A power series approximation in symmetry projected coupled cluster theory

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Projected Hartree-Fock (PHF) theory excels at strong electron correlations but struggles with weak correlations.
• Single-reference methods like configuration interaction and coupled cluster theory handle weak correlations but fail with strong ones.
• Combining these methods into a unified symmetry-projected coupled cluster approach is theoretically challenging.

Purpose of the Study:

• To develop an alternative formulation for combining the strengths of PHF and single-reference methods.
• To identify and isolate 'disentangled cluster operators' that bridge the gap between strong and weak correlation descriptions.
• To provide a more accurate and versatile theoretical framework for electronic structure calculations.

Main Methods:

• Developed a novel theoretical formulation to identify disentangled cluster operators.
• Applied the formulation to model systems and small molecules for validation.
• Focused on overcoming the difficulties in integrating different correlation treatment strategies.

Main Results:

• The proposed formulation successfully identifies disentangled cluster operators.
• Demonstrated promising accuracy in describing electronic correlations in test systems.
• The method offers a potential pathway to bridge the limitations of existing quantum chemical theories.

Conclusions:

• The new formulation provides a viable strategy for unifying strong and weak correlation treatments.
• The identified disentangled cluster operators are key to this unification.
• Further development and application to larger systems are warranted.