Combining Complex Conjugation, Time-Reversal, and Spin-Flip Symmetry Projection of Coupled Cluster Wave Functions.

Area of Science:

• Quantum chemistry
• Computational physics
• Theoretical chemistry

Background:

• Static correlation is crucial for accurately describing chemical bonds and molecular properties.
• Traditional methods like Hartree-Fock struggle to capture static correlation.
• Symmetry breaking and restoration offer a potential pathway to include static correlation.

Purpose of the Study:

• To investigate the use of complex conjugation symmetry breaking and restoration for static correlation.
• To evaluate the performance of symmetry-projected coupled cluster theory.
• To explore the recovery of time reversal and point group symmetries.

Main Methods:

• Hartree-Fock (HF) calculations with complex conjugation symmetry breaking.
• Spin-symmetry broken coupled cluster (CC) methods.
• Symmetry projection techniques (spin flip, time reversal, point group).

Main Results:

• Two nonorthogonal HF configurations naturally capture static correlation.
• Symmetry-projected CC wave functions show excellent agreement with full configuration interaction (FCI).
• Demonstrated accuracy in beryllium hydride insertion, lithium fluoride dissociation, and H4 symmetric stretching.
• Successfully recovered time reversal and point group symmetries.

Conclusions:

• Complex conjugation symmetry breaking is an effective strategy for static correlation.
• Symmetry-projected coupled cluster theory provides accurate and robust electronic structure calculations.
• This approach offers a promising route for treating strongly correlated systems in quantum chemistry.