3. Working with parameter files

Specifying the many parameters to the concept script via the -c option quickly becomes tiresome. A better solution is to write all parameters in a text file.

Tip

The param directory is meant as a dedicated place to store your parameter files. It comes with a few example parameter files included.

Copy the content below to a file named e.g. param/tutorial-3:

param/tutorial-3
# Non-parameter helper variable used to control the size of the simulation
_size = 64

# Input/output
initial_conditions = {
    'species': 'matter',
    'N'      : _size**3,
}
output_dirs = {
    'powerspec': f'{path.output_dir}/{param}',
}
output_times = {
    'powerspec': [a_begin, 0.3, 1],
}

# Numerics
boxsize = 4*_size*Mpc/h
potential_options = 2*_size

# Cosmology
H0      = 67*km/(s*Mpc)
Ωb      = 0.049
Ωcdm    = 0.27
a_begin = 0.02

To run CONCEPT using this parameter file, do

./concept \
    -p param/tutorial-3 \
    -n 4

The simulation specified by the above parameters is quite similar to the simulations of the previous section, though we’ve now been more explicit about the cosmology and the size of the simulation box, and also refined the output specifications. In doing so, we’ve made use of several helpful tricks:

  • The comments / section headings are there for organisational purposes only. These may be omitted, and the order in which the parameters are specified does not matter.

  • Parameter files are glorified Python scripts, and so we may utilize the full Python (3) language when defining the parameters. We can also define helper variables that are not themselves parameters, like _size above, which here is used to simultaneously adjust the number of particles 'N', the side length of the simulation box boxsize and the size of the potential grid via potential_options. Though not strictly necessary, it is preferable for such helper variables to be named with a leading underscore _, to separate them from actual parameters.

  • We have explicitly specified the directory for power spectrum output in the output_dirs parameter. The value is constructed using the magic path and param variables, available to all parameter files. The path variable holds absolute paths specified in the .path file. If you want to take a look at this .path file, do e.g.

    cat .path

    Writing path.output_dir then maps to whatever value is specified for output_dir in the .path file, which will be '/path/to/concept/output'. When used as is, the param variable maps to the file name of the parameter file in which it is used, in this case 'tutorial-3'. In total, the power spectrum output directory then gets set to '/path/to/concept/output/tutorial-3'. We could have gotten away with just writing 'output/tutorial-3' out statically. Such relative paths in parameter files should be given with respect to the installation directory.

  • We’ve specified multiple times at which to dump power spectra in output_times. Note that we use the parameter a_begin (creating a power spectrum of the initial conditions), the value of which is set further down. Generally, the order of variable definitions — even when one depends on another — is of no importance inside parameter files.

  • The simulation takes place in a cubic box, the side length of which is set by boxsize. In the parameter specification above, we’ve let boxsize be proportional to _size, which has the effect of keeping the particle density constant when varying _size: If e.g. _size is increased by a factor of 2, boxsize increases by a factor of 2 as well, meaning that the simulation volume goes up by a factor of 8, which is compensated by a similar increase in the number of particles by a factor \(2^3 = 8\).

    No default units are ever assumed by CONCEPT when it reads parameters, and so it’s critical that you explicitly tack on units on all dimensional parameters. For boxsize, the extra fancy unit of \(\text{Mpc}/h\) is used above, with \(h \equiv H_0/(100\, \text{km}\, \text{s}^{-1}\, \text{Mpc}^{-1})\) inferred dynamically from the Hubble constant H0 set further down.

  • The parameters Ωb and Ωcdm of course set the present amounts of baryons and cold dark matter, respectively. Together, these otherwise distinct species are collectively referred to as just matter. Thus, declaring the species to be 'matter' in the initial_conditions implies that the particles will represent both the baryonic and cold dark matter.

The -p (parameter file) option can be mixed with the -c (command-line parameter) option. As an example, consider leaving out the definition of _size from the parameter file and instead supplying it when running the code:

./concept \
    -p param/tutorial-3 \
    -c "_size = 64" \
    -n 4

If you forget to specify _size — or any other variable referenced by the parameter file — CONCEPT will exit with an error, letting you know.

Logged job information

For each CONCEPT run (or job) a lot of information is logged. Each new job gets a unique integer ID, which is stated in the beginning and end of the run (and further included in the header of the power spectrum data files). Each job gets its own subdirectory within the job directory, containing at least:

  • param: A copy of the parameter file used for the job.

    Note

    When mixing -p and -c, the combined parameters are what’s stored in the copied parameter file.

  • log: A record of the information printed to the screen during the job.

    Tip

    To view the logged output of e.g. CONCEPT job 1 with proper colouring, use

    less -rf job/1/log

    Arrow keys to navigate, Q to quit.

So far we’ve introduced only the most essential parameters. The remaining sections of this tutorial will introduce further parameters — and expand on already encountered ones — as needed. For more focused documentation on any specific parameter, consult Parameters.