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Excited states from modified coupled cluster methods: Are they any better than EOM CCSD?
Varun Rishi1, Ajith Perera1, Marcel Nooijen2
1Quantum Theory Project, Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA.
The Journal of Chemical Physics
|April 17, 2017
Summary
New equation of motion (EOM) methods extend coupled cluster (CC) approximations to accurately calculate excited states. These modified CC approaches, including 2CC and distinguishable cluster singles and doubles (DCSD), show promise for various molecular systems.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Theoretical Chemistry
Background:
- Coupled cluster (CC) methods, particularly coupled cluster singles and doubles (CCSD), are benchmarks for ground-state electronic structure calculations.
- Simplifications and modifications of CCSD, such as 2CC and distinguishable cluster singles and doubles (DCSD), offer improved efficiency and accuracy for ground-state properties.
- Extending these accurate ground-state methods to excited-state calculations is crucial for understanding molecular photophysics and photochemistry.
Purpose of the Study:
- To establish the theoretical framework for applying simplified ground-state coupled cluster methods to excited-state calculations using the equation of motion (EOM) formalism.
- To evaluate the performance of these novel EOM approaches for various types of excited states, including valence and Rydberg states.
- To assess the applicability of these methods to doubly excited states and their behavior under different molecular geometries.
Main Methods:
- Development of the theoretical foundation for equation of motion (EOM) extensions of parameterized CCSD generalizations and the distinguishable cluster singles and doubles (DCSD) method.
- Application of these novel EOM-CC methods to benchmark sets of singlet and triplet excited states in a diverse range of organic molecules.
- Testing the methods on doubly excited states, using the CH+ cation as a case study at equilibrium and stretched bond lengths.
Main Results:
- The developed EOM extensions of simplified CC methods inherit the exactness for two-electron systems from their ground-state counterparts.
- The methods demonstrate reliable performance across various excited states, including valence and Rydberg excitations in organic molecules.
- Performance evaluation for doubly excited states in CH+ indicates the potential of these methods for describing challenging electronic configurations.
Conclusions:
- The proposed equation of motion extensions of simplified coupled cluster methods provide a viable and potentially more efficient route to accurate excited-state energies.
- These methods offer a promising alternative to standard EOM-CCSD, with potential for consistent performance across different types of excited states.
- Further investigation is warranted to fully establish the advantages and limitations of these modified CC approaches compared to established EOM-CCSD.
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