Electric Field Dependence of EPR Hyperfine Coupling Constants

In this article, we introduce a simple but reliable method to calculate the electric field dependence of the isotropic hyperfine coupling tensor A_iso for free radicals. This dependence, also referred to as the Bloembergen effect, can be of interest in analyzing EPR experiments for solid-state materials but is rarely studied for isolated radicals in the gaseous phase. The proposed method uses the numerical differentiation of the field-perturbed A tensor and, consequently, as a purely numerical method, does not depend on a quantum chemical method used to determine the hyperfine tensor A. To test the performance of the proposed method, we used a set of 28 systems, including seven organic radicals, the majority of which were taken from a recent benchmark study by Bartlett's group. We employed the well-tested and robust density functional theory (DFT) functional, namely CAM-B3LYP, and the variant of the CCSD method based on local pair natural orbitals, namely DLPNO-CCSD to calculate the A_iso tensor itself, and all first derivatives or Bloembergen effect constants a⁽¹⁾.