Linear-response theory for Mukherjee's multireference coupled-cluster method: excitation energies

Area of Science:

• Computational Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of excitation energies is crucial for understanding molecular properties and reactions.
• Multireference character in ground states poses challenges for standard single-reference quantum chemical methods.

Purpose of the Study:

• To employ the linear-response function for Mukherjee's multireference coupled-cluster method (Mk-MRCC) for calculating vertical excitation energies.
• To assess the impact of multireference ground-state wavefunctions on excitation energies.
• To compare Mk-MRCC results with single-reference coupled-cluster (SR-CC) calculations.

Main Methods:

• Utilized the linear-response (LR) function within the singles and doubles approximation of the Mukherjee's multireference coupled-cluster method (Mk-MRCCSD-LR).
• Calculated vertical excitation energies for ozone and benzynes (o-, m-, p-benzyne).
• Compared results with SR-CC (singles and doubles, and singles, doubles, and triples approximations).

Main Results:

• Obtained vertical excitation energies for ozone and a series of benzynes with increasing multireference character.
• Demonstrated the influence of multireference ground-state wavefunctions on calculated excitation energies.
• Identified artificial splitting of excited states in Mk-MRCC theory due to intrinsic redundancy.

Conclusions:

• The Mk-MRCCSD-LR method provides a valuable tool for studying excitation energies in systems with multireference character.
• Comparison with SR-CC methods highlights the importance of accounting for multireference effects.
• The observed artificial state splitting in Mk-MRCC-LR requires careful consideration for straightforward application.