Varying the fraction of orbital exchange in density functional theory: Influence on nuclear magnetic resonance shielding constants

Abstract

A series of hybrid exchange-correlation functionals containing varying fractions of orbital exchange ξ=0.0, 0.1, 0.2, … 1.0, are determined using conventional molecular thermochemical fits. The functionals are used to determine Kohn–Sham nuclear magnetic resonance shielding tensors for a series of small molecules involving first- and second-row atoms; results are compared with experimental values. On average, isotropic and anisotropic shieldings determined using the conventional coupled approach become progressively less accurate as ξ increases from 0.0 to 1.0. By contrast, isotropic and anisotropic shieldings determined from the hybrid Kohn–Sham densities using the uncoupled multiplicative Kohn–Sham (MKS) approach [Chem. Phys. Lett. 337, 341 (2001)] improve significantly as ξ increases from 0.0 to 0.2–0.3; optimal results are more than three times as accurate as the corresponding coupled results. As ξ is further increased, the MKS results degrade. The quality of the Kohn–Sham highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) eigenvalue difference in the MKS calculations is investigated by comparing it with values determined from coupled cluster Brueckner doubles densities. In line with the shielding observations, optimal HOMO–LUMO differences are obtained near ξ=0.3.