The interaction between the spin magnetic moment of an unpaired electron and the nuclear spin magnetic moments resulting in the splitting of the α (spin up) and β (spin down) energy levels in an external magnetic field and, thus, in the multiplet pattern of the ESR spectra of radical-like species and transition metal compounds. Two main contributions to the hyperfine @[email protected] are usually considered, @[email protected] contact and dipolar interactions. The contact interaction is @[email protected] and related to the unpaired @[email protected] at the @[email protected], \(|\psi _{\unicode[Times]{x3C3}}|^{2}\). The dipolar interaction is anisotropic, and related to \(r^{-3}\), where \(r\) is the distance between the atom holding the unpaired electron and the @[email protected] with non-zero spin.