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Bridging Single- and Multireference Domains for Electron Correlation: Spin-Extended Coupled Electron Pair

Takashi Tsuchimochi1, Seiichiro Ten-No1

  • 1Graduate School of Science, Technology, and Innovation, and ‡Graduate School of System Informatics, Kobe University , Kobe, Hyogo 657-0025, Japan.

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We introduce a size-consistent spin-extended configuration interaction method that improves upon ECISD for quantum chemistry calculations. This approach enhances accuracy for molecular properties and geometric optimization by incorporating quadruple excitations.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Accurate calculation of molecular properties requires advanced quantum chemical methods.
  • Spin-extended configuration interaction with singles and doubles (ECISD) is a recent development.
  • Size-consistency is crucial for reliable theoretical predictions in quantum chemistry.

Purpose of the Study:

  • To develop a size-consistent generalization of spin-extended configuration interaction with singles and doubles (ECISD).
  • To incorporate the effects of quadruple excitations within a coupled electron pair approximation framework.
  • To enable accurate determination of molecular properties and geometric optimization for challenging systems.

Main Methods:

  • A size-consistent generalization of ECISD is proposed, treating quadruples via coupled electron pair approximation.
  • The method utilizes an orbital-invariant energy functional minimized by diagonalizing an effective Hamiltonian.
  • Analytical gradients are derived for efficient geometric optimization and property calculations.

Main Results:

  • The proposed methods generally yield improved results compared to standard ECISD.
  • Size-consistency is approximately retained, though not perfectly due to symmetry-projection operators.
  • The approach demonstrates capability in handling static correlation, suitable for multireference systems.

Conclusions:

  • The developed size-consistent spin-extended CI methods offer a promising advancement in quantum chemistry.
  • These methods provide a robust framework for studying molecular dissociation, electronic properties, and bonding.
  • Further investigation into size-consistency and size-extensivity is warranted for these novel approaches.