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

A method for computing NMR shieldings with the direct random phase approximation (RPA) and the closely related sigma-functionals [Trushin, E.; Thierbach, A.; Gorling, A. Toward chemical accuracy at low computational cost: density functional theory with sigma-functionals for the correlation energy. J. Chem. Phys. 2021, 154, 014104] is presented, which is based on a finite-difference approach. The accuracy is evaluated in benchmark calculations using high-quality coupled cluster values as a reference. Our results show that the accuracy of the computed NMR shieldings using direct RPA is strongly dependent on the density functional theory reference orbitals and improves with increasing amounts of exact Hartree-Fock exchange in the functional. NMR shieldings computed with direct RPA using a Hartree-Fock reference are significantly more accurate than MP2 shieldings and comparable to CCSD shieldings. Also, the basis set convergence is analyzed and it is shown that at least triple-zeta basis sets are required for reliable results.

Automated summary

A method for computing NMR shieldings using direct RPA is presented, with accuracy depending on the DFT reference orbitals and amount of Hartree-Fock exchange. Results show that shieldings with Hartree-Fock reference are more accurate than MP2 and comparable to CCSD shieldings. At least triple-zeta basis sets are required for reliable results in basis set convergence analysis.