Complex absorbing potentials within EOM-CC family of methods: theory, implementation, and benchmarks.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Metastable electronic states, such as shape resonances, are crucial in understanding molecular interactions and reactions.
• Traditional quantum chemical methods often struggle to accurately describe these short-lived, unstable states.
• The equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method is a powerful tool for calculating electronic excitation and attachment energies, but its application to metastable states is challenging.

Purpose of the Study:

• To present a production-level implementation of EOM-CCSD augmented with a complex absorbing potential (CAP) for the study of electron attachment and excitation energies.
• To enable the treatment of metastable electronic states within the EOM-CC formalism, similar to how bound states are handled.
• To investigate the numerical performance and sensitivity of resonance properties to CAP parameters and basis set choice.

Main Methods:

• Development and implementation of a complex absorbing potential (CAP) augmented equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method.
• Investigation of the method's numerical performance, including resonance positions and lifetimes.
• Evaluation of the sensitivity of results to CAP parameters and the selection of one-electron basis sets.
• Establishment of a protocol for studying molecular shape resonances using standard basis sets and a universal CAP parameter criterion.

Main Results:

• The CAP-augmented EOM-CCSD method successfully treats metastable electronic states, providing accurate electron attachment and excitation energies.
• The study characterized the sensitivity of resonance positions and lifetimes to CAP parameters and basis set choices.
• A reliable protocol for studying molecular shape resonances was developed and validated.
• The method demonstrated competitive performance compared to other theoretical approaches for resonance calculations.

Conclusions:

• CAP-augmented EOM-CCSD is a robust and accurate method for investigating molecular shape resonances.
• The developed protocol offers a practical approach for applying this method using standard computational chemistry tools.
• The findings suggest that this method is capable of reproducing experimental results for various molecular systems.