MOLECULAR-DYNAMICS SIMULATIONS OF THE BASAL PLANES OF NI AND CU USINGFINNIS-SINCLAIR POTENTIALS

Using MD simulations, we have calculated the surface phonon spectral d ensity functions for the (100), (110), and (111) surfaces of Ni and Cu using Finnis-Sinclair (FS) potentials. These simulated phonon spectra l densities are compared to the experimental inelastic helium atom sca ttering and HREELS data which are available for the three basal faces of Ni and Cu. We find that the overall shape of the calculated surface and second layer phonon spectral densities qualitatively reproduce th ose obtained from force constant fits, i.e. lattice dynamical modellin g, of the experimental phonon dispersion data. Good agreement is also found between the calculated and experimental geometric separations be tween the surface and second layer for a given interface. However, on all surfaces the phonon frequencies calculated with Finnis-Sinclair po tentials are lower than the experimentally measured values. The best a greement between our calculated results and the experimentally measure d phonon frequencies was for the (100) and (I 10) surfaces, while the poorest agreement was on the (111) surfaces. From this we conclude tha t Finnis-Sinclair model potentials derived from bulk properties system atically underestimate the many body binding potential at the surface. This underestimation of the many body binding term is also manifested in the magnitude of the calculated surface stress. Our results indica te that the Finnis-Sinclair model potentials are quite adequate for a good qualitative and semi-quantitative description of the bonding chan ges at the surfaces of Ni and Cu.