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The approximate second order coupled-cluster method based on a size-consistent Brillouin-Wigner partitioning.

Linus Bjarne Dittmer1,2, Nikolay V Tkachenko1, Martin Head-Gordon1,3

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We introduce BWs-CC2, a new computational chemistry method that improves upon coupled-cluster methods (CC2) for molecular property prediction. This variant offers better accuracy, especially for challenging molecules, by using a refined partitioning approach.

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Area of Science:

  • Computational chemistry
  • Quantum chemistry
  • Method development

Background:

  • Coupled-cluster methods (CC2) are widely used for electronic structure calculations.
  • The choice of partitioning for the unperturbed Hamiltonian impacts CC2 accuracy.
  • Møller-Plesset (MP) partitioning is standard, but alternatives may offer improvements.

Purpose of the Study:

  • To introduce and evaluate a novel variant of the CC2 method, termed BWs-CC2.
  • To compare the performance of BWs-CC2 against conventional CC2 and MP2 methods.
  • To assess the impact of different partitioning schemes on calculated molecular properties.

Main Methods:

  • Developed BWs-CC2 using a two-parameter size-consistent Brillouin-Wigner (BW-s) partitioning.
  • Assessed BWs-CC2, CC2, MP2, and BW-s2 variants on a 535-element database.
  • Utilized internally stable spin-polarized Hartree-Fock (HF) orbitals and the aug-cc-pVQZ basis set.

Main Results:

  • BWs-CC2 generally outperforms CC2 and BW-s2 with a single parameter choice.
  • CC2 with stable orbitals significantly outperforms MP2 for spin-contaminated systems.
  • BWs-CC2 shows promise for thermochemistry, non-covalent interactions, barrier heights, and isomerization energies.

Conclusions:

  • The BWs-CC2 variant offers a soundly based improvement over conventional CC2.
  • The perception of CC2 ground-state quality should be reevaluated.
  • BWs-CC2 demonstrates improved accuracy, particularly for challenging molecular systems.