  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 non-separable bispectra and the special class of diagonal-free
trispectra. The modal approach also allows for the reconstruction of the
spectra from given realisations, a fact which is exploited to provide an
accurate consistency check of the simulations.
