[CP2K:64] practical MD-barostat question

Teodoro Laino teodor... at gmail.com
Thu May 10 19:38:20 CEST 2007

To answer your question just think that at EQUILIBRIUM fluctuations  
of the Volume are
strictly connected with fluctuations of the Pressure and Temperature..
Now,  in an NPT ensemble, if you reached the statistical equilibrium,  
the fluctuations of the Volume
are Guassians-like  with width:

\sigma(V) = (V k_B T \beta_T)^(1/2)

where \beta_T is the isothermal compressibility.. Again this is  
something you may check if you've an idea of the
isothermal compressibility of your system to know if you're really in  
[At equilibrium fluctuations of temperature have obviously a sigma of  
the Gaussian distribution of: T(2/3N)^(1/2)...]

(so the answer to your question is: sure it is very system dependent)..

What may be more easy to check to know if you're in a correct NPT  
ensemble is to control the following quantity:

<(P - P_{ext}) V> = -K_B T

where P_ext is the external pressure applied on your system..
This is TRUE only  if you use the proper nose' hoover chains as  
corrected by by Martina, Tobias and Klein in 1994 (I'm mostly
sure this is the case for CP2K. Chris can, much more than I'm doing,  
comment on that  and correct me if I'm wrong...)
and if you're at equilibrium.
So fluctuations of 5KBar over 1 Bar can be quite normal.. what is  
important is that you end up with an equilibrated system..
just check the volume for that or do some statistical analysis on the  
value of instantaneous pressure (average, std, normal distribution..)..


On 10 May 2007, at 19:01, Toon wrote:

> Hi,
> When performing an MD simulation, using the NPT_I ensemble with the
> default settings and a target pressure of 1 bar, what is the order of
> magnitude of the oscillations in the instantaneous pressure that one
> should expect? (I have set PV_AVA to true in the input file to get the
> instantaneous pressure in the output file.) Since I'm new on this
> subject, I was performing a simple test run on a box of solid methane
> with 64 molecules. (low temperature, below 90K, zero diffusion) I see
> oscillations in the instantaneous pressure with an amplitude of 5000
> bar. Is this normal? Maybe the answer is very system dependent?
> best regards,
> Toon
> >

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