Capturing static and dynamic correlation with ΔNO-MP2 and ΔNO-CCSD.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurately describing electron correlation is crucial for predicting molecular properties.
• Static correlation (ΔNO method) and dynamic correlation (MP2, CCSD) require distinct treatments.
• Existing methods often struggle to efficiently combine these correlation types.

Purpose of the Study:

• To develop a unified computational method for both static and dynamic electron correlation.
• To improve the accuracy of electronic structure calculations for molecular systems.
• To provide a computationally feasible approach for complex chemical problems.

Main Methods:

• Combining the ΔNO method for static correlation with Møller-Plesset perturbation theory (MP2) and coupled-cluster singles and doubles (CCSD) for dynamic correlation.
• Adapting finite-temperature CCSD expressions, including orbital occupancies and vacancies.
• Incorporating damping factors into MP2 and CCSD residual equations to partition correlation effects.

Main Results:

• The developed method accurately calculates potential energy curves for diatomic molecules.
• Results show good agreement with extrapolated full configuration interaction (FCI) data.
• Performance is comparable to conventional multireference approaches.

Conclusions:

• The combined ΔNO-MP2/CCSD method offers a robust way to include both static and dynamic correlation.
• This approach provides accurate electronic structure data for molecular systems.
• The method is a promising alternative to traditional multireference techniques for studying chemical reactivity and spectroscopy.