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arXiv · 2504.04459

Excitation Energies from the Entanglement Coupled Cluster Model for Doublets

Sarai Dery Folkestad·Kristine Lauvstad Kruken·Henrik Koch
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Abstract

We present excitation energies for molecular doublets from a spin-adapted formulation of coupled cluster singles and doubles theory. The entanglement coupled cluster approach represents an unconventional take on the notorious problem of spin adaptation for open-shell species. In this approach, the high-spin open-shell molecular system is coupled to non-interacting bath orbitals to form a total closed-shell system. In entanglement coupled cluster theory, many of the attractive features of the spin-adapted closed-shell coupled cluster is retained: an unambiguous definition of the cluster operator and a terminating Baker-Campbell-Hausdorff expansion. The result is a spin-adapted coupled cluster theory for open-shell species. The model produces excitation energies of a quality comparable to the closed-shell counterpart. Additionally, some ionized states that cannot be modeled accurately with the alternative equation-of-motion approach for ionized states, can be described with the entanglement coupled cluster singles and doubles model.

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Sarai Dery Folkestad, Kristine Lauvstad Kruken, Henrik Koch. 2025-04-06. Excitation Energies from the Entanglement Coupled Cluster Model for Doublets. https://arxiv.org/abs/2504.04459

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