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| Main Authors: | , |
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| Format: | Preprint |
| Published: |
2025
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2507.13472 |
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| _version_ | 1866916849714724864 |
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| author | Lee, Nicholas Tew, David P. |
| author_facet | Lee, Nicholas Tew, David P. |
| contents | We present a spin-free, size-extensive, and size-consistent coupled cluster method based on a generalised normal ordered exponential ansatz. This approach is a natural generalisation of single-reference coupled cluster theory for arbitrary spin eigenfunctions. The working equations are size-extensive through the generalised normal order formalism, and made spin-free with the spin-ensemble approach. Redundancies amongst excitations are eliminated by selecting only those excitations that project the reference function onto the first-order interacting space. Furthermore, by utilising localised orbitals, the proposed method describes dissociation into open-shell fragments size-consistently. Numerical results on prototypical multireference systems at the singles and doubles level of theory are competitive with existing multireference approaches, yet with more compact working equations. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_13472 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Spin-free Generalised Normal Ordered Coupled Cluster Lee, Nicholas Tew, David P. Chemical Physics We present a spin-free, size-extensive, and size-consistent coupled cluster method based on a generalised normal ordered exponential ansatz. This approach is a natural generalisation of single-reference coupled cluster theory for arbitrary spin eigenfunctions. The working equations are size-extensive through the generalised normal order formalism, and made spin-free with the spin-ensemble approach. Redundancies amongst excitations are eliminated by selecting only those excitations that project the reference function onto the first-order interacting space. Furthermore, by utilising localised orbitals, the proposed method describes dissociation into open-shell fragments size-consistently. Numerical results on prototypical multireference systems at the singles and doubles level of theory are competitive with existing multireference approaches, yet with more compact working equations. |
| title | Spin-free Generalised Normal Ordered Coupled Cluster |
| topic | Chemical Physics |
| url | https://arxiv.org/abs/2507.13472 |