sym2quantized-sapt

A SymPy-based package for second-quantized operator algebra in SAPT (Symmetry-Adapted Perturbation Theory of intermolecular interactions).

It extends SymPy's sympy.physics.secondquant module to work in the double Fermi vacuum — the product of two independent Fermi vacuums, one for each monomer (A and B) of a dimer. On top of that it provides a generalized Wick's theorem, dummy-index canonicalization, RHF spin integration, and numpy.einsum code generation, which together let you derive closed-form SAPT energy expressions symbolically.

Installation

Python 3.8 is the reference interpreter (it is what CI runs); 3.9-3.12 also work. setup.py is the single source of truth for dependencies - the dev extra adds the test, lint and format tooling.

python3 -m venv venv
source venv/bin/activate
python3 -m pip install -e ".[dev]"

Or with conda:

conda env create -f environment.yml
conda activate sym2quantized-sapt

Quick start

Derive the first-order SAPT electrostatic energy (E_pol^{(10)}):

from sympy import symbols, Dummy, latex
from sym2quantized_sapt.sapt_utils import get_V_operator
from sym2quantized_sapt.double_fermi_vac import wicks_double_vac
from sym2quantized_sapt.spin_integrator import spin_integration

V = get_V_operator()
E10 = wicks_double_vac(V, keep_only_fully_contracted=True)
E10 = spin_integration(E10)
print(latex(E10))

See the examples/ directory for full derivations of the electrostatic, induction, dispersion, and exchange energies.

Starting a project with this package? GETTING_STARTED.md covers the environment, the shape of a derivation script, and the conventions that cost debugging time when they are violated.

Running the tests

# fast tests (example runners and other heavy cases are marked `slow`)
python3 -m pytest ./tests/

# full suite, including the slow tests — note the custom `--slow` flag
python3 -m pytest ./tests/ --slow

Many tests assert exact sympy.latex(...) output, so they are sensitive to the SymPy version. setup.py caps it at sympy>=1.8.0,<1.13.0: 1.13 changed Float/int equality and breaks coefficient formatting in code_generator.

Contributing

See CONTRIBUTING.md for the development setup, style checks, and how derivations under examples/ are tested.

License

Distributed under the BSD 3-Clause License. See LICENSE for details.

Citation

If you use this software in academic work, please cite it — see CITATION.cff. Each GitHub release is archived on Zenodo with a version-specific DOI; the concept DOI 10.5281/zenodo.21215565 always resolves to the latest version. The underlying SAPT formalism follows S. Rybak, B. Jeziorski, K. Szalewicz, J. Chem. Phys. 95, 6576 (1991).