RMSFs

Root mean square fluctuation (RMSF) is a metric used in molecular dynamics and structural biology to quantify how much each atom or residue moves about its average position throughout a simulation. RMSF characterizes local flexibility and complements static structural data.

To compute RMSF for atom i, positions r_i(t) are collected over the trajectory of length T frames. The mean position r_i^avg is calculated across frames, and the RMSF is defined as RMSF_i = sqrt( (1/T) sum_t | r_i(t) - r_i^avg |^2 ). Some analyses use the initial structure as reference, yielding RMSF_i = sqrt( (1/T) sum_t | r_i(t) - r_i^init |^2 ). In practice, RMSF is often reported per residue by averaging RMSF over all heavy atoms in that residue, frequently using Cα atoms to obtain a per-residue profile. The trajectory is usually aligned to remove overall translation and rotation before calculation.

Applications include identifying flexible loops, termini, or binding-site regions, comparing structural flexibility under different ligands, mutations, or environmental conditions, and comparing with experimental B-factors as a rough validation of mobility.

Considerations include choice of atom set, alignment procedure, trajectory length and sampling frequency, and convergence. RMSF reflects sampling and can be influenced by simulation conditions and force fields; it does not provide direct timescale information about motions.