Multireference state-specific coupled-cluster methods. State-of-the-art and perspectives

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Accurate quantum mechanical calculations are crucial for understanding atomic and molecular systems.
• Quasidegenerate states, requiring multiple determinants, pose significant challenges for traditional computational methods.
• Coupled-cluster (CC) theory is a powerful method for electron correlation but struggles with quasidegenerate states.

Purpose of the Study:

• To review state-specific multireference coupled-cluster (CC) approaches.
• To address the challenges of applying CC theory to quasidegenerate ground and excited states.
• To present methods for accurate, size-extensive, and routine calculations on small to medium-size molecular systems.

Main Methods:

• Focus on state-specific multireference coupled-cluster (CC) theories.
• Development of approximate CC methods for quasidegenerate systems.
• Utilizing single CC calculations to focus on a specific state of the system.

Main Results:

• State-specific CC approaches provide a viable solution for high-level quantum mechanical calculations on quasidegenerate states.
• These methods enable accurate description of electron correlation effects in complex wavefunctions.
• The reviewed approaches are designed for routine application to small and medium-size molecular systems.

Conclusions:

• State-specific multireference CC methods are effective for studying quasidegenerate states in atomic and molecular systems.
• These techniques balance accuracy, size-extensivity, and computational simplicity.
• The reviewed methods facilitate routine high-level quantum mechanical investigations of challenging electronic structures.