[gmx-users] Pressure variation in NPT ensemble

Mark Abraham Mark.Abraham at anu.edu.au
Thu Dec 14 05:27:20 CET 2006

nur avneet wrote:
> Dear all
> I set the pressure in NPT ensemble to 1bar..and run the simulation.
> But, the log file does not show the same pressure...for some cases it 
> does remain close to 1 but in others it even shows around 4 bars of 
> pressure.
> What is the significance of this variation...and what should i do to 
> remaove this variation.

This gets "reported" every few months on this list. You are not 
"setting" the pressure to 1bar, you are specifying a negative-feedback 
system to move the system towards 1bar. The variation is normal and not 
indicative of a problem unless the time-average of that value is not 
near the chosen value.

Think about how you might define an instantaneous pressure in 
statistical mechanics...


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