Thesis
Spin-adapted coupled cluster theory for open-shell molecules using the normal-ordered exponential ansatz
- Abstract:
- A new unlinked formulation of Lindgren's normal-ordered exponential coupled cluster (NOECC) ansatz is devised, with a careful truncation procedure that includes terms in the exponential only up to linear or quadratic order in the amplitudes while minimising size inconsistency errors. This is implemented in a new python package which we have named Open-shell Coupled Cluster using Spin Free Diagrammatic construction (OCCSFrD), which automatically generates, and then solves, spin-adapted coupled cluster equations with an arbitrary configuration state function (CSF) as reference. After verifying that the quadratic-truncated unlinked NOECC faithfully replicates the traditional (quartic-terminating) coupled cluster for single-reference closed shells, the open-shell NOECC energies are found for several high-spin doublets, without the spin contamination introduced in traditional methods. The NOECC framework is then tested in several single-reference multi-determinant cases: the splitting of the 2-electron open-shell singlet from its corresponding high-spin triplet in the beryllium atom, the size consistency of two open shell doublet lithium atoms, the low-lying multi-determinant singlet state in the Oxygen molecule. Some potential energy surfaces are also calculated using NOECCSD for the methylene diradical and for H4 in a range of rectangular geometries, and the 3-electron doublet and quartet energies of the nitrogen atom are calculated with the linear-truncated NOECC. As a contribution towards a general theoretical model for quantum chemistry, this framework achieves a systematically improvable spin-adapted treatment of dynamic correlation for the general statically correlated single-reference wavefunction, clarifying the `genuine' multireference problem that remains to be solved.
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Authors
Contributors
+ Lee, N
- Institution:
- University of Oxford
- Division:
- MPLS
- Department:
- Chemistry
- Sub department:
- Sub-Department of Physical and Theoretical Chemistry
- Research group:
- Tew Research Group
- Oxford college:
- Pembroke College
- Role:
- Contributor
- ORCID:
- 0000-0003-1945-7847
+ Tew, D
- Institution:
- University of Oxford
- Division:
- MPLS
- Department:
- Chemistry
- Sub department:
- Sub-Department of Physical and Theoretical Chemistry
- Research group:
- Tew Research Group
- Oxford college:
- St Hilda's College
- Role:
- Supervisor
- ORCID:
- 0000-0002-3220-4177
+ McGrady, J
- Institution:
- University of Oxford
- Division:
- MPLS
- Department:
- Chemistry
- Sub department:
- Inorganic Chemistry
- Research group:
- Computational Inorganic Chemistry Group
- Oxford college:
- New College
- Role:
- Examiner
- ORCID:
- 0000-0002-8991-1921
+ Koehn, A
- Institution:
- University of Stuttgart
- Research group:
- Institute for Theoretical Chemistry
- Role:
- Examiner
- ORCID:
- 0000-0002-0844-842X
+ Engineering and Physical Sciences Research Council
More from this funder
- Funder identifier:
- https://ror.org/0439y7842
- Grant:
- EP/L015722/1
- Programme:
- Theory and Modelling in the Chemical Sciences (Centre for Doctoral Training)
- DOI:
- Type of award:
- DPhil
- Level of award:
- Doctoral
- Awarding institution:
- University of Oxford
- Language:
-
English
- Keywords:
- Subjects:
- Deposit date:
-
2026-06-17
- ARK identifier:
Terms of use
- Copyright holder:
- Alexander D. Gunasekera
- Copyright date:
- 2025
- Licence:
- CC Attribution (CC BY)
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