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Related Experiment Video

Updated: Apr 8, 2026

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

Published on: April 8, 2020

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Spin-free generalized normal ordered coupled cluster.

Nicholas Lee1, David P Tew1

  • 1Physical and Theoretical Chemical Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.

The Journal of Chemical Physics
|April 7, 2026
PubMed
Summary
This summary is machine-generated.

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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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We developed a new coupled cluster method that is spin-free and size-consistent. This computational chemistry approach accurately models complex molecular systems, offering a more efficient alternative to existing methods.

Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Coupled cluster (CC) theory is a powerful quantum chemical method for electronic structure calculations.
  • Standard CC methods face challenges with systems exhibiting strong electron correlation or multiple important electronic configurations (multireference systems).
  • Existing multireference methods can be computationally expensive and complex.

Purpose of the Study:

  • To develop a novel spin-free, size-extensive, and numerically size-consistent coupled cluster (CC) method.
  • To generalize single-reference CC theory for arbitrary spin eigenfunctions, addressing limitations in describing systems with complex spin states.
  • To improve the computational efficiency and accuracy of theoretical models for multireference systems.

Main Methods:

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  • A generalized normal ordered exponential ansatz was employed for the coupled cluster wave function.
  • The spin-ensemble approach was utilized to achieve spin-free working equations.
  • Excitations were carefully selected to eliminate redundancies and ensure projection onto the first-order interacting space.
  • Localized orbitals were incorporated to achieve size consistency in dissociation processes.

Main Results:

  • The developed method demonstrates size-extensivity and numerical size-consistency through the generalized normal order formalism.
  • The spin-ensemble approach successfully eliminates spin contamination.
  • The method accurately describes the dissociation of molecules into open-shell fragments.
  • Numerical results for multireference systems at the singles and doubles level are competitive with established methods.

Conclusions:

  • The proposed generalized coupled cluster method offers a robust and efficient approach for studying challenging quantum chemical systems.
  • This method provides a more compact formulation of working equations compared to existing multireference techniques.
  • The spin-free and size-consistent nature makes it suitable for a wider range of chemical problems, particularly those involving bond breaking and complex electronic structures.