[MMTK] Barostat not working properly (Argon example)

[Andreas Kring]

> The quantity called simply "pressure" in the trajectories should more 
> precisely be called "instantaneous pressure". It is a microscopic 
> quantity, not a thermodynamic one. It fluctuates wildly and even becomes 
> negative. Only its time average over times much longer than the barostat 
> relaxation time can be interpreted as a thermodynamic pressure.
> 

Ok, that explains it. I have read about this in M. P. Allen and D. J. 
Tildesley book, 'Computer Simulation of Liquids'  :-)

Is there a quick answer to how one can determine the relaxation time of 
the barostat?

>> 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
> 
> This is perfectly correct. You can also retrieve the whole pressure time 
> series at once using
> 
>     p = tr.pressure/Units.atm
> 

Ok, this is nice to know  :-)

Thanks for the quick answer.

Kind regards
Andreas