Benchmarking the Accuracy of the Direct Random Phase Approximation and σ-Functionals for NMR Shieldings

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Nuclear Magnetic Resonance (NMR) shielding calculations are crucial for understanding molecular structure and dynamics.
• Accurate and computationally efficient methods for predicting NMR shieldings are highly sought after in chemistry.
• Density Functional Theory (DFT) approximations, including random phase approximation (RPA) and σ-functionals, offer potential for cost-effective calculations.

Purpose of the Study:

• To present and evaluate a finite-difference based method for computing NMR shieldings using direct random phase approximation (RPA).
• To assess the accuracy of the direct RPA method by comparing it against high-quality coupled cluster (CC) reference data.
• To investigate the influence of DFT reference orbitals and the amount of exact Hartree-Fock (HF) exchange on the accuracy of computed NMR shieldings.

Main Methods:

• Implementation of a finite-difference approach for direct random phase approximation (RPA) calculations of NMR shieldings.
• Benchmark calculations comparing direct RPA results with high-accuracy coupled cluster (CC) data.
• Analysis of the impact of varying density functional theory (DFT) reference orbitals and Hartree-Fock (HF) exchange content on shielding accuracy.
• Examination of basis set convergence for reliable NMR shielding predictions.

Main Results:

• The accuracy of direct RPA NMR shielding calculations is highly sensitive to the choice of DFT reference orbitals.
• Incorporating more exact Hartree-Fock (HF) exchange into the DFT functional significantly improves the accuracy of direct RPA shieldings.
• Direct RPA calculations using a Hartree-Fock reference yield NMR shieldings that are more accurate than MP2 and comparable to CCSD results.
• Reliable NMR shielding predictions necessitate the use of at least triple-zeta (TZ) basis sets.

Conclusions:

• The direct RPA method, particularly with a Hartree-Fock reference, provides a computationally efficient and accurate approach for NMR shielding calculations.
• The findings highlight the importance of selecting appropriate DFT reference orbitals and incorporating exact exchange for high-fidelity NMR shielding predictions.
• The study establishes that direct RPA with HF reference orbitals offers a viable alternative to more computationally expensive methods like CCSD for obtaining accurate NMR shieldings, provided sufficient basis sets are employed.