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Abstract

In this short article, the relationship of the spin‐Hamiltonian parameters occurring in electron paramagnetic resonance spectroscopy with the geometric and electronic structure of molecules is described. Starting from the nonrelativistic many‐particle Hamiltonian and the necessary relativistic extensions, an effective Hamiltonian approach is introduced that leads to closed‐form expressions for all spin‐Hamiltonian (SH) parameters that are correct to second order. The translation into an analytic derivative linear‐response treatment is briefly described. The latter is the method of choice for actual implementations in the framework of quantum chemical approximations. Of the myriad of possible approximations, Hartree–Fock theory, coupled‐cluster theory, and density functional theory are briefly reviewed. To illustrate the subtleties met in calculating SH parameters, the issues related to calculating isotropic and dipolar hyperfine interactions are discussed in some detail. Methods to treat large systems and environment effects are briefly touched before concluding the article with an illustrative example of a combined EPR spectroscopic and quantum chemical study on an enzyme active site.