Re: Pressure fluctuations

From: Mark Abraham (
Date: Thu Apr 05 2007 - 22:09:25 CDT

Cyril Falvo wrote:
> Hi,
> I don't have any experience in molecular dynamics. I am just trying to
> equilibrate a solvent box (chloroform) in a NPT simulation.
> I am using Langevin piston with the typical parameters
> (*LangevinPistonTarget = 1.01325 bar, **LangevinPistonPeriod = 200 fs,
> **LangevinPistonDecay = 100 fs*).
> During my simulation the temperature fluctuations are reasonable but the
> pressure fluctuations are very very large. I found in namd mailing list
> some messages
> about pressure fluctuations,
> In
> this last message it is suggested that these fluctuations are normal
> I understand that both T and P have to fluctuate. However, when the
> fluctuations are much higher than the targeted value, I don't understand
> the physical meaning of this.
> Does these fluctuations are normal ? Do you think I made a mistake in my
> simulation ?

How do you define an instantaneous pressure? Even after you achieve
that, all it does is reflect the momentary tendency for the system to
expand or contract. A time-averaged pressure is the one that is
meaningful for comparison with your piston target. There's no reason for
the fluctuations to be of any particular size.


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