  We discuss how simulations of mechanical properties of materials require
descriptions at many different length scales --- from the nanoscale where an
atomic description is appropriate, through a mesoscale where dislocation based
descriptions may be useful, to macroscopic length scales. In some materials,
such as nanocrystalline metals, the range of length scales is compressed and a
polycrystalline material may be simulated at the atomic scale. The first part
of the paper describes such simulations of nanocrystalline copper. We observe
how the grain boundaries contribute actively to the deformation. At grain sizes
below 10-15 nm deformation in the grain boundaries dominate over the
traditional dislocation-based deformation mechanisms. This results in a
reversal of the normal grain size dependence of the yield stress: we observe
that the material becomes softer when the grain size is reduced. The second
part of the paper gives an overview over simulation techniques appropriate for
problems too large to be treated in atomic-scale simulations, and how they can
be combined to describe the interplay between phenomena at different length
scales through multiscale modelling.
