CCR rates are governed by the cross-TCF, which is modulated by the geometry of the methyl group. For illustration, Figure shows the internal TCFs and corresponding spectral densities of two selected methyl groups with markedly different dynamics, ILE61-C γ,2 and ILE61-C δ (the methyl axis order parameters are given in Figure and Table in the supporting information). At very short picosecond time scales, the cross-TCF is anti-correlated due to the 109.5°angle between the two C-H bond vectors in the methyl group. At longer times, the two ILE61 methyl groups display a different dynamic behavior. The cross-TCF of the ILE61-C γ,2 methyl group shows a rapid increase (decorrelation) within less than 100 ps, then it reaches a constant plateau. ILE61-C γ,2 thus appears to have dynamics that occur on a single time scale, which can be described with a single exponential function (the LS2 model, equation 7) with a time constant of ca. 30 ps (Figure ). In contrast, ILE61-C δ has more complicated dynamics (Figure ). The cross-TCF shows an initial fast rise, similar to C γ,2 . However, the TCF of C δ does not show a plateau at a stable value, but it has a second, slower motional component that leads to a decay of the cross-TCF on the nanosecond time scale. To describe such more complicated motion, two exponential functions are needed (the 4P model, equation 17). The presence of these different types of dynamics, as described above for ILE61 as an illustrative case, was found to be consistently captured with the two force fields used in the present study (the internal and overall TCFs of all methyl groups are shown in Figures and, respectively, and the spectral densities are shown in Figure ). Thus, at least at this qualitative level, the AMBER ff99SB*-ILDN and CHARMM36 force fields provide at consistent picture of the side chain dynamics in ubiquitin. Out of 50 methyl groups in ubiquitin, 25/26 were fitted with LS2 for the AMBER/CHARMM simulations, and the rest were fitted with the 4P model. Figure as well as Figures and furthermore show that for all methyl groups, the time correlation functions obtained from fitting to the (assumed) analytical model closely match the actual TCFs that result directly from the MD simulations (that is, without any fitting). However, these "raw" TCFs are subject to statistical noise, especially at long lag times, which is eliminated by the fitting.