Universal non-Gaussian initial conditions for N -body simulations

In this paper we present the implementation of an efficient formalism for the generation of arbitrary non-Gaussian initial conditions for use in N-body simulations. The methodology involves the use of a separable modal approach for decomposing a primordial bispectrum or trispectrum. This approach allows for the far more efficient generation of the non-Gaussian initial conditions already described in the literature, as well as the generation for the first time of nonseparable bispectra and the special class of diagonal-free trispectra. The modal approach also allows for the reconstruction of the spectra from given realizations, a fact which is exploited to provide an accurate consistency check of the simulations.