Expressions are presented for calculation of the matrix elements of the energy operator for the spinspin interaction (direct and exchange terms in the single-configuration approximation) in the LSJM representation (the LS-coupling scheme) and in the representation of uncoupled angular momenta. The formulas for the case of permutation of electrons (the fp configuration) are discussed in detail. The matrix elements for the electron pf configuration are derived in both representations, while the matrix elements for the hole p⁵f configuration are obtained in the uncoupled angular momentum representation only. All the calculations are performed in the formalism of irreducible tensor operators. The spin-spin interaction is represented in the two-electron energy matrix by four radial integrals of the Marvin spin interactions: by three direct integrals S₁, S₂, and S¹₂ and one exchange integral S₃.