Initial Conditions to CosmologicalN‐Body Simulations, or, How to Run an Ensemble of Simulations

The conventional method of generating initial conditions for cosmological N-body simulations introduces a significant error in the real-space statistical properties of the particles. More specifically, the finite box size leads to a significant underestimate of sigma₈, the correlation function, and nonlinear effects. I implement a method of generating initial conditions for N-body simulations that accurately models the real-space statistical properties, such as the mass variance in spheres and the correlation function. The method requires running ensembles of simulations because the power in the DC mode is no longer assumed to be zero. For moderately sized boxes, I demonstrate that the new method corrects the underestimate in the mass variance in spheres and the shape of the correlation function. I also argue that subtracting Poisson noise from the power spectrum is a dangerous practice. Code to generate initial conditions to second order in Lagrangian perturbation theory with the new method is available at http: //www. astro. princeton. edu/~esirko/ic.