On the definition of local spin in relativistic and nonrelativistic quantum chemistry

In this work, a survey is given on the definition of electron spin as a local property in nonrelativistic and relativistic quantum chemistry and in wave-function- and density-based theories. In particular, the transition from four-component theory to the two- and one-component Douglas-Kroll-Hess framework with respect to the electronic density is considered. Local spin is an important concept in chemistry comparable to the partial charge concept. It is often applied in transition metal chemistry and especially needed for the description and understanding of the electronic structure of spin-spin interactions in polynuclear clusters. The relevance of spin contamination and the effect of the approximate nature of contemporary density functionals is discussed in the light of results obtained for dinuclear manganese and rhenium model clusters.