Introduction

Description of the method

MUSIC (MUlti-Scale Initial Conditions) generates cosmological initial conditions for a hierarchical set of nested regions. A detailed description of some of the methods can be found in the code paper Hahn & Abel (2011). There have been notable changes however with respect to the original version. Notably,

• the real space transfer functions are no longer used, cosmological perturbations are set in Fourier space
• the original double padding of nested grids is no longer necessary, the grids are composited now by using a Meyer wavelet orthogonal scale-space composition

Cookbook for setting up a zoom simulation with MUSIC

The procedure for setting up a zoom simulation follows typically the procedure of 4 steps, given below. Note that resolution levels in MUSIC are specified by their linear power-2 exponent, i.e. a resolution of $128^3$ cells or particles corresponds to level 7 ($\log_2 128=7$). We use the term “lower” for levels synonymously with “coarser” and “higher” with “finer”

Method 1) Use the cosmICweb database

Select the parent simulation and object of interest from the cosmICweb interface, then download the associated MUSIC2 config file and generate the ICs.

Method 2) DIY -- run parent yourself

Run a unigrid dark matter-only pre-flight simulation

In order to set up unigrid initial conditions with MUSIC, select first the desired resolution for this pre-flight simulation. Assume we want to run a 1283 simulation, the coarse grid level has to be set to log2 128=7. Since we want to run a unigrid simulation, both levelmin and levelmax in section [setup] should be set to 7. Also the coarse grid seed needs to be chosen now and must not be changed afterwards. To do this, we set seed[7] in section [random] to the desired random seed. This seed determines the large scale structure and we will only add subgrid noise when performing refinement later. Now, set the box size, starting redshift, all the cosmological parameters and the input transfer function in the respective sections. Finally, select the output plugin in section [output] for the code with which you wish to perform this pre-flight simulation. Finally run MUSIC with the configuration file that contains all your settings and start your simulation. Note that you also have to explicitly specify a redshift at which to generate the initial conditions.

Identify a region of interest (e.g. a halo) and trace back the region to the initial conditions in order to obtain the extent of its Lagrangian patch.

Once the simulation has finished, the region of interest has to be identified. Save all dark matter particle IDs contained in the volume. In the next step, identify the positions of these particles in the initial conditions file as output by MUSIC. Compute the bounding box of these particles and allow for some safety boundary.

Set up MUSIC to resample this Lagrangian patch at the desired resolution and possibly with baryons.

Insert the positions of the particles into a file TBD ...

Set the maximum refinement level in levelmax. Do not change the seed you had specified for the full box simulation nor add new seeds for lower levels! You can adjust levelmin if you do not specify seeds for these new coarser levels. Finally, add seeds for all additional finer levels including levelmax as seed[...] to section [random]. Add baryons, if desired, by saying baryons=yes in section [setup] and run MUSIC with this new configuration file.