where the order m corresponds to the three position coordinates defining the dipoles in the lab frame, namely the nuclear spin separation r ij (t) and the polar and azimuthal angles θ ij (t) and ϕ ij (t) with respect to the applied static magnetic field that is parallel to e z . N R and N T are the number of intramolecular and intermolecular degrees of freedom, respectively, where R denotes the rotational and T the translational relaxation modes, see Ref. for more details. A typical assumption is the stochastic independence of rotational and translational molecular diffusion, where the first corresponds to fast molecular reorientations while the latter is due to molecular translations over distances much longer than the molecular sizes . The ensemble average is performed by a double summation over spin pair i and j with i = j. The functions F (m) ij (t) are dependent on the vector r ij which specifies the position of spin j with respect to spin i, and read