Using EXP potentials with Gala#

Gala supports EXP as a backend for representing flexible and time-dependent gravitational potentials, typically constructed from N-body simulation snapshots. This requires:

  1. building EXP,

  2. building Gala with EXP support,

  3. and setting up a EXPPotential object using the user’s EXP config and coefficient files.

Note that EXP support currently requires building Gala (and EXP) from source. Additionally, this workflow has only been tested on Linux and MacOS with the setups seen in the GitHub actions test config file.

Building EXP#

The EXP documentation is the authoritative source on how to build EXP. Currently, the only Gala-specific addition to the instructions is that Gala expects the build directory to be present in the EXP root directory. The install directory will be looked for in the EXP root directory too, or one can set GALA_EXP_LIB_PATH (see below).

To install EXP’s dependencies, here is one recipe that we have found to work on Ubuntu 24.04:

sudo apt-get install build-essential cmake gfortran git libeigen3-dev libfftw3-dev libhdf5-dev libomp-dev libopenmpi-dev ninja-build
# install uv python, only needed if you don't already have python:
# curl -LsSf https://astral.sh/uv/install.sh | sh

Here is another recipe using modules that has been found to work on Flatiron Institute’s rusty cluster:

module load modules/2.3 cmake gcc openmpi hdf5 libtirpc eigen fftw git python

EXP also builds on Mac by installing the dependencies with Homebrew:

brew install cmake eigen fftw hdf5 open-mpi git ninja

After installing the dependencies, one can download and build EXP on Linux with:

git clone --recursive https://github.com/EXP-code/EXP.git
cd EXP
cmake -G Ninja -B build -DCMAKE_INSTALL_RPATH=$PWD/install/lib --install-prefix $PWD/install
cmake --build build
cmake --install build

For a full example of how to build EXP on Mac, see this build recipe.

Note that building pyEXP is not required. However, some tests will use pyEXP if it is present.

Building Gala with EXP support#

Building Gala with the GALA_EXP_PREFIX environment variable set to the EXP root dir will trigger compilation of the Gala’s EXP Cython extensions. For example:

git clone https://github.com/adrn/gala.git
cd gala
export GALA_EXP_PREFIX=/path/to/EXP

If you build and install EXP following the instructions above, the EXP libraries will be located in EXP/install/lib and the Gala build process knows to look there by default. If you installed EXP to a different location, you can set the GALA_EXP_LIB_PATH environment variable to point to the lib directory of the EXP install:

# Only do this if the install location is not $GALA_EXP_PREFIX/install
# export GALA_EXP_LIB_PATH=/path/to/EXP-install/lib

That is, GALA_EXP_LIB_PATH can be set if the CMake --install-prefix was set to a location other than GALA_EXP_PREFIX/install. GALA_EXP_LIB_PATH should be the directory that contains the .so or .dylib files.

Now you can run the Gala build. For example, using uv:

uv venv
uv pip install -ve .

Or using venv:

python -m venv .venv
. .venv/bin/activate
python -m pip install -ve .

In either case, the pip output should show a message like Gala: installing with EXP support.

Running Gala with an EXP potential#

To use an EXP potential with Gala, first you’ll need a config file, a basis file, and a coefficients file from EXP. We have included example files with this tutorial, produced by constructing a basis and computing coefficients with particle data from a single snapshot of the dark matter halo of the m12m simulation in the Latte suite of the FIRE-2 simulations. In particular, the relevant files are:

  • m12m-basis.yml - the basis configuration file

  • m12m_basis_table.model - the basis table (density and potential evaluated on a grid of spherical radii)

  • m12m-coef.hdf5 - the coefficients file

The basis was generated with a unit system in which G=1 (standard for EXP), the mass unit is \(10^{12}~\mathrm{M}_\odot\), and the length unit is 10 kpc. Setting up an EXPPotential object with these files is as easy as specifying the unit system and EXP files:

import astropy.units as u
import gala.potential as gp
from gala.units import SimulationUnitSystem

exp_units = SimulationUnitSystem(mass=1e12 * u.Msun, length=10 * u.kpc, G=1)

exp_pot = gp.EXPPotential(
    units=exp_units,
    config_file="data/m12m-basis.yml",
    coef_file="data/m12m-coef.hdf5",
)

