Ab-initio electronic structure calculations have been used to evaluate the binding energy of atomic and molecular hydrogen to graphite lattice defects. Results show that graphite defects (Stone- Wales, vacancy) are preferred binding sites with respect to regular lattice sites. We find that molecular hydrogen is physisorbed between the graphite planes, but cannot diffuse across a graphitic plane.

Ab-initio electronic structure calculations have been used to evaluate the binding energy of atomic and molecular hydrogen to graphite lattice defects. Results show that graphite defects (Stone- Wales, vacancy) are preferred binding sites with respect to regular lattice sites. We find that molecular hydrogen can be physisorbed between the graphite planes, but cannot diffuse across a graphitic plane.

We studied the nucleation mechanism of carbon nanotubes based on the hypothesis that the starting nanotube seed can be nucleated by rolling a small fragment of a graphite sheet (graphene) under thermal fluctuations. The energy barriers for rolling a graphene along different crystallographic directions are calculated from a tight-binding model,. We then estimate the relative weight of the large-amplitude fluctuations corresponding to low-frequency vibrational modes of graphene sheets of increasing size. Direct molecular dynamics simulation of the high- temperature fluctuation of a pair of parallel graphenes demonstrates that a nanotube closed at one end can spontaneously form. We discuss the combined effects due to: (a) the decrease of the energy barriers against rolling with increasing nanotube radius, and (b) the increase of random fluctuations with increasing size of the graphene sheet. The superposition of such effects may lead to a preferential range of nanotube diameters which could nucleate more abundantly than others.

Tight Binding molecular dynamics simulations have been performed on single wall carbon nanotubes, in order to evaluate thermal stability and the effect of the most relevant defects (the single vacancy and a Stone-Wales -SW- defect). The nanotubes are stable up to the graphite instability temperature. Both the considered defects have a large formation energy (EF(vac)=6.10 eV, EF(SW)= 5.55 eV).

An atomic-scale model of liquid silicon has been cooled from high temperatures down in the temperature range between the amorphous and the crystalline melting temperatures by nanosecond scale molecular dynamics simulations with the Stillinger-Weber potential. Tetrahedrally coordinated sites have been identified, in the supercooled liquids, by using a few structural order parameters. The local structure and the stability of these crystalline-like regions (c-type sites and clusters) have been characterized. These have been regarded as candidates for crystalline embryos.

The crystalline disordered phase obtained by mechanical alloying of elemental 75 at. % Ni and 25 at. % Al powders has been investigated. The stability of this phase with respect to the thermal reordering process leading to the L12 structure has been analyzed by means of x-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni3Al model, reproduced via molecular dynamics using a many-body potential, have been used to interpret experimental data. The ordering transformation takes place in an extended range of temperature (from 320 to 600 °C) and occurs simultaneously with the release of internal strain. Numerical simulations performed under different conditions show that the activation energy of the Ni-vacancy migration mechanism responsible for the ordering process depends on the local state of strain, thus suggesting an explanation for the considerable lowering of this energy in samples obtained by ball milling.

The order-disorder transformation in CU3Au has been investigated using a combined Molecular Dynamics and Cluster Variation Method approach. Free energy minimization has been performed using the Natural Iteration technique. The calculated temperature dependence of enthalpy, lattice dynamics, short-range and long-range order parameters have been successfully compared with experimental data.

The behaviour of two different systems, NiZr2 and Cu3Au, submitted to a chemical disordering process under various thermodynamical conditions has been investigated by Molecular Dynamics simulations. The role played by volume expansion and elastic softening in the onset of the transition has been discussed in the framework of the percolation model (C.Massobrio and V.Pontikis, Phys.Rev.B 45 (1992) 2484), correlating the elastic instability with the percolation of the distorted regions which establishes in the lattice as a consequence of the external perturbations (introduction of defects, volume expansion, chemical disorder etc.).

We report a first observation, of the behavior of clusters of deuterium atoms confined in a lattice vacancy in a Pd crystal, either at equilibrium or under the influence of externally applied lattice strains. Molecular dynamics simulations have been performed on both Pd perfect crystals and Pd crystals with a single vacancy, showing the energetic behavior and the dynamical values of the interatomic distance among deuterium atoms belonging to clusters of different sizes confined within the vacancy. At low temperatures, the smallest interatomic D-D distances are of the order of the D2 molecular bond length and are obtained during the transitory regimes induced by hydrostatic strains applied to the lattice.

We calculate the shear elastic constants of the alloy NiZr2 by molecular dynamics simulations in the crystalline and amorphous phases as well as upon introduction of antisite defects in the crystal at T=300K. For S (long range order parameter) equal to 0.5, the system is amorphous and C' is larger than the same quantity relative to the crystal whereas C44 and C66 are smaller.

We have performed a systematic study to evaluate the structural response of a {110} fcc surface to an externally imposed elastic deformation, by Molecular Dynamics at a fixed temperature T=0.48T and T=0.68T (T melting point). For high contractions along the dense axis [110], surface concentration of defects (SCD) and diffusion coefficient increase with respect to the equilibrium values. The resulting surface disorder is similar to the one found, at equilibrium, at higher temperatures.