barostat problem NPT

[simin pahlavi]

Dear Cp2k usrs,

I am new in CP2K and I am trying to simulate 190 water molecule in T=800 k and P=4500 bar by the means of AIMD.

I have a question about the barostat in CP2k. According the previous articles I have used CSVR barostat with timecon 2000 fs.

As I have attached the plot of instantaneous pressure, the fluctuation in pressure are about -4500 to 4500.

I have tried to change the timecon to the smaller value after 4000 fs relaxation to 1000 but the fluctuation did not change that much.

I appreciate it if you recommend me a convenient barostat and thermostat for the system in hand.

Thanks in advance 

simin

[Matt W]

I don't really see a problem.

I'd suggest running a classical simulation (should be water setups in the tests) to get an idea of equilibriation of an NPT system, and what the fluctuations would look like.

Matt

[Yingchun Zhang]

Hi simin,

I usually don't run AIMD under NPT ensemble. But I remember another thermostat should be added into the barostat section. And I recommend use CELL_REF with slightly larger dimension, say 15 A for your case in a NPT simulation.

BR,

Yingchun

[Mohammad Hellani]

Hello,

Can you please tell me how you have calculated the pressure?

i Added in the print of Motion section STRESS_TENSOR but it is writing 1/3 Trace of Stress Tensor.

is 1/3 Trace of STress Tensor the same as Pressure?

Thanks for your help.

Mohammad

[Thomas Kühne]

Dear Simin, 

due to the fact that the fluctuations are proportional to the square root of the number of 

particles your pressure as a function of time seems to me very reasonable. I hence, 

would only suggest to substantially reduce the timecon to a much lower value, e.g. 

50 - 100fs. Also, please keep in mind that the region in the thermostat section is by 

default global, which to the best of my knowledge, only couples to 3 degrees of freedom 

of the whole system! So better use massive to equilibrate your system. 

Best, 

Thomas

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<pressure.png><water.inp>

==============================

Prof. Dr. Thomas D. Kühne

Dynamics of Condensed Matter

Chair of Theoretical Chemistry

University of Paderborn

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