[MMTK] Barostat not working properly (Argon example)

[Andreas Kring]

Hi everyone.

I'm using MMTK-2.5.20 and I have experienced a little trouble with the 
barostat-object (the pressure is not constant) when working within the 
NPT-ensemble. Therefor, I tried to run the argon example included with 
the MMTK modules.

There are no problem running the argon.py script - it generates a 
argon_npt.nc file. When I extract the pressure (and the temperature) as 
a function of time from the trajectory in this file, I get the following 
result:

http://www.fys.ku.dk/~akring/argon_NPT_864_PT_10fs.png

As seen in this plot the pressure it not constant even though a barostat 
object is added to the universe in the argon.py file? Why is this the case?

I extract the pressure values (in units of atm) in the following way 
(pressure value number n from the trajectory in this example):

tr = Trajectory(None,'argon_npt.nc')
P[n] = tr[n]['pressure']/Units.atm

Any ideas for what could be wrong?

Kind regards
Andreas Kring