large temperature fluctuation in NVT AIMD

[Mostafa Abedi]

Hi Everybody,

I am trying to do NVT AIMD simulations for a system consisting of 16 H2O molecule. Strangely, the temperature fluctuation is very large (see attached "WATER-1.ener" file). I know a little bit temperature fluctuation is acceptable, but mine is really large. This is the first time I am trying CP2K for doing AIMD and not familiar with structure of the input file. Maybe something is wrong or missing in the input file (see attached "water.inp" file). I should say I've already done this calculation with ORCA for non-periodic water system. Now, I want to see the effect of periodicity with cp2k. Any help would greatly appropriated. Many thanks.

Best,

Mostafa 

[Thomas Kühne]

Dear Mostafa, 

you are relying on the default values for the thermostat. For such a small system (the fluctuations scale with the sqrt 

of the number of particles), a strong coupling is necessary. Try for instance REGION MASSIVE, TYPE CSVR and 

TIMECON 50. More importantly, however, the dimension of your cell is too large resulting in a way too low density. 

Cheers, 

Thomas

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<WATER-1.ener><water.inp><water.xyz>

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

Thomas D. Kühne

Dynamics of Condensed Matter

Chair of Theoretical Chemistry

University of Paderborn

Warburger Str. 100

D-33098 Paderborn

Germany

tdku...@mail.upb.de

+49/(0)5251/60-5726

[Mostafa Abedi]

Dear Thomas,

Many thanks for your help. I've changed the input file according to your suggestion. However, the temperature fluctuation is getting larger. I attached the corresponding files. I really appropriate if you could have another look into this. Thanks.

Best,

Mostafa

On Tuesday, April 7, 2020 at 4:14:44 PM UTC-4, tkuehne wrote:

Dear Mostafa, 

you are relying on the default values for the thermostat. For such a small system (the fluctuations scale with the sqrt 

of the number of particles), a strong coupling is necessary. Try for instance REGION MASSIVE, TYPE CSVR and 

TIMECON 50. More importantly, however, the dimension of your cell is too large resulting in a way too low density. 

Cheers, 

Thomas

Am 07.04.2020 um 21:51 schrieb Mostafa Abedi <abedimo...@gmail.com>:

Hi Everybody,

I am trying to do NVT AIMD simulations for a system consisting of 16 H2O molecule. Strangely, the temperature fluctuation is very large (see attached "WATER-1.ener" file). I know a little bit temperature fluctuation is acceptable, but mine is really large. This is the first time I am trying CP2K for doing AIMD and not familiar with structure of the input file. Maybe something is wrong or missing in the input file (see attached "water.inp" file). I should say I've already done this calculation with ORCA for non-periodic water system. Now, I want to see the effect of periodicity with cp2k. Any help would greatly appropriated. Many thanks.

Best,

Mostafa 

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<WATER-1.ener><water.inp><water.xyz>

[Thomas Kühne]

Have you also adapted the coordinates, or simply modified the cell geometry? 

If not you may first try to carefully prepare your initial state taken the presence 

of periodic boundary conditions into account ...

Cheers, 

Thomas

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<water.inp><WATER-1.ener>

[Abedi, Mostafa]

You are right, I just changed the unit cell in the input file to the one that fits the structure. Is there any program/visualizer for preparing the initial state that takes the presence of periodic boundary conditions into account?

Mostafa 

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[Mostafa Abedi]

You are right, I just changed the unit cell in the input file to the one that fits the structure. Is there any program/visualizer for preparing the initial state that takes the presence of periodic boundary conditions into account?

Mostafa 

Thomas

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<water.inp><WATER-1.ener>

[Thomas Kühne]

I was told that Packmol is very helpful: http://m3g.iqm.unicamp.br/packmol/home.shtml. 

Also, at tests/QS/benchmark (or regtests in general) you will find equilibrated water 

configurations for all sort of system sizes starting from 32 molecules. 

Greetings, 

Thomas

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[Rakesh]

Hi Mostafa,

What changes did you observe by including periodicity in the calculations (cp2k vs ORCA).

Thanks,

Rakesh