Problem Generators

A problem generator (pgen) sets up the initial conditions and any problem-specific runtime behavior for a simulation. AthenaK decides which problem generator to run either at build time (for custom problems) or at runtime through the input file (for the built-in test problems).

Selecting a problem generator

There are two ways to choose a problem generator.

  1. Built-in problem generator, selected at runtime. When AthenaK is built without a custom problem, the pgen_name parameter in the <problem> block selects one of the problem generators that ship with the code:

    <problem>
pgen_name = linear_wave

  2. Custom problem generator, selected at build time. Compile a specific problem generator source file from src/pgen/ (or your own) with the PROBLEM option:

    cmake -B build -D PROBLEM=my_problem_file
cmake --build build -j

    This compiles src/pgen/my_problem_file.cpp and wires it in through the user-problem hook. A build configured this way always runs that problem generator and ignores pgen_name.

If pgen_name is not recognized and no custom -D PROBLEM=... build was used, AthenaK aborts with a fatal error.

Built-in pgen_name values

The problem generators dispatched in src/pgen/pgen.cpp are:

  • advection

  • cpaw

  • gr_bondi

  • cshock

  • diffusion

  • linear_wave

  • implode

  • gr_monopole

  • mri3d

  • orszag_tang

  • rad_linear_wave

  • rad_beam

  • shock_tube

  • shwave

  • z4c_boosted_puncture

  • z4c_linear_wave

In addition, two pre-defined unit-test problem generators are built in by default:

  • eos_compose — exercises the tabulated (CompOSE) equation-of-state machinery.

  • gauss_legendre — exercises the Gauss–Legendre quadrature used for spherical grids and surface integrals.

Where to find existing examples

  • Core pgen sources: src/pgen/

  • Test-oriented pgens: src/pgen/tests/

  • Ready-to-run input files: inputs/ and inputs/tests/

Common workflow

# Build (default built-in pgens)
cmake -B build
cmake --build build -j

# Parse-only check for the chosen pgen
./build/src/athena -i inputs/tests/linear_wave_hydro.athinput -n

# Run
./build/src/athena -i inputs/tests/linear_wave_hydro.athinput

Tips for custom pgens

  • Start from a nearby file in src/pgen/ or src/pgen/tests/.

  • Keep all run-time knobs in the <problem> block so users can override them easily.

  • Validate with -n (parse only) before launching long runs.