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Re: [lammps-users] Atoms in the lennard-jones droplet fly away!!
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Re: [lammps-users] Atoms in the lennard-jones droplet fly away!!


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Sun, 20 May 2018 16:59:49 -0400



On Sun, May 20, 2018 at 4:32 PM, saeed alborzi <alborzi.saeed6@...24...> wrote:
Could you please explain more obvious? How should I change it?

​it is annoying, when people don't tell you the whole story right away, isn't it?
i was giving you a demonstration of how it felt reading your original post.​
 

I think I should put air atoms in the space to represent the air pressure on the droplet but it costs increased simulation time.

​this makes no sense. at the volume you are simulating, there are effectively no atoms there in the corresponding volume of air. that won't hold atoms back.

the problem is your initial geometry and your simulation protocol.
you start with a "brick" of atoms. is this physically meaningful? no! is this what a droplet looks like? no!!
especially the atoms at the corners and edges have very li​ttle that holds them back. ever heard of surface tension? ever though about what the reason for that is?
so a more realistic starting geometry would be a whole or a section of a sphere.
but it doesn't stop here. you are placing atoms, assign a (randomized) temperature and start your simulation right away. is this likely to be a realistic starting point? no!
so you have lots of atoms with high potential energy or potential energy that isn't consistent with the kinetic energy and so on. you need to get rid of some of these inconsistencies. i.e. you have to relax and equilibrate your system. and have to do it carefully.

finally, if you are simulating a droplet with a soft interaction (as opposed to, say, a water droplet, which has strong water-water forces between them), you have to expect a high fugacity, i.e. atoms may easily leave the droplet, since it doesn't require much kinetic overcome the surface tension (there is rather little of it). so you have to start your simulation *very* soft and carefully, e.g. equilibrate it at a much lower temperature and very carefully heat it up. then put a reflecting wall around your system to contain any atoms leaving the droplet and building suitable liquid/vapor coexistence conditions.

i strongly suggest, you discuss the "design" of your simulation with your adviser, since it looks a lot, like you haven't fully thought through what you are trying to do and your problems are not problems of missing LAMMPS experience or incorrect usage of LAMMPS commands, but rather bad choices for how to set up and run your system. you'd have the same problems with any MD code, if you'd use the same starting geometry and simulation settings.

axel.


 

On Sun, May 20, 2018 at 7:23 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:


On Sun, May 20, 2018 at 9:45 AM, saeed alborzi <alborzi.saeed6@...24...> wrote:
Dear Axel

Sorry for insufficient details.
The attached is the picture of my initial geometry which is simple cube for the droplet and also the input script.
I used lj/cut force field with pair coeffs of 0.2398 for liquid-liquid and 0.1 for solid-liquid.

​as i had suspected. you get out what you put in​. your choice of initial geometry and your simulation protocol make it inevitable that atoms are flying away.

axel.

 

On Sun, May 20, 2018 at 5:38 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:


On Sat, May 19, 2018 at 3:30 PM, Stefan Paquay <stefanpaquay@...29....> wrote:
There will probably be a vapor liquid coexistence. 

​or the initial geometry or choice of force field parameters or general setup ​is messed up.
nobody can make any specific statement based on a so incredibly vague description.

axel.



On Sat, May 19, 2018, 2:30 AM saeed alborzi <alborzi.saeed6@...24...> wrote:
Dear lammps-users

I am simulating a droplet on a fixed surface with fix nvt until it is equilibriated but the droplet atoms continuously separate from the droplet and fly away in the space. Is it correct? Or how can I fix it?

Any help would be appreciated.
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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@...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@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.