PyCC: A Reference Quantum Chemistry Python Package for Education, Validation, and Rapid Prototyping

T. Daniel Crawford; Benjamin G. Peyton; Zhe Wang; Aparna Krishnan; Brendan M. Shumberger; Jose Madriaga; Joshua Atta-Kumi; Aaron De Clercq; Ritvik Prabhu

arXiv · 2610.04107

PyCC: A Reference Quantum Chemistry Python Package for Education, Validation, and Rapid Prototyping

T. Daniel Crawford·Benjamin G. Peyton·Zhe Wang·Aparna Krishnan·Brendan M. Shumberger·Jose Madriaga·Joshua Atta-Kumi·Aaron De Clercq·Ritvik Prabhu

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Abstract

PyCC is a Python-based quantum chemistry package that emphasizes coupled cluster and related methods. Developed over the last five years, PyCC's purpose is to provide a clear reference implementation that will serve as a validation suite for production code, a platform for rapid prototyping and testing of new methods, and an educational resource for newcomers to the field. The package covers both closed- and open-shell single-determinant references in spatial- and spin-orbital formulations for the coupled cluster hierarchy through triples-including methods such as CC3, as well as for MP2 and CI methods. PyCC provides a broad range of molecular properties such as analytic first and second energy derivatives including gradients, Hessians, polarizabilities and atomic polar tensors, as well as non-energy-derivative properties, such as velocity-gauge atomic polar tensors and diagonal Born-Oppenheimer corrections, plus dynamic linear response functions, and excitation energies. The package includes a number of unique capabilities, such as real-time CC propagation (up through CC3), atomic axial tensors for simulating vibrational circular dichroism spectra at the MP2 and CI levels, and local correlation methods including perturbation-aware correlation domains. The code is designed around a shared Wavefunction base and a method-agnostic derivative layer that enables a new method to inherit the full property set once it supplies its reduced densities and their first-order responses. PyCC is fully open source and available on GitHub under the BSD 3-clause license.

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T. Daniel Crawford, Benjamin G. Peyton, Zhe Wang, Aparna Krishnan, Brendan M. Shumberger, Jose Madriaga, Joshua Atta-Kumi, Aaron De Clercq, Ritvik Prabhu. 2026-10-02. PyCC: A Reference Quantum Chemistry Python Package for Education, Validation, and Rapid Prototyping. https://arxiv.org/abs/2610.04107

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