  Molecular-dynamics simulation can give atomistic information on the processes
occurring in nanoindentation experiments. In particular, the nucleation of
dislocation loops, their growth, interaction and motion can be studied. We
investigate how realistic the interatomic potentials underlying the simulations
have to be in order to describe these complex processes. Specifically we
investigate nanoindentation into a Cu single crystal. We compare simulations
based on a realistic many-body interaction potential of the
embedded-atom-method type with two simple pair potentials, a Lennard-Jones and
a Morse potential. We find that qualitatively many aspects of nanoindentation
are fairly well reproduced by the simple pair potentials: elastic regime,
critical stress and indentation depth for yielding, dependence on the crystal
orientation, and even the level of the hardness. The quantitative deficits of
the pair potential predictions can be traced back (i) to the fact that the pair
potentials are unable in principle to model the elastic anisotropy of cubic
crystals; (ii) as the major drawback of pair potentials we identify the gross
underestimation of the stable stacking fault energy. As a consequence these
potentials predict the formation of too large dislocation loops, the too rapid
expansion of partials, too little cross slip and in consequence a severe
overestimation of work hardening.
