  We study the effect of baryonic processes on the halo mass function in the
galaxy cluster mass range using a catalogue of 153 high resolution cosmological
hydrodynamical simulations performed with the AMR code ramses. We use the
results of our simulations within a simple analytical model to gauge the
effects of baryon physics on the halo mass function. Neglect of AGN feedback
leads to a significant boost in the cluster mass function similar to that
reported by other authors. However, including AGN feedback not only gives rise
to systems that are similar to observed galaxy clusters, but they also reverse
the global baryonic effects on the clusters. The resulting mass function is
closer to the unmodified dark matter halo mass function but still contains a
mass dependent bias at the 5-10% level. These effects bias measurements of the
cosmological parameters, such as $\sigma_8$ and $\Omega_m$. For current cluster
surveys baryonic effects are within the noise for current survey volumes, but
forthcoming and planned large SZ, X-ray and multi-wavelength surveys will be
biased at the percent level by these processes. The predictions for the halo
mass function including baryonic effects need to be carefully studied with
larger and improved simulations. However, simulations of full cosmological
boxes with the resolution we achieve and including AGN feedback are still
computationally challenging.
