STEPSIC: Initial condition generator for stereographic cosmological simulations

TL;DR

STEPSIC is an open-source tool that generates initial conditions for stereographic cosmological simulations, supporting various geometries and ensuring accurate matter power spectra.

Contribution

It extends Lagrangian perturbation theory-based initial conditions to non-periodic spherical and cylindrical geometries used by StePS.

Findings

Power spectrum matches linear theory within 0.5% up to Nyquist frequency.

Second-order LPT suppresses transient power excess in simulations.

The code supports multiresolution mapping for stereographic geometries.

Abstract

Conventional cosmological initial condition generators are designed exclusively for fully periodic cubic domains and cannot produce the non-periodic, observer-centric configurations required by stereographically projected N-body codes such as StePS. We present STEPSIC, an open-source initial condition generator that extends Lagrangian perturbation theory-based initial conditions to the spherical and cylindrical geometries used by StePS, while also supporting cuboid domains with arbitrary aspect ratios. The code constructs Gaussian random density fields on anisotropy-free Fourier grids with cubic voxels, applies first- and second-order LPT to obtain displacement and velocity fields, and interpolates these onto particles via B-spline mass-assignment kernels with Fourier-space deconvolution. For stereographic geometries, a multiresolution scheme maps displacement fields across the radially varying particle mass resolution intrinsic to the projection. Both standard and paired-and-fixed variance-reduced realizations are supported. In periodic cubic boxes, the recovered matter power spectrum agrees with the input linear theory prediction to better than 0.5% up to half the Nyquist wavenumber, independent of box aspect ratio (tested up to 10:1). Cross-validation against monofonic using identical white noise fields yields sub-percent power spectrum agreement, with a small residual offset consistent with differences between two independent implementations. Full N-body evolution of matched cylindrical StePS runs confirms that second-order LPT correctly suppresses the 2-3% transient power excess present in first-order initial conditions.