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

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.