ace-jax

Build, fit and evaluate Atomic Cluster Expansion (ACE) interatomic potentials in Python with JAX, on CPU or GPU, all from one pip install.

Documentation: https://acesuit.github.io/ace-jax/: installation, quickstart, tutorials, how-to guides, and the CLI and API reference.

What it does

• Build a basis and fit it in one command. aj fit --order 3 --max-degree 10 --train train.xyz --out fit builds the symmetry-adapted ACE basis and fits it. The fit.yaml it writes reproduces the run.
• Bayesian fits. Linear ACE is fitted by Bayesian linear regression, with the energy, force and virial noise levels and the prior scale chosen by maximising the evidence: no hand-tuned weights. A hybrid ACE + Gaussian-process arm adds a calibrated uncertainty ladder (MAP, Laplace, Pathfinder, VI, NUTS), and the linear model has POPS and ARD uncertainties. The radial basis can be learned as part of the fit (--learn-radial).
• Fast evaluation. ACECalculator and GPCalculator are ASE calculators, fast enough for molecular dynamics, with predicted energy_std and forces_std for GP and ARD models. export_lammps deploys a model to LAMMPS through lammps-jax.
• PACE potentials. pacemaker .yace files load, evaluate and write back.
• Data in, data out. Training data is extended XYZ or a list of ase.Atoms (stress labels are converted to virials). aj eval writes predictions as extended XYZ, with an RMSE table per config type.

Install

pip install ace-jax

Extras: "ace-jax[cuda]" (CUDA 12 JAX), "ace-jax[gp]" (the Pathfinder rung) and "ace-jax[fast-neighbours]" (a C++ neighbour list). Building new bases needs the ace-jax-coupling wheel, a core dependency available for Linux x86_64 and aarch64, macOS arm64 and Windows x64; elsewhere, fit and evaluate from an existing basis file. See Installation.

Quickstart

Fit a linear ACE model to labelled data, check it on a test set, and evaluate it:

aj fit --order 3 --max-degree 10 --train train.xyz --test test.xyz \
    --e0 lsq --m-per-species 0 --opt lbfgs --out fit
aj eval --model fit/model.npz --data test.xyz --out predictions.xyz

Use the fitted model from Python:

import jax
jax.config.update("jax_enable_x64", True)

from ase.build import bulk
from ace_jax import ACECalculator

atoms = bulk("Si", "diamond", a=5.43, cubic=True)
atoms.calc = ACECalculator("fit/model.npz")
print(atoms.get_potential_energy(), atoms.get_forces())

The Quickstart runs this on a small silicon data set, and the tutorials are notebooks that run on a laptop CPU, including ones adapted from the MLIP school 2026. For coding agents, skills/ace-jax/SKILL.md is a compact usage guide.

Performance and validation

Fits reproduce ACEfit's design matrix and least-squares solve to 1e-8, and PACE evaluation matches the ML-PACE C++ code and python-ace; CI checks both. The throughput benchmarks against LAMMPS ML-PACE are in docs/dev/benchmarks.md.

Contributing

See CONTRIBUTING.md for the development setup, tests and the reference-parity jobs, and the changelog for releases. ace-jax is MIT-licensed and part of ACEsuit.