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Re: [lammps-users] Fix NPT aniso stabilises at incorrect pressure
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Re: [lammps-users] Fix NPT aniso stabilises at incorrect pressure


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Wed, 21 Jun 2017 06:08:08 -0400



On Wed, Jun 21, 2017 at 5:59 AM, Dzepina, Branislav <b.dzepina15@...68...> wrote:

I’m looking at pressure as output by :

thermo_style custom press


​that is the total averaged pressure over Pxx, Pyy, Pzz.
however with your fix npt command, you are only adjusting​ the x-dimension of your box according to Pxx.
so you should output and monitor pxx to determine whether fix npt is working as expected.

axel.

 

Brani

From: Axel Kohlmeyer [mailto:akohlmey@...24...]
Sent: 21 June 2017 10:56
To: Dzepina, Branislav <b.dzepina15@...68...>
Cc: lammps-users@...396...sourceforge.net
Subject: Re: [lammps-users] Fix NPT aniso stabilises at incorrect pressure

 

 

 

On Wed, Jun 21, 2017 at 5:13 AM, Dzepina, Branislav <b.dzepina15@...36.....68...> wrote:

Dear Lammps users,

 

I am encountering some strange behaviour regarding an NPT fix.

I am trying to simulate a box of ~54,000 Fe atoms at high pressure (7.7GPa) and temperature which are described using a MEAM potential (Phys. Rev. B 89, 094102).

I run the simulation for 200k timesteps with periodic boundaries in all directions.

If I use the fix:

 

fix 2 all npt temp 1860 1860  0.1 iso 77000 77000 1

 

the pressure stabilises within 2000 timesteps to the correct value.

However, If I try applying the same pressure only in the x direction:

 

fix 2 all npt temp 1860 1860  0.1 x 77000 77000 1

 

the pressure  consistently stabilizes at ~10GPa throughout the entirety of the run.

 

A search through the forums brings up a user experiencing exactly the same problem (http://lammps.sandia.gov/threads/msg66409.html) . The problem in their thread

it would seem was the core-shell package.

 

Could I have a similar problem with the MEAM potential? Or is there something I am misunderstanding.

Any advice would be greatly appreciated.

 

are you looking at the total pressure, or at the pxx component?

 

axel.

 

 

 

Cheers,

Brani

 

 

 

 


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--

Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.




--
Dr. Axel Kohlmeyer  akohlmey@...92......  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.