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PreprintCC BY 4.0arXiv

Coupled cluster theory for multireference and open-shell molecules

  • Sarai Dery Folkestad
  • Henrik Koch
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Abstract

We introduce a simple coupled cluster approach for multireference and open-shell systems. In the restricted active space coupled cluster framework, the cluster operator only includes excitations from doubly occupied to virtual orbitals, and this ensures the individual operators commute among each other. The ground and excited states are obtained by diagonalisation of the coupled cluster similarity transformed Hamiltonian in a restricted active space. We demonstrate the framework for the RAS-CCSD model, where single and double excitations are included in the cluster operator, and likewise for a complete active space. We consider systems with multireference character and open-shell systems in a spin-adapted manner. The approach captures significantly more dynamical correlation than without the coupled cluster similarity transformation.

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Cite

BibTeXRIS
BibTeX
@misc{folkestad2026coupled,
  title = {{Coupled cluster theory for multireference and open-shell molecules}},
  author = {Sarai Dery Folkestad and Henrik Koch},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.07440},
  eprint = {2610.07440},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.07440},
  note = {CC BY 4.0},
}
RIS
TY  - UNPB
TI  - Coupled cluster theory for multireference and open-shell molecules
AU  - Sarai Dery Folkestad
AU  - Henrik Koch
PY  - 2026
DA  - 2026/10/05
PB  - arXiv
DO  - 10.48550/arxiv.2610.07440
UR  - https://arxiv.org/abs/2610.07440
AB  - We introduce a simple coupled cluster approach for multireference and open-shell systems. In the restricted active space coupled cluster framework, the cluster operator only includes excitations from doubly occupied to virtual orbitals, and this ensures the individual operators commute among each other. The ground and excited states are obtained by diagonalisation of the coupled cluster similarity transformed Hamiltonian in a restricted active space. We demonstrate the framework for the RAS-CCSD model, where single and double excitations are included in the cluster operator, and likewise for a complete active space. We consider systems with multireference character and open-shell systems in a spin-adapted manner. The approach captures significantly more dynamical correlation than without the coupled cluster similarity transformation.
ER  -