Interacting Quantum Atoms (IQA) energy decomposition analysis partitions the molecular energy into intra- and interatomic contributions, with physically meaningful components such as kinetic, electron–electron, electron–nucleus, and nucleus–nucleus terms [1]. In this work, we present a Python implementation and performance assessment of the IQA framework developed within the QC-Devs Software Consortium [2]. Our modular and flexible implementation can be combined with any atoms-in-molecules partitioning scheme. In particular, we couple IQA with variational Hirshfeld methods [3] and compare the results against Quantum Theory of Atoms in Molecules (QTAIM) [4] for a series of diatomic molecules. We show that the resulting energy components generally align with chemical expectations, with Hirshfeld-based IQA offering improvements over QTAIM in some cases.

Because IQA analysis is computationally demanding, we present several performance optimization strategies, including just-in-time compilation, multiprocessing, and vectorization, that significantly accelerate the calculations. This achieves speedups of up to two orders of magnitude compared to our baseline implementations. The resulting implementation is computationally efficient, flexible, and currently supports a wide variety of Hirshfeld-based partitioning schemes. Through the QC-Devs ecosystem, the IQA module interfaces seamlessly with other packages, enabling straightforward use with different wavefunction file formats. Moreover, its modular Python architecture allows additional partitioning schemes to be incorporated with minimal effort. To the best of our knowledge, this is the first IQA implementation to support multiple Hirshfeld-based partitioning methods within a module and an extensible framework.

[1] Guevara-Vela, J. M.; Francisco, E.; Rocha-Rinza, T.; Martın Pendas, A. Interacting Quantum Atoms–A review. Molecules 2020, 25, 4028.
[2] Chan, M.; Verstraelen, T.; Tehrani, A.; Richer, M.; Yang, X. D.; Kim, T. D.; Vohringer-Martinez, E.; Heidar-Zadeh, F.; Ayers, P. W. The tale of HORTON: Lessons learned in a decade of scientific software development. J. Chem. Phys. 2024, 160.
[3] Heidar-Zadeh, F.; Ayers, P. W.; Verstraelen, T.; Vinogradov, I.; Vohringer-Martinez, E.; Bultinck, P. Information theoretic approaches to atoms-in-molecules: Hirshfeld family of partitioning schemes. J. Phys. Chem. A 2018, 122, 4219–4245.
[4] Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford University PressOxford, 1990.