Then one can use the potential object like any other Gala potential. For example, to integrate and plot an orbit:

import gala.dynamics as gd

w0 = gd.PhaseSpacePosition(
    pos=[8, 0.0, 1.0] * u.kpc,
    vel=[0.0, 220, 0.0] * u.km / u.s,
)
orbit = gp.Hamiltonian(exp_pot).integrate_orbit(w0, dt=1 * u.Myr, t1=0, t2=6 * u.Gyr)
fig = orbit.plot(units=u.kpc, linestyle="-", alpha=0.5, label="orbit in m12m")

Units#

Gala generally works in physical units (e.g., kpc, solar mass, etc.), whereas EXP typically works in user-defined simulation units. To use EXP with Gala, one must define a SimulationUnitSystem and specify this when creating the potential (as demonstrated above). If the basis was computed from a scale-dependent potential, the simulation unit system must match the units used to generate the basis. If the potential was computed from a scale-independent model, the simulation unit system can be arbitrary, but it can be used to set physical scales to the simulations.

Time Evolution#

An EXPPotential may be time-evolving or static. If the coefficient file has only one snapshot, the potential will be static. Likewise, if tmin/tmax are passed such that only one snapshot from the coefs falls within that range, the potential will be static. For the examples below, we use hypothetical files config.yml and coefs.h5 that contain coefficients for multiple snapshots.

One can always check if an EXPPotential is static with:

exp_pot.static

One can also “freeze” make a multi-snapshot potential (i.e. make it static) by selecting a single snapshot with the snapshot_index parameter:

exp_pot = gp.EXPPotential(
    units=exp_units,
    config_file="config.yml",
    coef_file="coefs.h5",
    snapshot_index=0,
)

Important

For time-evolving potentials, if one tries to evaluate the potential outside of the time range stored in the coefficients file (even indirectly, such as during an orbit integration), currently a NAN will be returned (after printing an error message to stderr). Proper exception propagation is a planned feature.

If the coefficients file stores a very large time range but the user is only interested in a smaller range, one can specify tmin and/or tmax to load a smaller subset of the coefficient data (for memory efficiency):

exp_pot = gp.EXPPotential(
    units=exp_units,
    config_file="config.yml",
    coef_file="coefs.h5",
    tmin=1.0,
    tmax=2.0,
)

Note that, as mentioned above, subsequently using a time outside this range will result in a NAN being returned (with an associated error printed to stderr). Or more precisely: using a time outside the range of snapshots that this tmin/tmax caused to be loaded will cause such an error. One can check the loaded range of snapshots with:

exp_pot.tmin_exp
exp_pot.tmax_exp

tmin and tmax should not be passed for single-snapshot coefficient files.

File Paths#

EXPPotential takes config_file and coef_file as file path arguments. These can be absolute paths, or paths relative to the current working directory.

The config file itself may reference file paths like the modelname and cachename. These paths can be absolute paths, or paths relative to the config file.

Testing#

The tests for EXP are all in the dedicated test_exp.py file. The EXP tests will be run by default if Gala was built with EXP (use GALA_FORCE_EXP_TEST=1 to always test EXP). Similarly, some of the tests will compare against pyEXP if it is available (use GALA_FORCE_PYEXP_TEST=1 to always test this).

With the test dependencies installed (see Running the tests), to run just the EXP tests, one can run the following from the repo root:

pytest gala/potential/potential/tests/test_exp.py

Limitations#

The EXPPotential currently has the following limitations:

  • Hessian evaluation is not supported.

  • Pickling, saving, and loading is not supported.

  • Performance may currently not be as high as native Gala potentials

  • Evaluating the potential at a time outside the loaded time range will result in NANs

API#

See EXPPotential for the complete API documentation.