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Re: [lammps-users] NVT/NVE Ensemble for flow system
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Re: [lammps-users] NVT/NVE Ensemble for flow system


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Sat, 5 May 2018 13:41:31 -0400



On Sat, May 5, 2018 at 10:20 AM, Yakang JIN <yjinam@...6715...> wrote:

Dear LAMMPS Users and Prof. Axel,

 

In order to realize the poiseuille flow, generally, we build a system including fluids and medium. In most cases, we only fix the medium and impose NVT ensemble to fluids. Herein, I am confused about the choice of ensemble for flexible system.


​as has been explained here many, many times. using fix nve or fix nvt does *NOT* automatically mean, you have an NVE ensemble or an NVT ensemble. with a flow simulation, you​ have a non-equilibrium system, so neither of those ensembles apply (as they require 3d periodicity and no other manipulation of system forces or velocities to the system outside the integrator).

 

 

Basically we have two choices:

1: Just give NVT ensemble to the moving part, and modify the temperature by water temperature excluding the moving direction, by the following command

 

compute         Twater water temp/partial 1 1 0

fix 1 movepart nvt temp 300.0 300.0 100.0

fix_modify   1 temp Twater

 

2. Give NVT ensemble to the flexible underlying medium to control the temperature, and meanwhile give NVE to fluids .

 

fix 1 surface nvt temp 300.0 300.0 100.0

fix 2 water nve

 

which one should be better and physically reasonable?


​both have advantages and disadvantages. the closest to an experimental environment would be a bit more complex variant of your second example.
you partition your surfaces in 3 regions: the region facing the flow is only time integrated by fix nve, same is the flow; the region to the outside it not ​time integrated at all (it has to be thick enough to resemble bulk), the region in between is thermalized with a dissipative thermostat (langevin, csld, gle, gld, ...) to mimic the heat exchange with the missing bulk of the environment.

there is still one slightly unphysical component: the force inducing the flow in combination with periodic boundaries will generate heat. but if the force is small enough (i.e. the flow reasonably slow) and after you have reached a steady state, you should be ok. finite size effects and limitations of classical potentials, and incomplete statistics are likely to induce larger errors.
 

 

And I have tested these two cases, it seems that the outcome temperature from the second one gives better results.


​making tests yourself and choosing the option that is acceptable for your needs is the way to go. nobody can make that choice for you. you cannot write in a paper "some dude on the mailing list told me to do this". either you can convince yourself and others, that you made the best (or good enough) choice or you have to find some other approach.

axel.​

 

 

Hope to hear from soon. Thank you very much.

 

Best Regards,

Timmy

 


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--
Dr. Axel Kohlmeyer  akohlmey@...12...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.