  The increasing sensitivity of current experiments, which nowadays routinely
measure the thermal SZ effect within galaxy clusters, provide the hope that
peculiar velocities of individual clusters of galaxies will be measured rather
soon using the kinematic SZ effect. Also next generation of X-ray telescopes
with microcalorimeters, promise first detections of the motion of the intra
cluster medium (ICM) within clusters. We used a large set of cosmological,
hydrodynamical simulations, which cover very large cosmological volume, hosting
a large number of rich clusters of galaxies, as well as moderate volumes where
the internal structures of individual galaxy clusters can be resolved with very
high resolution to investigate, how the presence of baryons and their
associated physical processes like cooling and star-formation are affecting the
systematic difference between mass averaged velocities of dark matter and the
ICM inside a cluster. We, for the first time, quantify the peculiar motion of
galaxy clusters as function of the large scale environment. We also demonstrate
that especially in very massive systems, the relative velocity of the ICM
compared to the cluster peculiar velocity add significant scatter onto the
inferred peculiar velocity, especially when measurements are limited to the
central regions of the cluster. Depending on the aperture used, this scatter
varies between 50% and 20%, when going from the core (e.g. ten percent of the
virial radius) to the full cluster (e.g. the virial radius).
