A nested-grid particle-mesh code for high-resolution simulations of gravitational instability in cosmology

I describe a nested-grid particle-mesh (NGPM) code designed to study gravitational instability in three dimensions. The code is based upon a standard PM code. Within the parent grid I am able to define smaller subgrids, allowing a substantial extension of the dynamical range in mass and length. I treat the fields on the parent grid as background fields and utilize one-way interactive meshing. Waves on the coarse parent grid are allowed to enter and exit the subgrid, but waves from the subgrid are precluded from affecting the dynamics of the parent grid. On the parent grid the potential is computed using a standard multiple Fourier transform technique. On the subgrid a Fourier transform technique is used to compute the subgrid potential at high resolution. Quasi-isolated boundary conditions are imposed on the subgrid using the standard method for generating isolated boundary conditions, but, rather than using the isolated Green function, the Ewald method is used to compute a Green function on the subgrid which possesses the full periodicity of the parent grid. I present a detailed discussion of my methodology and a series of code tests.