Approximating electronically excited states with equation-of-motion linear coupled-cluster theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Canonical equation-of-motion coupled-cluster (EOM-CC) theory is a standard method for calculating electronic excitation energies.
• Perturbative approaches offer a computationally less expensive alternative to high-level CC methods.
• Developing accurate and efficient methods for excited state calculations remains a key challenge in computational chemistry.

Purpose of the Study:

• To introduce a new perturbative approach to canonical equation-of-motion coupled-cluster (EOM-CC) theory.
• To develop novel EOM-CC methods based on linear coupled-cluster doubles (LCCD) and linear coupled-cluster singles and doubles (LCCSD) wavefunctions.
• To benchmark the performance of these new methods against experimental and high-accuracy theoretical data.

Main Methods:

• A second-order Møller-Plesset partitioning of the Hamiltonian was employed.
• The approach yields established equation-of-motion many-body perturbation theory (EOM-MBPT) equations.
• Two new EOM-CC methods were derived utilizing LCCD and LCCSD wavefunctions.

Main Results:

• The proposed methods were tested on 25 small organic molecules.
• Excellent agreement was observed with canonical EOM-CC singles and doubles (EOM-CCSD) for state ordering and relative excitation energies.
• Acceptable quantitative agreement was achieved for absolute excitation energies compared to experimental and best-estimate data.

Conclusions:

• The new perturbative EOM-CC methods provide accurate excited-state properties for organic molecules.
• These methods offer a promising balance between accuracy and computational cost.
• The findings contribute to the advancement of theoretical tools for spectroscopic predictions.