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arXiv · 2504.13585

First-Order Symmetry-Adapted Perturbation Theory with Double Exchange for Multireference Systems

Dominik Cieśliński·Michał Przybytek·Grzegorz Chałasiński·Michał Hapka
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Abstract

We extend first-order multiconfigurational symmetry-adapted perturbation theory, SAPT(MC), [Hapka M. et al. JCTC, 2021, 17], to account for double-exchange effects, where up to two electron pairs are exchanged between interacting monomers. To achieve this, we derive density-matrix-based expressions for the first-order exchange energy to arbitrary orders in the overlap expansion. As a numerical demonstration, we apply the double-exchange approximation to strongly orthogonal geminal wave functions. Additionally, we propose an approximate method for evaluating double-exchange energy with complete active space (CAS) wave functions of the valence type, i.e., with n active electrons distributed over n orbitals. We analyze the performance of these methods on model dimers in both ground and excited states.

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Dominik Cieśliński, Michał Przybytek, Grzegorz Chałasiński, Michał Hapka. 2025-04-18. First-Order Symmetry-Adapted Perturbation Theory with Double Exchange for Multireference Systems. https://arxiv.org/abs/2504.13585

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