pyramids are more stable than Frank-van der Merwe (FM) films due to the partial strain relief.

Molecular dynamics simulations by a tight binding potential provide new interesting information on the ground state properties of the TiSi2 phases. In particular, we have compared some structural, elastic, thermodynamic and electronic properties of the C49, the C54 and the amorphous phases. It turns out that the C49 structure is much softer than the C54, also displaying a melting temperature some 300-400 K below the one for the C54, in agreement to very recent experimental results. The amorphous phase is energetically and structurally more akin to the C49 than to the C54 phase. On the basis of these results we suggest the higher formation kinetics of the former to be related to an intrinsic advantage in the growth stage. (C) 2000 Elsevier Science B.V. All rights reserved.

Epitaxially stabilized iron monosilicide films having the CsCl structure (c-FeSi) have been investigated by conversion electron Mossbauer spectroscopy and transmission electron microscopy. The (57)Fe Mossbauer parameters (isomer shift delta, linewidth Gamma, and quadrupole splitting Delta) are reported and discussed in terms of the local surrounding of the Fe nucleus. High statistical accuracy and resolution allowed a detailed investigation of the effects of strain and of the structural phase transformation from the epitaxially stabilized to the bulk stable phase, The phase transformation was found to proceed in a rather surprising layer by layer mechanism with smooth interfaces between the epitaxially stabilized, the bulk stable, and a third phase. Results from a molecular-dynamics simulation at constant pressure and temperature of the structural phase transition are presented and compared with the experimental findings. The isomer shift and the electric-field gradient at the Fe nucleus in the strained c-FeSi and in the third phase have been calculated using the ab initio full potential linear muffin-tin orbital method. The Mossbauer parameters of some relevant point defects in c-FeSi have likewise been calculated within this framework. [S0163-1829(99)01302-8].

In this letter, we show that the polymorphic C49 form of TiSi2 has much smaller elastic constants than those of the C54 bulk-stable structure and that its melting temperature is about 300 degrees C lower. These issues supply intriguing hints in explaining the kinetic advantage of the C49 over the C54 phase and in understanding the role of the elastic energy in the phase transformation from C49 to C54. (c) 1999 American Institute of Physics. [S0003-6951(99)03023-5].

Molecular Dynamics simulations in the Tight-Binding approach allow the study of the ionic and electronic structures of semiconductors. The Tight-Binding codes are characterized by inhomogeneous data distribution and require the repeated diagonalization of a large sparse matrix to compute the whole body of its eigenvalues and eigenvectors. We describe the porting of this code on a parallel computer: we show the parallelization strategy for both the Molecular Dynamics part of the code and for the diagonalization needed at each time step. The parallelization has been carried out within the High Performance Fortran (HPF) environment, and tested on IBM SP architectures. The integration of optimized parallel mathematical routines is also described.

The High Performance Fortran (HPF) environment has been used to efficiently parallelize a Molecular Dynamics (MD) code. The MD code is a Tight-Binding code, properly specialized for semiconductor materials and characterized by inhomogeneous data distribution. Furthermore, the electronic properties are taken into account during the atomic dynamics: for this reason a large sparse matrix is built and diagonalized to compute the whole body of its eigenvalues and eigenvectors at each time step. For the diagonalization task, a parallel mathematical routine implemented. The strategy of parallelization, the integration of the mathematical routine in the HPF code are described and discussed. All the benchmark are performed on IBM SP architecture.

We show that tight-binding molecular dynamics provides a detailed description of the relations between structural deformations and changes in the electronic features during a Jahn-Teller process. In this case the metal-semiconductor displacitive phase transition occurring in epitaxial FeSi2 with film thickness can be correctly reproduced and interpreted by variable cell molecular dynamics for the bulk configuration. We show that it actually corresponds to a pattern of local Jahn-Teller distortions occurring at selected sites in different times, so that the configurational evolution cannot be described by a global coordinate.