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Multireference Dynamic Correlation Energy from the Combined Particle-Hole and Particle-Particle Adiabatic Connection

Aleksandra Tucholska1, Katarzyna Pernal2

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|January 14, 2026
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Summary
This summary is machine-generated.

We developed a new adiabatic connection (AC) framework for calculating electron correlation energy in multireference systems. The combined ffAC0 method offers high accuracy and efficiency for diverse chemical challenges.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Accurate calculation of electron correlation energy is crucial for multireference systems.
  • Existing adiabatic connection (AC) formalisms have limitations in capturing correlation effects.
  • Particle-hole (ph) and particle-particle (pp) representations offer distinct perspectives on correlation.

Purpose of the Study:

  • To develop a formally exact adiabatic connection (AC) framework for multireference correlation energy.
  • To derive and implement the particle-particle (pp) AC formalism.
  • To propose and test a combined particle-hole and particle-particle (ph-pp) AC method (ffAC).

Main Methods:

  • Derivation of the ppAC formalism using fermionic operator algebra.
  • Implementation based on the extended random-phase approximation.
  • Development of a combined ffAC method by merging ph and pp correlation amplitudes.

Main Results:

  • The second-order multireference pp correlation energy naturally arises within the ppAC framework.
  • The combined ffAC method successfully incorporates both ph and pp correlation contributions, avoiding double counting.
  • The linearized ffAC0 method demonstrated superior accuracy across various challenging chemical systems, matching or exceeding NEVPT2.
  • ffAC0 is computationally more efficient than NEVPT2, requiring only one- and two-electron reduced density matrices.

Conclusions:

  • The derived ppAC formalism provides a robust framework for multireference correlation.
  • The combined ffAC method offers a balanced and accurate approach to electron correlation.
  • The ffAC0 method represents a significant advancement in computational efficiency and accuracy for multireference correlation energy calculations.