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Extended Koopmans' theorem at the second-order perturbation theory.
Yonghao Gu1, Xin Xu1
1Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Ministry of Education Key Laboratory of Computational Physical Sciences, Department of Chemistry, Fudan University, Shanghai, China.
Journal of Computational Chemistry
|February 1, 2020
Summary
This study introduces a novel scheme to fix the negative occupation problem in extended Koopmans
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Theoretical Chemistry
Background:
- The extended Koopmans' theorem (EKT) combined with second-order Møller-Plesset (MP2) perturbation theory offers a direct method for calculating ionization potentials (IPs).
- A significant limitation of the EKT-MP2 approach is the 'negative occupation problem', which can lead to incomplete or unphysical IP spectra.
- Understanding the contributions of relaxation and correlation effects to IP calculations is crucial for accurate theoretical predictions.
Purpose of the Study:
- To address and resolve the negative occupation problem encountered in EKT-MP2 calculations for ionization potentials.
- To provide a deeper physical interpretation of the EKT-MP2 method by dissecting relaxation and correlation contributions.
- To establish a connection between EKT-MP2 and derivative-based MP2 methods for ionization potentials.
Main Methods:
- Implementation of a 'small positive number scheme' to correct the negative occupation problem in EKT-MP2.
- Introduction of a new Koopmans-type quantity, KT-MP2, to analyze relaxation and correlation effects.
- Comparison of EKT-MP2 with the derivative approach of MP2 energy with respect to orbital occupation numbers.
Main Results:
- The proposed scheme effectively eliminates unphysical results arising from the negative occupation problem.
- The KT-MP2 quantity successfully distinguishes the roles of relaxation and correlation in ionization potential calculations.
- A clear relationship between EKT-MP2 and MP2 energy derivative methods is established, offering complementary insights.
Conclusions:
- The modified EKT-MP2 method provides a robust and physically meaningful approach for calculating ionization potentials.
- The study enhances the understanding of electron ionization processes by clarifying the impact of relaxation and correlation.
- The developed methods offer valuable tools for theoretical investigations of electronic properties in atoms and molecules.
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