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Improving upon CCSD(T): LambdaCCSD(T). I. Potential energy surfaces
Andrew G Taube1, Rodney J Bartlett
1Quantum Theory Project, University of Florida, Gainesville, Florida 32608, USA.
The Journal of Chemical Physics
|February 6, 2008
Summary
Coupled-cluster singles, doubles, and perturbative triples (CCSD(T)) struggles with systems away from equilibrium. A new method, LambdaCCSD(T), significantly improves accuracy for bond breaking and dissociation curves.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Theoretical Chemistry
Background:
- Coupled-cluster singles, doubles, and perturbative triples (CCSD(T)) is a highly accurate quantum chemistry method.
- Standard CCSD(T) exhibits limitations for systems deviating from equilibrium, particularly in bond-breaking scenarios.
Purpose of the Study:
- To develop and implement an improved quantum chemical method, LambdaCCSD(T), to address the shortcomings of CCSD(T) for systems away from equilibrium.
- To evaluate the performance of LambdaCCSD(T) for various bond-breaking processes and compare it with standard CCSD(T).
Main Methods:
- Implementation of LambdaCCSD(T) for open and closed shells with arbitrary single determinant reference functions.
- Application of LambdaCCSD(T) to diverse bond-breaking examples, including diatomic molecules (HF, C2, N2) and H2O.
- Assessment of LambdaCCSD(T) performance using weighted average nonparallelity errors and comparison with CCSD(T).
- Investigation of the concerted transition state of 1,3,5-trinitrohexahydro-1,3,5-triazine (RDX) using both CCSD(T) and LambdaCCSD(T).
Main Results:
- LambdaCCSD(T) substantially enhances the description of systems at long bond lengths compared to CCSD(T).
- Weighted average nonparallelity errors were reduced from 22 mhartree (CCSD(T)) to 10 mhartree (LambdaCCSD(T)) for HF, C2, N2, and H2O.
- LambdaCCSD(T) with a Brueckner reference provides the most accurate single-reference coupled-cluster description of N2 dissociation to date.
- Both CCSD(T) and LambdaCCSD(T) indicate that the RDX transition state is well described, with a high activation barrier suggesting it's not a primary decomposition pathway.
Conclusions:
- LambdaCCSD(T) offers a significant improvement over CCSD(T) for describing bond dissociation and systems away from equilibrium.
- The method maintains the accuracy of CCSD(T) for equilibrium geometries.
- LambdaCCSD(T) represents a valuable advancement in quantum chemical methods for studying chemical reactions and molecular properties.
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