RE: Pressure is not constant in NPT ensemble

From: Parisa Akhski (Parisa.Akhshi_at_chem.queensu.ca)
Date: Sun Aug 01 2010 - 13:31:11 CDT

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Hi,

I am just wondering how much deviation form the target pressure is allowed in NPT ensemble by NAMD when using "LANGEVIN PISTON PRESSURE"?

According to the

http://www.ks.uiuc.edu/Research/namd/2.6/ug/node32.html

it seems that in order to check the reliability of the pressure, we need to run a constant volume cutoff simulation in a cell that is larger than the molecular system by at least the cutoff distance; so, the pressure for this isolated system should average to zero over time. I set the pressure on 1bar and in the output file it starts form a big negative value and goes up but fluctuates between -50 and 50 bar (does not get zero finally). Is this OK or does anyone have any suggestion?

Thanks,

Parisa

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