Variational Monte Carlo for quantum many-body systems

tachys is a JAX library for finding the ground states of quantum lattice models with neural-network wavefunctions. Build a Hamiltonian by composing spin and fermionic operators, use any Flax network as the wavefunction, and optimize it with variational Monte Carlo — the same code runs on a laptop and across a multi-GPU cluster. It also trains foundation models: a single network covering a whole family of Hamiltonians, rather than one run per coupling.

Documentation: https://tachys.org/quickstart.html

Installation

CPU:

pip install tachys

CUDA (GPU):

pip install "tachys[cuda]"

This installs JAX with CUDA 12 support via the cuda optional dependency.

Install from source

git clone https://github.com/tachys-dev/tachys.git
cd tachys
pip install .

Development install

To install in editable mode (changes to the source are reflected immediately without reinstalling):

pip install -e ".[test]"          # CPU
pip install -e ".[cuda,test]"     # CUDA
pytest tests/

Authors

• Riccardo Rende
• Luciano Loris Viteritti

License

Copyright 2026 The Simons Foundation, Inc.

Licensed under the Apache License, Version 2.0.