Including relativistic and primordial Non-Gaussianity contributions in
  cosmological simulations by modifying the initial condition
  We present a method to implement relativistic corrections to the evolution of
dark matter structures in Newtonian simulations of a LCDM universe via the
initial conditions. We take the nonlinear correspondence between the Lagrangian
(Newtonian) evolution of dark matter inhomogeneities and the
synchronous-comoving (relativistic) matter density description, and use it to
promote the relativistic constraint as the initial condition for numerical
simulations of structure formation. In this case, the incorporation of
Primordial non-Gaussianity (PNG) contributions as initial conditions is
straightforward. We implement the relativistic,fNL and gNLcontributions as
initial conditions for the L-PICOLA code, and compute the power spectrum and
bispectrum of the evolved matter field. We focus specifically on the case of
largest values of non-Gaussianity allowed at 1-sigma by Planck
observations(fNL=-4.2 and gNL=-7000). As a checkup, we show consistency with
the one-loop perturbative prescription and with a fully relativistic simulation
(GRAMSES) on the adequate scales. Our results confirm that both relativistic
and PNG features are most prominent at very large scales and for squeezed
triangulations. We discuss future prospects to probe these two contributions in
the bispectrum of the matter density distribution
