18 EPR Properties
The electron paramagnetic resonance (EPR) spectra are characterized by the electron–nucleus hyperfine coupling (HFC) and the g-tensor. For systems with a spin \(S > 1/2\), the zero-field splitting (ZFS) tensor is important as well. Additionally, the electric field gradient is used to compute the nuclear quadrupole interaction tensor. These quantities are calculated with the modules dscf, ridft, and mpshift. The HFC and g-tensor are also used for the contact and the pseudo-contact term of paramagnetic NMR shielding constants and shifts. Here, we describe the computational work flow, while we refer to Refs. [337], [85], [187], [188], and [338] for the theory.
The HFC, the g-tensor, and the EFG are available within a non-relativistic or scalar-relativistic one-component framework and a spin–orbit two-component framework. For the g-tensor and the paramagnetic spin–orbit part of HFC tensor, spin–orbit perturbation theory is applied in the one-component framework. The ZFS tensor is only available within a non- or scalar-relativistic one-component framework using spin–orbit perturbation theory. Effective core potentials (ECPs) should not be used to compute the HFC, ZFS, or g-tensor, as this may lead to unphysical results due to the lack of explicit core electrons.
If you are interested in all EPR parameters, we recommend to use the flag $epr and the mpshift module. In a one-component framework, this requires three linear response frameworks, i.e. the same-spin spin–orbit and spin-flip spin–orbit perturbations for the HFC and ZFS tensors as well as the magnetic field treatment for the g-tensor. For convenience, the EPR calculations support a restart option at all stages. With the two-component version, this requires to run three non-collinear SCF calculations as described below. In case of a two-component calculation, the ZFS tensor is not calculated. Note that the m-grids such as gridsize m3 should never be used for EPR calculations. Please make sure to always use the full grid for the SCF iterations, i.e. gridsize 3 or 3a.