Perspective on Coupled-cluster Theory. The evolution toward simplicity in quantum chemistry.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Electron correlation has been a central challenge in quantum chemistry for over 60 years.
• Traditional methods struggle to accurately account for electron correlation, limiting predictive accuracy.
• Coupled-cluster (CC) theory has emerged as a leading computational approach.

Purpose of the Study:

• To highlight the conceptual advancements of coupled-cluster (CC) theory.
• To emphasize the 'evolution toward simplicity' in CC theory and its extensions.
• To underscore the importance of CC theory for achieving predictive quality in quantum chemistry.

Main Methods:

• Review of coupled-cluster (CC) theory as a many-body method.
• Discussion of its application to solve the Schrödinger and Dirac equations.
• Consideration of basis set convergence and higher cluster excitations.

Main Results:

• CC theory effectively addresses electron correlation, enabling highly accurate *ab initio* calculations.
• It provides predictive quality results for various chemical properties.
• The theory offers a conceptually new, many-body foundation for quantum chemistry.

Conclusions:

• Coupled-cluster theory has revolutionized quantum chemistry by providing a robust framework for electron correlation.
• Its numerical performance and conceptual simplicity make it invaluable for predictive theoretical chemistry.
• Equation-of-motion coupled-cluster (EOM-CC) extensions further enhance its capabilities.