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Re: [lammps-users] Fixing the temperature of an oscillating wall
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Re: [lammps-users] Fixing the temperature of an oscillating wall

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Sat, 26 May 2018 14:27:05 -0400

On Sat, May 26, 2018 at 12:28 PM, Kanka Ghosh <kanka.kankaghosh.ghosh@...24...> wrote:
I want to simulate an oscillating wall made of particles which will create a sound wave in the domain containing argon gas using LAMMPS. However, I want to thermostat the wall-particles as otherwise, it is giving rise to a heat flux to the argon gas particles in the domain.

​you are not making much sense here. the heat exchange of gas atoms with a wall due to thermal motions of the wall atoms is negligible compared to the kinetic energy transferred due to your wall movements. ​


I'm using fix_move wiggle to oscillate particles of the wall. On the top of it, I am using Berendsen (Not NVT as it will time integrate) thermostat to it. But it is not working and I'm unable to thermostat the wall particles.

​fix move completely ignores forces and velocities on atoms and just moves them following a prescribed trajectory, so a thermostat fix *cannot* have an impact.​

Is it not possible simultaneously to oscillate the wall particles (using fix_move) and maintaining the temperature of the wall-particles using a thermostat (Berendsen or other thermostats other than NVT)?

​first of all, the "wall temperature" is not a well defined concept here as the wiggle oscillation are kinetic energy as well. so you have subtract that bias out. or do the precribed motion in one direction and free time integration and thermalization in the other two directions. this is all very complicated and it is questionable whether this actually has a meaningful impact.


Here is my test lammps- script. Right now, I just want to keep the temperature of the oscillating wall fixed. Is there a way to get it?

#-----------initialize simulation--------------
units                       metal
dimension                3
boundary                    p p m
atom_style                sphere
atom_modify                map array

#-----------create atoms--------------------

region                    box block -60.0 60.0 -60.0 60.0 -52.0 50.0
region                    inbox block -60.0 60.0 -60.0 60.0 -40.0 50.0
region                    wall block -60.0 60.0 -60.0 60.0 -52.0 -50.0
create_box                2 box
create_atoms            1 random 5000 125698 inbox
lattice                    fcc 3.0
create_atoms            2 region wall

fix                         walls all wall/reflect zlo -52.0  zhi 50.0 units box

#------M &  D--------------------------------------------------

set                        type 1 mass 39.95
set                        type 1 diameter 3.4
set                        type 2 mass 40.078
set                        type 2 diameter 3.6          


group                         immobile region wall
group                         mobile  subtract all immobile

set                        group mobile type 1
set                        group immobile type 2

# ---------- Define Interatomic Potential ---------------------
pair_style               lj/cut 3.4
pair_coeff               1 1 0.0103 3.4  3.81
pair_modify                 shift yes
pair_coeff                 2 2 0.2152 3.6  3.81
pair_modify                 shift yes
pair_coeff                 1 2 0.0103 3.5  8.5
pair_modify                 shift yes
neighbor                  6 multi
neigh_modify             delay 10 check yes

# ----------------minimization of Ar---------------------

fix                        1 immobile setforce 0.0 0.0 0.0
thermo                    100
thermo_style            custom step temp pe ke etotal press vol enthalpy
min_style                cg
minimize                    1e-25 1e-25 10000 1000000
unfix                        1


timestep             0.001
velocity             mobile create 300.0 123475 dist gaussian mom yes rot yes
fix                 3 mobile nvt temp 300.0 300.0 0.01
fix             5 immobile move wiggle 0.0 0.0 3.0 0.5 units box
fix             7 immobile temp/berendsen 300.0 300.0 0.1
compute            mytemp mobile temp
compute                 mytemp1 immobile temp
thermo                 100
thermo_style          custom step time c_mytemp c_mytemp1 pe ke etotal press vol
log                      lognew.txt
#----------------- Run-------------------

run_style         verlet
run               1000000

Thanks & Regards,
Kanka Ghosh
Senior Research Fellow
Dept.of Physics
IIT Madras

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Dr. Axel Kohlmeyer  akohlmey@...12...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.