Accurate Electron Affinities from the Extended Koopmans' Theorem Based on Orbital-Optimized Methods.

Area of Science:

• Quantum Chemistry
• Computational Chemistry

Background:

• The extended Koopmans' theorem (EKT) offers a systematic method for calculating electron affinities (EAs) across various theoretical levels.
• While EKT is commonly used for ionization potentials, its application to electron affinity computations remains less explored.
• A comprehensive benchmarking study is needed to evaluate EKT's performance for EA predictions.

Purpose of the Study:

• To conduct the first benchmarking study on the performance of extended Koopmans' theorem (EKT) methods for predicting electron affinities (EAs).
• To assess EKT approaches, including orbital-optimized methods and their standard counterparts, for EA calculations.
• To evaluate the accuracy of EKT for atoms, closed-shell, and open-shell molecules.

Main Methods:

• Application of orbital-optimized methods, specifically orbital-optimized third-order Møller-Plesset perturbation theory and orbital-optimized coupled-electron pair theory (OCEPA(0)).
• Comparison of these orbital-optimized methods with their standard counterparts for EA calculations.
• Utilizing the aug-cc-pVQZ basis set for the OCEPA(0) method.

Main Results:

• The study presents a benchmarking of EKT methods for electron affinity predictions.
• Orbital-optimized methods, including OCEPA(0), were assessed alongside standard EKT approaches.
• The OCEPA(0) method demonstrated highly promising results for the electron affinities of atoms and molecules, with mean absolute errors of 0.14 eV and 0.17 eV, respectively.

Conclusions:

• The extended Koopmans' theorem (EKT) is a viable method for calculating electron affinities (EAs).
• Orbital-optimized coupled-electron pair theory (OCEPA(0)) shows excellent performance for EA predictions.
• The findings support the use of EKT, particularly OCEPA(0), for accurate EA calculations in computational chemistry.