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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry

Background:

  • Multireference coupled cluster (MRCC) theories are crucial for describing complex electronic structures.
  • Approximate MRCC methods often require corrections to accurately capture dynamic correlation.
  • Quasidegenerate electronic situations pose challenges for standard quantum chemical methods.

Purpose of the Study:

  • To develop a new asymmetric energy functional for multireference coupled cluster (MRCC) theories.
  • To define non-iterative corrections for approximate MRCC methods in quasidegenerate systems.
  • To encapsulate dynamic correlation within a correlation Hamiltonian for improved accuracy.

Main Methods:

  • An energy expansion approach for the exact electronic energy in quasidegenerate situations.
  • Definition of a correlation Hamiltonian within the model space (M(0)).
  • Parametrization of wavefunctions to ensure connected forms of the correlation Hamiltonian and size-extensive energies.

Main Results:

  • The proposed framework allows dynamic correlation to be captured by the correlation Hamiltonian.
  • A universal correction applicable to various approximate MRCC methods is derived.
  • Analogies with existing MRCC triples corrections, including Λ-Mk-MRCCSD(T), are established.

Conclusions:

  • The new energy functional provides a robust way to improve approximate MRCC theories.
  • The correlation Hamiltonian effectively incorporates dynamic correlation effects.
  • The method offers potential for parallel implementation due to independent reference-related contributions.