Linear-Response Density Cumulant Theory for Excited Electronic States

Area of Science:

• Quantum chemistry
• Theoretical chemistry
• Computational physics

Background:

• Traditional Density Cumulant Theory (DCT) primarily describes single electronic states.
• Accurate calculation of molecular excited states is crucial for understanding chemical processes.
• Existing methods like Equation-of-Motion Coupled Cluster (EOM-CCSD) face challenges with certain excited states.

Purpose of the Study:

• To develop and implement a linear-response formulation of Density Cumulant Theory (DCT).
• To evaluate the performance of the new linear-response DCT method (LR-ODC-12) for calculating excitation energies.
• To compare LR-ODC-12 against established methods like EOM-CCSD for various molecular systems.

Main Methods:

• Derivation of the linear-response DCT formalism.
• Implementation of the LR-ODC-12 method.
• Benchmarking against high-level theoretical calculations (EOM-CCSDT, Heat-Bath Configuration Interaction).

Main Results:

• LR-ODC-12 demonstrates smaller mean absolute errors in excitation energies for small molecules compared to EOM-CCSD.
• The method accurately describes challenging singly and doubly excited states in polyenes (butadiene, hexatriene).
• LR-ODC-12 shows excellent agreement with accurate reference data, outperforming EOM-CCSD for specific excited states.

Conclusions:

• Linear-response DCT provides a balanced and accurate description of multiple electronic states.
• LR-ODC-12 is a promising theoretical tool for studying molecular excited states.
• The developed method offers a viable alternative for accurate electronic structure calculations of excited states.