The parameters of many-body potentials for fcc and hcp transition metals, based on the second-moment approximation of a tight-binding Hamiltonian, have been systematically evaluated. The potential scheme, cast in analytical form, allows us to reproduce correctly the thermal behavior of transition metals making use of a small set of adjustable parameters. The large cutoff, which extends the range of the interactions up to the fifth-neighbor distance, ensures good quantitative agreement with the experimental data up to temperatures close to the melting point. The ability of the potentials to describe real systems has been checked by calculating point-defect properties, lattice dynamics, and finite-temperature behavior, and by comparing the results with other potential schemes. Application of this scheme to bcc transition metals has proved unsuccessful. Examples of derivation of many-body potentials for a few transition-metal alloys with cubic structure are also reported.

A semi-empirical model to simulate thermodynamic properties of f.c.c. transition metals is proposed. The attractive energy is derived within a second-moment approximation of the tight-binding scheme. The repulsive pairwise interaction is assumed to be of a Born-Mayer type and the parameters of the potential have been adjusted on the bulk modulus, the elastic constants and the cohesive energy. Bulk vibrational properties and defect energies for vacancies, interstitials and surfaces are estimated. Moreover, molecular dynamics simulations with this potential model to evaluate the high-temperature behaviour are presented. In particular, we have calculated the root-mean-square displacement and the thermal expansion as a function of temperature for Cu and Ni. These results allow us to test the anharmonicity of the potential. The agreement with experiment is quite good for low and intermediate temperatures. At high temperatures (T0·65T_m where T _m is the melting temperature) the mean-square displacement increases too rapidly. Some possible origins of this discrepancy are discussed.

We investigate the consequence of failures, occurring on the electrical grid, on a telecommunication network. We have focused on the Italian electrical transmission network and the backbone of the internet network for research (GARR). Electrical network has been simulated using the DC power flow method; data traffic on GARR by a model of the TCP/IP basic features. The status of GARR nodes has been related to the power level of the (geographically) neighbouring electrical nodes (if the power level of a node is lower than a threshold, all communication nodes depending on it are switched off). The electrical network has been perturbed by lines removal: the consequent re-dispatching reduces the power level in all nodes. This reduces the number of active GARR nodes and, thus, its Quality of Service (QoS). Averaging over many configurations of perturbed electrical network, we have correlated the degradation of the electrical network with that of the communication network. Results point to a sizeable amplification of the effects of faults on the electrical network on the communication network, also in the case of a moderate coupling between the two networks.

The thermodynamical and structural behavior of a (110) face of a fcc (12-6) Lennard-Jones solid has been investigated by molecular-dynamics simulation on the solid-gas coexistence line. The temperature dependence of the relevant structural and mass-transport properties shows the following. (a) Despite the high degree of disorder which gradually appears on surface layers when the temperature is increased, the surface retains its solidlike character up to temperatures (T≊0.64ɛ/$k_B$) very close to the triple point ($T_t$=0.68ɛ/$k_B$). This conclusion does not confirm the findings of previous theoretical work predicting the formation of a liquid surface layer well below the bulk melting point. (b) The large concentration of vacancy-adatom pairs, produced at the surface in the high-temperature range, accounts for the high values of the surface diffusivity. (c) The Arrhenius plot of defect concentration indicates a progressive decrease of their formation energy for temperatures ranging from $T_r$=0.8$T_m$ to the melting point. Consistently, the order parameter decreases slowly with increasing temperature up to $T_r$ but from T=$T_r$ to the melting point it decreases much more rapidly than predicted by the extrapolation of the low-temperature data. These results are qualitatively consistent with the onset of a surface-roughening transition, in agreement with recent experimental results obtained from helium scattering on (110) copper surfaces.