**G. Ziegenhain and H. Urbassek**

Oliver and Pharr (1992); Hertz (1882); Chaudhri (2000) one can determine the contact hardness

Fischer-Cripps (2004) of materials.

For small length scales Gane and Bowden (1968) the material properties differ from the macroscopic expectations. Particularly the force-depth curve has characteristic dips (the so-called pop-ins) at the elastic-plastic transition Göken and Kempf (2001). This and the overestimated theoretical shear stress Frenkel (1926) suggest that atomistic effects play an important role for the macroscopic length scale material behavior; namely these atomistic effects are dislocations Phillips (2001); Hull and Bacon (1992). The indented surface itself is also of interest: pile-up effects and regeneration are only two important topics.

Therefore an understanding of the atomistic plasticity is of great importance for material modeling on larger length scales and for understanding the material properties (hardness, elasticity) at all. Apart from that this the current length scale of electronic devices has already reached the nm-scale and atomistic effects itself are becoming important for industrial production.

In our simulations we model the indenter as an external constraint using the potential proposed in Kelchner et al. (1998):

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G. Ziegenhain - 24.10.2007