Approximating CCSD(T) Nuclear Magnetic Shielding Calculations Using Composite Methods.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Accurate calculation of nuclear magnetic resonance (NMR) shieldings is crucial for molecular structure determination.
• High-level computational methods like coupled cluster singles and doubles with perturbative triples (CCSD(T)) provide accurate NMR shieldings but are computationally expensive.
• Development of efficient approximations is needed to make these calculations more accessible.

Purpose of the Study:

• To develop and evaluate composite method approximations for computationally efficient calculation of NMR shieldings.
• To assess the convergence of the pcS-n basis set series towards experimental gas-phase shieldings at the CCSD(T) level.
• To investigate the accuracy of using simpler methods (HF, B3LYP, KT3, MP2) to approximate CCSD(T) NMR shieldings.

Main Methods:

• Utilized composite method approximations with the pcS-n basis set series.
• Performed calculations at various levels of theory, including Hartree-Fock (HF), B3LYP, KT3, and Møller–Plesset perturbation theory (MP2).
• Compared results against coupled cluster singles and doubles with perturbative triples (CCSD(T)) calculations and experimental gas-phase shieldings.

Main Results:

• The pcS-n basis sets demonstrate rapid convergence toward experimental gas-phase NMR shieldings at the CCSD(T) level.
• Approximations using HF and MP2 significantly reduce computational time (CPU time).
• The combination of HF and MP2 methods provides a minimal impact on the accuracy of predicted NMR shieldings compared to CCSD(T).

Conclusions:

• Composite method approximations, particularly using HF and MP2, offer an efficient and accurate strategy for calculating NMR shieldings.
• This approach balances computational cost and accuracy, making advanced NMR shielding calculations more feasible.
• The findings facilitate broader application of computational NMR spectroscopy in chemical research.