Improving Aufbau Suppressed Coupled Cluster through Perturbative Analysis

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Electronic Excited States

Background:

• Accurate calculation of electronic excited states is crucial for understanding photophysical and photochemical processes.
• Existing methods like equation-of-motion coupled cluster theory face challenges in accuracy and computational cost for certain excitation types.
• Aufbau suppressed coupled cluster (ASCC) theory offers a promising framework but requires further refinement for excited states.

Purpose of the Study:

• To improve the accuracy of Aufbau suppressed coupled cluster theory for various types of electronic excitations.
• To maintain the computational efficiency of the leading-order terms compared to ground-state coupled cluster methods.
• To achieve high accuracy for charge transfer excitations, outperforming established methods.

Main Methods:

• Perturbative analysis to guide the improvement of ASCC theory.
• Development of a spin-adapted and more efficient implementation of ASCC.
• Systematic identification and prioritization of amplitudes for excited state calculations.
• Partial linearization of the theory to mitigate side effects of Aufbau suppression.

Main Results:

• Achieved high accuracy for simple single excitations, multiconfigurational single excitations, and charge transfer excitations.
• Maintained the computational cost of leading-order terms comparable to ground-state coupled cluster.
• Demonstrated a mean unsigned error for charge transfer states that is 0.25 eV lower than equation-of-motion coupled cluster theory.
• Identified key differences in amplitude prioritization between excited-state and ground-state ASCC theory.

Conclusions:

• The enhanced ASCC theory provides a computationally efficient and highly accurate method for calculating electronic excited states.
• The developed approach offers a significant improvement for charge transfer excitations compared to existing state-of-the-art methods.
• The findings highlight the importance of amplitude prioritization and partial linearization for accurate excited-state calculations in ASCC theory.