The DFT calculations of the spin–spin coupling constants have been carried out using the approach of Helgaker, Watson and Handy [

14]. In particular, the sum-over-state contributions to the spin–spin coupling constants are obtained analytically, using the linear-response formalism [

14]. More-over, unlike the original implementations of Malkina, Salahub and Malkin [

11,

12] and of Dickson and Ziegler [

15], in which only the local-density approximation (LDA) and the generalized-gradient approximation (GGA) can be used, the implementation of Helgaker and coworkers allows for the use of the exact Hartree–Fock exchange. The latter is an important point since the use of hybrid functionals such as B3LYP improves considerably the calculated spin–spin coupling constants [

14]. Accordingly, all DFT results presented here have been obtained with the Becke three-parameter Lee–Yang–Parr (B3LYP) functional [

17,

18]. The DFT calculations of spin–spin coupling con-stants have been carried out using an experimental version of the dalton program [

19]. For details on the DFT implementation, see Ref. [

14]. The calculations of the spin–spin coupling constants at the MCSCF level [

5,

20] have been carried out by means of dalton 1.2 [

19]. The restricted active-space (RAS) approach has been employed, with the nonhydrogen 1

s orbitals in the inactive subspace and with the seven Hartree–Fock-occupied valence orbitals in the RAS2 sub-space. While the RAS-0 wave function contains in the RAS3 subspace 20 orbitals (for CH

_{3}CH

_{3}) or 19 orbitals (for CH

_{3}NH

_{2} and CH

_{3}OH), the RAS-I wave function contains 30 (for CH

_{3}CH

_{3}) or 27 orbitals (for CH

_{3}NH

_{2} and CH

_{3}OH) in the RAS3 subspace. The RAS1 subspace is empty and a maximum of two electrons are allowed to be excited from RAS2 to RAS3. This construction of the MCSCF active space has previously been shown to provide good results for spin–spin coupling constants [

21,

22]. Some of the MCSCF results have already been reported in Refs. [

21,

22]. The CCSD spin–spin calculations have been carried out using linear-response theory—see Ref. [

23] and references therein—as implemented in a program based on Aces II [

24]. For a description of CCSD second derivatives in the unrestricted Hartree–Fock framework, see Ref. [

25]. In the calculations presented here, all four terms—Fermi contact (FC), spin–dipole (SD), paramagnetic spin-orbit (PSO) and diamagnetic spin–orbit (DSO)—that contribute to nuclear spin–spin cou-pling constants in nonrelativistic theory have been calculated. The reported coupling constants have been averaged assuming free internal molecular rotation, except when the dependence on the dihedral angle is discussed. The experimental equilibrium geometry [

26,

27,

28] has been employed. The Karplus curves have been obtained changing only the dihedral angle, i.e. without taking into account geometry relaxation. It has been demonstrated before [

22] that for the

^{3}J(HH) couplings in the moleculea under study the results obtained with and without geometry relaxation are close to each other. The nitrogen coupling constants are given for isotope

^{15}N. All calculations have been carried out in the IGLO-III basis [

29]. Although not large, this basis should be sufficient for a qualitative prediction of spin–spin coupling constants, at least for the DFT calculations. How-ever, for the MCSCF and in particular the CCSD calculations, in which dynamical correlation is described by means of virtual excitations, this basis is certainly not sufficiently large to recover the full effects of electron correlation. We therefore expect the CCSD calculations to be further away from the basis-set limit than are the DFT calculations. In addition, we have ignored the effect of triple excitations in the MCSCF and coupled-cluster calculations. Although we do not claim that the presented MCSCF and CCSD spin–spin couplings are close to the true nonrelativistic results, they are representative of the best spin–spin calculations that can nowadays be carried out for molecules of this size.