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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
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The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
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Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
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Renormalized Internally Contracted Multireference Coupled Cluster with Perturbative Triples.

Robin Feldmann1, Markus Reiher1

  • 1Department of Chemistry and Applied Biosciences, ETH Zürich,, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland.

Journal of Chemical Theory and Computation
|August 19, 2024
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This summary is machine-generated.

This study introduces the renormalized internally contracted multireference coupled cluster (ric-MRCC) method, combining DSRG and ic-MRCC theories. The new approach offers accurate calculations for complex molecular systems with improved computational efficiency.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • The driven similarity renormalization group (DSRG) offers a unitary multireference coupled cluster theory.
  • Numerical instabilities can arise in coupled cluster methods.
  • Accurate calculations for strongly correlated systems remain a challenge.

Purpose of the Study:

  • To develop a novel computational method for accurate electronic structure calculations.
  • To extend the DSRG approach to nonunitary transformations for broader applicability.
  • To improve the efficiency of multireference coupled cluster calculations.

Main Methods:

  • Combined internally contracted multireference coupled cluster (ic-MRCC) with the driven similarity renormalization group (DSRG).
  • Adapted the unitary flow equation approach for nonunitary transformations, creating the renormalized ic-MRCC (ric-MRCC) method.
  • Introduced approximations to the Baker-Campbell-Hausdorff expansion and approximate perturbative triples for ric-MRCCSD[T].

Main Results:

  • Demonstrated the accuracy of ric-MRCC methods for potential energy curves of H8, F2, H2O, N2, and Cr2.
  • Achieved accuracy comparable to advanced multireference methods.
  • ric-MRCCSD and ric-MRCCSD[T] matched CCSD(T) and full configuration interaction results for spectroscopic constants and energies.

Conclusions:

  • The developed renormalized ic-MRCC (ric-MRCC) method provides a computationally efficient and accurate approach for electronic structure calculations.
  • This method effectively addresses challenges in multireference systems.
  • The findings pave the way for more reliable predictions in quantum chemistry.