Calculations of the EPR <b>g</b>-tensor using unrestricted two- and four-component relativistic approaches within the HF and DFT frameworks

Approaches and programs for calculations of the EPR <b>g</b>-tensor in the framework of the two- and four-component methods are still very rare. There are three main reasons for this: the wider community's unawareness of the importance of second- and higher order spin–orbit effects on the <b>g</b>-tensor, the methodological problems associated with performing such calculations and the lack of understanding of these problems. This paper reports on the implementation of a method for calculation of the <b>g</b>-tensor in the framework of the relativistic unrestricted two- and four-component Hartree–Fock and density functional theory approaches based on the Kramers pair formalism. This implementation allows us to analyse problems which arise when the <b>g</b>-tensor is calculated via Kramers pairs in the unrestricted framework.