  Here we present a simple, parameter-free, non-perturbative algorithm that
gives low-redshift cosmological particle realizations accurate to
few-Megaparsec scales, called MUSCLE (MUltiscale Spherical ColLapse Evolution).
It has virtually the same cost as producing N-body-simulation initial
conditions, since it works with the 'stretch' parameter {\psi}, the Lagrangian
divergence of the displacement field. It promises to be useful in quickly
producing mock catalogs, and to simplify computationally intensive
reconstructions of galaxy surveys. MUSCLE applies a spherical-collapse
prescription on multiple Gaussian-smoothed scales. It achieves higher accuracy
than perturbative schemes (Zel'dovich and 2LPT), and, by including the
void-in-cloud process (voids in large-scale collapsing regions), solves
problems with a single-scale spherical-collapse scheme. Slight further
improvement is possible by mixing in the 2LPT estimate on large scales.
Additionally, we show the behavior of {\psi} for different morphologies (voids,
walls, filaments, and haloes). A Python code to produce these realizations is
available at http://skysrv.pha.jhu.edu/~neyrinck/muscle.html.
