Dear all,

I'm trying to estimate the correlation time for my system, for the purpose of 
selecting a number of uncorrelated configurations and setting a good value for 
nstxout.

My systems are usually small molecules in molecular liquids (water, methanol, 
acetonitrile, cyclohexane...). As a first approach, I calculate the velocity 
autocorrelation function for the pure liquid (or for a group containing only 
the solvent in a simulation of the full system).

I expect to find a function more or less like the one shown here: 
<http://www.compsoc.man.ac.uk/~lucky/Democritus/Theory/vaf.html>. But instead I 
get a line that rapidly oscillates around 0, with an amplitude that 
progressively vanishes. I think I understand this behaviour, as it would be due 
to intramolecular vibrations or librations, but I don't find samples of this in 
the literature.

My question is how can analyze this and extract some correlation time? How 
could I get some smooth function like the one shown in the above link? Am I 
doing something wrong? Is there another way to estimate a good value for 
nstxout?

Any help or pointers to where to find it would be appreciated.

Thank you,
Ignacio
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