Recoupling the singlet- and triplet-pairing channels in single-reference coupled cluster theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Single-reference coupled cluster (CC) theory accurately describes dynamical correlation but fails for strongly correlated systems.
• Restricted CC methods often break down with strong correlation, necessitating alternative approaches like symmetry breaking or multi-reference methods.
• Simplifying the T2 operator or doubles amplitude equations has shown promise in improving CC performance for strong correlation.

Purpose of the Study:

• To investigate the coupling between singlet- and triplet-pairing channels within the T2 operator in coupled cluster theory.
• To develop a method for recoupling these channels to restore dynamical correlation without compromising accuracy in strongly correlated systems.
• To analyze the behavior of this recoupling strategy across different correlation regimes (weak vs. strong).

Main Methods:

• Exploration of the coupling between singlet- and triplet-pairing channels in the T2 operator.
• Development and application of a recoupling strategy to recover dynamical correlation.
• Assessment of the method's performance in both weakly and strongly correlated systems.

Main Results:

• In weakly correlated systems, the singlet- and triplet-pairing channels exhibit weak coupling, and simple recoupling yields accurate results.
• Strong correlation significantly increases the coupling strength between singlet- and triplet-pairing channels.
• Perturbatively recoupling these channels becomes challenging in the strongly correlated regime due to this increased coupling.

Conclusions:

• Singlet-paired coupled cluster offers qualitative improvements for strong correlation but sacrifices dynamical correlation accuracy.
• Recoupling singlet and triplet pairing channels is a promising avenue for recovering dynamical correlation in coupled cluster theory.
• The effectiveness of perturbative recoupling is limited by the strong coupling between channels in highly correlated systems, posing an ongoing theoretical challenge.