[CP2K-user] [CP2K:19784] Cons. Qty for different ensembles + Pressure fluctuation

[Léon Luntadila Lufungula]

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:
[image: NVE.png]
NVT:
[image: NVT.png]
NPT:
[image: NPT.png]
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....
[image: Pressure.png]

Thanks in advance for the help!

Kind regards,
Léon

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