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An "ultimate" coupled cluster method based entirely on T2.

Zachary W Windom1, Ajith Perera1, Rodney J Bartlett1

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This study introduces a new hierarchy of coupled cluster methods using only double-electron excitations (T2) to efficiently model complex electron correlations in molecules. These methods offer accurate results comparable to more complex calculations with reduced computational cost.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Coupled cluster methods are essential for accurate electronic structure calculations.
  • Existing methods struggle with diverse electron correlation types.
  • Higher-rank excitations (T2m) improve accuracy but increase computational cost.

Purpose of the Study:

  • Develop a computationally efficient "ultimate" coupled cluster method based on T2 operators.
  • Emulate the accuracy of methods including higher-rank excitations.
  • Systematically account for higher-order electron correlations.

Main Methods:

  • Introduced a hierarchy of augmented T2 methods.
  • Utilized factorization theorem of many-body perturbation theory (MBPT).
  • Applied expectation-value coupled cluster theory.

Main Results:

  • Methods are correct through a specified order in MBPT.
  • Hierarchy reduces to standard Coupled-Dipole- গ্রহণযোগ্য (CCD) for dynamic correlations.
  • Improved accuracy for systems with non-dynamic and static correlations.

Conclusions:

  • The proposed augmented T2 methods provide a computationally efficient alternative.
  • Cheapest methods emulate O(N10) CCDQ accuracy with O(N6) cost.
  • Effective for diverse electron correlation regimes in molecules.