Source code: https://github.com/pyscf/pyscf

Short description: The Python-based Simulations of Chemistry Framework (PySCF) is an open-source collection of electronic structure modules powered by Python. The package provides a simple, lightweight, and efficient platform for quantum chemistry calculations and methodology development. PySCF can be used to simulate the properties of molecules, crystals, and custom Hamiltonians using mean-field and post-mean-field methods. To ensure ease of extensibility, almost all of the features in PySCF are implemented in Python, while computationally critical parts are implemented and optimized in C.