xnn

Machine-learning interatomic potentials for molecules and materials behind one PyTorch interface.

x in xnn refers to the neural network model architectures: g stands for graph, d for dense, c for convolutional, ff for classical force-fields, sqm for semi-empirical quantum mechanics, trf for transformers, hyb for hybrid models, all sharing one data object, one trainer and one deployment path. This categorization is reflected in the package structure.

Documentation: https://molssi-ai.github.io/xnn/

Installation

pip install xnns                  # core (torch, numpy, pyyaml)
pip install "xnns[gnn,ase]"       # + e3nn for NequIP/MACE/Allegro, + ASE calculator
pip install -e ".[all]"           # from a clone: everything, for development

The distribution is xnns; the import name and the command line are xnn.

Design principles

1. One data object. Every model takes an AtomicGraph and returns {"node_energy", "energy"}. Periodicity lives only in the edge vectors, so molecules and crystals look the same to a model.
2. Featurizers are first-class. Descriptors (symmetry functions, AEV) and equivariant edge features (spherical harmonics, Cartesian monomials) are standalone modules that models compose and you can inspect on their own.
3. Forces and stress in one place. ForceStressOutput differentiates any model's energy for forces and stress; models never implement them. Wrappers in the same style add long-range electrostatics (LES) and D3 / D4 dispersion to any model.
4. One config, three frontends. @register_model exposes a model to the YAML, argparse and Hydra interfaces, all of which fill one Config dataclass.

Load data

Structures are plain dictionaries (pos, atomic_numbers, optional cell, pbc and energy / forces / stress targets), any ASE-readable file, or a dataset from the hub:

from xnn.common.data import AtomicDataset, load_dataset

# From a list of structures dict
data = AtomicDataset(structures, cutoff=5.0)

# From a file (e.g., .extxyz, .cif, VASP, ...)
data = AtomicDataset.from_file("trajectory.extxyz", cutoff=5.0)

# From the xnn data hub (e.g., RMD17 dataset)
data = load_dataset("rmd17", molecule="aspirin", split="train", cutoff=5.0)

Train

from xnn.common.config import Config
from xnn.common.train import Trainer

# Initialize the configuration
cfg = Config()

# Set the model and data configuration
cfg.model.name = "nequip"
cfg.model.extra = {"species": [1, 6, 8], "l_max": 2}
cfg.data.batch_size = 16

# Train the model
Trainer(cfg, data).fit()

or from the command line: xnn train --config configs/train.yaml --set optim.epochs=50.

Pretrained models

from xnn.common.models import from_pretrained, list_models

# Foundation models, xnn models, local cache
list_models()

# MACE-MP / MACE-OFF, converted once and cached
model = from_pretrained("mace-off23-small")

# Your own checkpoints
model = from_pretrained("runs/exp/best.pt")

Fine-tune

Any pretrained model continues training through the same Trainer: naive fine-tuning, LoRA, multi-head replay (with original or pseudolabelled replay data), layer freezing and model-aware reference-energy reestimation, all from the config:

model:
  pretrained: mace-off23-small
  cutoff: 5.0
  lora: {rank: 16}                      # or heads: [pt_head, Default] with data.replay_path
  atomic_energies: estimated

Benchmark

Score pre-trained checkpoints on one dataset (energy / force / stress MAE, MSE or RMSE, per atom or as atomization energies), tabulated per model and written to CSV, JSON or Markdown:

xnn benchmark --config configs/benchmark.yaml

Deploy

from xnn.common.deploy import XNNCalculator, export_to_lammps

# Export as ASE calculator
atoms.calc = XNNCalculator(model, cutoff=5.0)

# Export to LAMMPS (TorchScript)
export_to_lammps(model, cutoff=5.0, path="deployed.pt")

xnn mdi --ckpt runs/exp/best.pt serves a model to an external MD code through the MolSSI Driver Interface.

Models

Each model is checked against its reference code or paper (notebooks/fidelity_checks/). SchNet, DimeNet, GemNet, PaiNN, TensorNet, ET, SpookyNet, NequIP, MACE, Allegro and AIMNet2 also export to TorchScript / LAMMPS; every model deploys through ASE and the MDI engine.

Examples and tests

Executed notebooks for every model are under notebooks/examples/ (pip install -e ".[examples]"); notebooks/examples/quickstart.py is a minimal script. Run the tests with pytest tests/.

Contributing

Bug reports, fixes and new models are welcome. Read CONTRIBUTING.md first: it explains how to report a bug with a reproducible example and what a pull request needs to be merged.

License

MIT

How to cite

If you use xnn in your work, please cite it as:

@software{Mostafanejad:2026:xnn,
  author  = {Mostafanejad, Mohammad},
  title   = {xnn: Machine-learning interatomic potentials for molecules and materials},
  year    = {2026},
  version = {0.19.0},
  url     = {https://github.com/molssi-ai/xnn}
}

Author: Mohammad Mostafanejad, The Molecular Sciences Software Institute (MolSSI)