Problem equilibrating with low pressure

From: santanu chatterjee (
Date: Fri Jun 30 2006 - 16:31:55 CDT

   I am trying to equilibrate a box of water at 300K
and a very low pressure (0.05 bar). I need to
calculate the density of the gas phase at this
temperature. I used small pressure in order to allow
the box to expand. I am using NPT simulation with NAMD
using Langevin Piston method. I found that unless I
use small values for the parameters
LangevinPistonPeriod and LangevinPistonDecay, I can
not control the pressure into such a small value. The
water box has nearly 650 atoms and I tried to
equilibrate for 250 ps.

  But if I do that, although the water box
expands...the kinetic energy of the system is blowing
up. I tried the same exercise at 350K and for a
pressure equal to 0.1 bar and I see similar problem.
And if I use a larger value for the parameters
LangevinPistonPeriod and LangevinPistonDecay, the
water box does not expand and the I get a very high
pressure fluctuation.

  I would appreciate if anyone can give me any
suggestions regarding how to equilibrate this system
with small system pressure.


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