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Different equation-of-motion coupled cluster methods with different reference functions: the formyl radical
Tomasz Kuś1, Rodney J Bartlett
1Quantum Theory Project, Department of Chemistry, University of Florida Gainesville, Florida 32611, USA. kus@qtp.ufl.edu
The Journal of Chemical Physics
|December 3, 2008
Summary
This study investigates excited states of the formyl radical using equation-of-motion coupled cluster methods. Choosing the correct reference function significantly improves the accuracy of excited state calculations.
Area of Science:
- Quantum Chemistry
- Theoretical Chemistry
- Computational Chemistry
Background:
- The formyl radical (HCO) is a key intermediate in combustion and atmospheric chemistry.
- Accurate theoretical descriptions of its excited states are crucial for understanding chemical reaction dynamics.
- Previous studies have shown limitations in describing excited states with certain computational methods.
Purpose of the Study:
- To investigate the doublet and quartet excited states of the formyl radical.
- To evaluate the performance of different equation-of-motion coupled cluster (EOM-CC) methods.
- To assess the impact of reference function choice on the accuracy of excited state calculations.
Main Methods:
- Equation-of-Motion Coupled Cluster (EOM-CC) methods, including singles and doubles (EOM-EE-CCSD), singles, doubles, and triples (EOM-EE-CCSDT), and spin-flipped singles and doubles (EOM-SF-CCSD).
- Application of these methods with various reference functions: unrestricted Hartree-Fock (HF), restricted open-shell HF, and quasirestricted HF.
- Calculation of structural parameters, vertical and adiabatic excitation energies, and harmonic vibrational frequencies.
Main Results:
- The choice of reference function critically impacts the accuracy of EOM-CC calculations for excited states.
- When target states primarily involve single excitations, both EOM-EE-CCSD and EOM-SF-CCSD provide accurate descriptions.
- For states with significant doubly excited character, EOM-SF-CCSD with an appropriate reference function often outperforms EOM-EE-CCSD.
Conclusions:
- The spin-flipped (SF) method, when coupled with a suitable reference function, offers a robust approach for describing excited states of the formyl radical.
- Careful selection of the reference function is paramount for achieving reliable results in excited state calculations.
- This work provides valuable insights for computational chemists aiming to accurately model excited states of small radicals.
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