with γ i being the gyromagnetic ratio of spin i, h the reduced Planck constant, r the dis-tance between the nuclei, and µ 0 the vacuum permeability. Here, the prefactor (2/5) was included in the SDF instead of in the relaxation rate prefactors, as done in several publications. At high static magnetic field strengths, the contribution of the second term in equation 11 decreases (it is usually less than 0.2 s -1 ), and the CCR rate is well approximated by the first term. The second term increases with decreasing S 2 axis . For very flexible methyl groups (S 2 axis < 0.3), CCR rates are small and the contribution of J HH,HH (ω H ) + J HH (2ω H ) to Γ can exceed 10%. Since the value of the CCR rate is determined by internal motion of the methyl group and the overall tumbling of the protein, the extraction of the internal dynamics information can be approximated by