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Linear and quadratic internally contracted multireference coupled-cluster approximations.

Joshua A Black1, Andreas Köhn1

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This study analyzes approximations to internally contracted multireference coupled-cluster (icMRCC) methods. While multireference CEPA(0) shows promise for potential energy curves, it lacks size consistency, hindering the development of accurate icMRCC methods.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry

Background:

  • The internally contracted multireference coupled-cluster (icMRCC) method is crucial for accurately describing systems with complex electronic structures.
  • Developing computationally efficient and accurate approximations to icMRCC is an ongoing challenge in quantum chemistry.

Purpose of the Study:

  • To implement and analyze linear and quadratic approximations to the icMRCC method.
  • To investigate the methodological peculiarities of the icMRCC ansatz through comparative analysis of different approximations.
  • To identify sources of discontinuities in potential energy curves calculated with linear icMRCC.

Main Methods:

  • Implementation of linear and quadratic approximations to icMRCC using linked and unlinked coupled-cluster formalisms.
  • Application of perturbation theory and coupled-electron pair approximation (CEPA(0)).
  • Diagrammatic analysis to identify terms causing and reducing discontinuities in potential energy curves.

Main Results:

  • Discontinuities were observed in potential energy curves calculated with linear icMRCC.
  • Diagrammatic analysis identified specific terms responsible for and mitigating these discontinuities.
  • Multireference CEPA(0) performed well for benchmark calculations (PECs, singlet-triplet splittings, barrier heights).

Conclusions:

  • The multireference CEPA(0) approximation, despite good performance on benchmark tests, lacks size consistency.
  • This lack of size consistency prevents it from being a definitive step towards a computationally inexpensive and accurate icMRCC method.
  • Further development is needed to overcome limitations and achieve a robust icMRCC approach.