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Is Externally Corrected Coupled Cluster Always Better Than the Underlying Truncated Configuration Interaction?
Ilias Magoulas1, Karthik Gururangan1, Piotr Piecuch1,2
1Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States.
Journal of Chemical Theory and Computation
|June 23, 2021
Summary
Externally corrected coupled-cluster (ec-CC) methods do not always improve truncated configuration interaction (CI) calculations. A new ec-CC approach using selected CI (CIPSI) amplitudes with moment expansions is introduced for better accuracy.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Theoretical Chemistry
Background:
- Configuration interaction (CI) and coupled-cluster (CC) methods are fundamental in quantum chemistry for describing electron correlation.
- Externally corrected (ec-CC) methods aim to improve upon truncated CI or CC wave functions by incorporating information from other methods.
- The effectiveness of ec-CC, particularly with specific cluster operator components (T3 and T4), on truncated CI remains an area of active research.
Purpose of the Study:
- To investigate the limitations of externally corrected coupled-cluster (ec-CC) approaches when applied to truncated configuration interaction (CI) wave functions.
- To identify specific classes of truncated CI wave functions where ec-CC, using T3 and T4 components, fails to provide improvements.
- To introduce a novel ec-CC method that addresses limitations in selected CI calculations.
Main Methods:
- Analysis of truncated configuration interaction (CI) wave functions.
- Application of externally corrected coupled-cluster (ec-CC) methods utilizing three-body (T3) and four-body (T4) cluster components.
- Development and implementation of a new ec-CC approach based on selected CI (CIPSI) amplitudes.
- Utilizing moment expansions to correct for missing correlations in the novel ec-CC method.
Main Results:
- Identified specific classes of truncated CI wave functions where ec-CC with T3 and T4 components does not enhance results.
- Demonstrated through numerical examples the implications for ec-CC computations with truncated and selected CI methods.
- Introduced a novel ec-CC approach using CIPSI-derived T3 and T4 amplitudes.
Conclusions:
- The standard externally corrected coupled-cluster (ec-CC) approach is not universally beneficial for all truncated configuration interaction (CI) calculations.
- A new ec-CC method, leveraging selected CI (CIPSI) and moment expansions, offers a promising avenue for more accurate energy calculations.
- Further research is warranted to fully explore the capabilities of the novel ec-CC approach in quantum chemical applications.
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