Are the halo occupation predictions consistent with large scale galaxy
  clustering?
  We study how well we can reconstruct the 2-point clustering of galaxies on
linear scales, as a function of mass and luminosity, using the halo occupation
distribution (HOD) in several semi-analytical models (SAMs) of galaxy formation
from the Millennium Simulation. We find that HOD with Friends of Friends groups
can reproduce galaxy clustering better than gravitationally bound haloes. This
indicates that Friends of Friends groups are more directly related to the
clustering of these regions than the bound particles of the overdensities. In
general we find that the reconstruction works at best to 5% accuracy: it
underestimates the bias for bright galaxies. This translates to an
overestimation of 50% in the halo mass when we use clustering to calibrate
mass. We also found a degeneracy on the mass prediction from the clustering
amplitude that affects all the masses. This effect is due to the clustering
dependence on the host halo substructure, an indication of assembly bias. We
show that the clustering of haloes of a given mass increases with the number of
subhaloes, a result that only depends on the underlying matter distribution. As
the number of galaxies increases with the number of subhaloes in SAMs, this
results in a low bias for the HOD reconstruction. We expect this effect to
apply to other models of galaxy formation, including the real universe, as long
as the number of galaxies incresases with the number of subhaloes. We have also
found that the reconstructions of galaxy bias from the HOD model fails for low
mass haloes with M = 3-5x10^11 Msun/h. We find that this is because galaxy
clustering is more strongly affected by assembly bias for these low masses.
