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Re: [lammps-users] Setting temperature during the pure shear's run
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Re: [lammps-users] Setting temperature during the pure shear's run

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Mon, 3 Jul 2017 09:43:28 -0400

On Mon, Jul 3, 2017 at 6:12 AM, Asadollahzadeh <niliasadollahzadeh@...24...> wrote:
Hi my friends!
I am using LAMMPS to simulate an  eam  system recently.My system was relaxed on 100 K, but when I applying pure shear , it has notable fluctuation and I don't know how can I set it on my desired temp??!!

​there is no indication in the input lines below, that you are doing what you claim you are doing.
i only see a one-time velocity assignment and a number of integrator/thermostat assignments, the last of which is not correct syntax.
with the assignment of fix nvt to the "middle" group, you are effectively annihilating your initial velocity ramp.
if you want to thermostat a shearing system, you have to very carefully think which degrees of freedom of your system, you need to thermostat, if at all.

also, in microscopic simulations, instantaneous temperature *always* fluctuates and the magnitude of the fluctuation depends on the size of the system. question is, does it fluctuate suitable around the desired average. ...and are you monitoring the correct temperature?


p.s.: fix temp/rescale is very bad for a production simulation and should not be used


May anyone give some advice to make the system on the fix temp ?

Thank you in advance.


velocity        lower set  0          0    0   units box
velocity        upper set  0.0006     0    0   units box

fix             3 lower setforce 0 0 0
fix             4 upper setforce 0 0 0

group           middle subtract all upper lower 
velocity    middle ramp vx 0.0 0.0006 y ${lim02} ${lim03} sum yes

compute    mystress middle stress/atom NULL virial
compute    reducedstress middle reduce sum c_mystress[4]     # press*volume

variable    middleVolume equal (${length_x}*${midlength}*${length_z})      
variable    stress equal (c_reducedstress)/(10^4*${middleVolume})
variable        strain equal (0.5*(1-(1/((step*0.001*0.0006/(${midlength}))+1)^2)))

fix        5    middle  nvt temp 100 100 0.1  
fix        6    lower   nve
fix        7    upper   nve
fix                    8    temp/rescale  10  100  100  0.1  1

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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.