[CP2K-user] [CP2K:19787] Cons. Qty for different ensembles + Pressure fluctuation

[Léon Luntadila Lufungula]

Dear Matthias,

I have similar results for a box of 256 water molecules:
NVT:
[image: NVT-256.png]
NPT: (fluctuations are a bit smaller but still in the range of several 
kbar...)
[image: NPT-256.png][image: Pressure-256.png]

You say that the sample should be equilibrated well in advance before 
starting the sampling run, so do you mean I should run an NVT simulation 
for longer until it equilibrates before running an NPT from the thermalized 
end structure? Or do you mean both NVT and NPT should be run for longer 
until the Cons. Qty. is also equilibrated? Because in both cases the 
temperature seems to be equilibrated quite well and the pressure in the NPT 
ensemble is also constant albeit with very large oscillations...

Kind regards,
Léon


On Monday 22 January 2024 at 13:40:05 UTC+1 Krack Matthias wrote:

> Dear Léon
>
>  
>
> a 32 H2O cell is still a relatively small system which can cause large 
> fluctuations. I suggest to try at least the 6x6x6 = 216 H2O input 
> <https://github.com/cp2k/cp2k/blob/master/tests/Fist/regtest-water/H2O-TIP3P-6x6x6.inp>. 
> The sample should be equilibrated well in advance before starting the 
> sampling run.
>
>  
>
> HTH
>
>  
>
> Matthias
>
>  
>
> *From: *cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Léon 
> Luntadila Lufungula <Leon.luntad... at uantwerpen.be>
> *Date: *Monday, 22 January 2024 at 12:56
> *To: *cp2k <cp... at googlegroups.com>
> *Subject: *[CP2K:19784] Cons. Qty for different ensembles + Pressure 
> fluctuation
>
> Dear all,
>
>  
>
> I ran several simulations on a box of 32 water molecules with the TIP3P 
> force field:
>
> 1) 1 ns NVE trajectory with timestep 0.5 fs
>
> 2) 1 ns NVT trajectory at 298.15K with timestep 0.5 fs and thermostat time 
> constant of 100fs
>
>  3) 1 ns NPT_I trajectory restarted from the final structure of the NVT 
> trajectory at 298.15K and 1 bar with timestep 0.5 and thermostat and 
> barostat time constants of 100 fs
>
>  
>
> I plotted the Cons. Qty [a.u.] vs. Time [fs] for the three simulations and 
> saw that the constant of motion was only more or less constant for the NVE 
> trajectory. The NVT and NPT trajectories both had strong oscillations and 
> significant drift of the constant of motion (see below) and I was wondering 
> if this is normal behaviour or that this means that the system is not yet 
> equilibrated? I would think that the Cons. Qty. should remain constant but 
> I can also understand that the thermostat and barostat might influence its 
> behaviour in a way.
>
> NVE:
>
> NVT:
>
> NPT:
>
> The simulations of the water box were small test cases, but I also need to 
> know what behaviour to expect when going to my production runs with my 
> systems of interest...
>
>  
>
> Additionally, in the NPT ensemble, the pressure seems to fluctuate very 
> strongly from -20000 to +20000 bar which seems to be quite excessive to me. 
> Is this normal? I've also seen this asked in another post 
> <https://groups.google.com/g/cp2k/c/gxA8r6wmyF4/m/k8QT7iglAAAJ>, but that 
> person did not get a reply unfortunately....
>
>  
>
> Thanks in advance for the help!
>
>  
>
> Kind regards,
>
> Léon
>
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