Approximate equation-of-motion coupled-cluster methods for electron affinities of closed-shell molecules

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Accurate calculation of electron affinities (EAs) is crucial for understanding molecular properties.
• Equation-of-motion coupled-cluster methods, like EOM-CCSD, provide high accuracy but are computationally expensive.
• Approximate methods are needed for larger systems and broader applicability.

Purpose of the Study:

• To investigate the performance of various approximate equation-of-motion coupled-cluster (EOM-CCSD) based methods for calculating electron affinities (EAs).
• To identify an economical and reliable method for determining EAs of closed-shell cations and neutral molecules.
• To assess the importance of specific terms in the EOM-CCSD formalism for EA calculations.

Main Methods:

• Evaluation of the equation-of-motion coupled-cluster method at the single and doubles level (EOM-CCSD).
• Assessment of approximate methods including P-EOM-MBPT2 and CIS(D∞).
• Development and testing of a new perturbative correction method, corr-CIS(D∞), based on key EOM-CCSD terms.

Main Results:

• P-EOM-MBPT2 provides reasonable EAs with a mean absolute error < 0.2 eV for molecules without significant multireference character, comparable to EOM-CCSD(2).
• The [[H, a_c ⁺], T₂] term in the 1h2p-1h block is identified as crucial for EA accuracy.
• The proposed corr-CIS(D∞) method yields EAs with excellent agreement (< 0.02 eV difference) with P-EOM-MBPT2, featuring N⁴/N⁵ scaling and OV³ storage.

Conclusions:

• The corr-CIS(D∞) method is an economical and reliable approach for calculating electron affinities.
• This method demonstrates high accuracy comparable to more computationally demanding techniques.
• corr-CIS(D∞) is suitable for application to electron affinities of large molecules.