Modern biomolecular force fields are usually able to accurately capture the backbone dynamics, for example in terms of NMR parameters such as generalized order parameters of amide N-H vectors (S 2 N H ) and associated correlation times (τ f ), as defined by the framework of Lipari-Szabo. The agreement between MD simulations and NMR experiments is typically especially good for structurally rigid elements in which fast internal dynamics dominate, whereas flexible regions that undergo slow dynamic transitions can be more challenging. Advances have also been made in the description of molecular rotational diffusion. While simulations with the widely-used TIP3P water model show too fast rotational diffusion (tumbling) of the protein, in line with the too low viscosity of TIP3P water, latergeneration water models such as TIP4P/2005 or SPC/E b provide more realistic protein tumbling times. Alternatively, one can use the rotational tumbling correlation time from NMR experiments, or the rotational velocity from the simulation can be rescaled to match the experimental time scale. However, despite recent progress, accurately predicting the dynamics of side chains from MD simulations is still a challenge. This is especially the case for C-H bond vectors in methyl groups, which cover a broad range of time scales and amplitudes, including jumps between multiple rotamer states. Recently, the description of methyl group dynamics in AMBER and CHARMM force fields was improved by reparametrization of the methyl group rotation barriers against high-level coupled cluster quantum chemistry calculations. Side chain carbon-13 nuclei provide a great source of dynamic information via C relaxation due to dipolar auto-correlation. Unfortunately, the utility of spin-spin (transverse) relaxation is greatly limited, due to interference effects from C- C coupling and crosscorrelated relaxation between C-H dipoles. Although this can be partly alleviated by generating C-labeled, fractionally deuterated proteins (for example CH 2 D and CHD 2 ), a more cost-effective approach would omit deuteration while uniform C-incorporation ensures that all methyl probes are available, rather than a subset (like in ILV labeling 34 ). At the same time, C-1 H correlation spectroscopy is highly sensitive for CH 3 groups. Fortunately, cross-correlated relaxation (CCR) from interference between C-H dipolar relaxation mechanisms offers such a route. Whereas CCR needs to be repressed in auto-relaxation measurements, it itself contains dynamic information. Methyl groups provide a comprehensive and well-dispersed set of probes throughout proteins. Methyl groups have particularly favorable properties as well, such as high Boltzmann polarization of the three attached hydrogen nuclei, while methyl axis rotation eliminates complications that arise from spin-spin couplings within the methyl group. The three-fold rotation leads to line narrowing.