Root mean square fluctuation
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The mean square fluctuation (MSF) is a measure of the deviation between the position of particle i and some reference position.
,
where T is the time over which one wants to average, and
is the reference postion of particle i. Typically this reference position will be the time-averaged position of the same particle i, ie. 
Note that where the difference between RMSD and MSF is that with the later the average is taken over time, giving a value for each particle i. With RMSD the average is taken over the particles, giving time specific values.
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