  We discuss the relation between the output of Newtonian N-body simulations on
scales that approach or exceed the particle horizon to the description of
General Relativity. At leading order, the Zeldovich approximation is correct on
large scales, coinciding with the General Relativistic result. At second order
in the initial metric potential, the trajectories of particles deviate from the
second order Newtonian result and hence the validity of 2LPT initial conditions
should be reassessed when used in very large simulations. We also advocate
using the expression for the synchronous gauge density as a well behaved
measure of density fluctuations on such scales.
