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Re: [lammps-users] force calculation in tersoff potential
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Re: [lammps-users] force calculation in tersoff potential

From: Nader Ameli <nader.mechanic@...24...>
Date: Sun, 26 Nov 2017 18:26:56 +0330

The core of my script is as bellow. I have deleted commands around variables, groups, etc (due to shorten it for reading). Many thanks for your time.

units metal
dimension 3
boundary p s s
atom_style atomic

lattice diamond ${latparam}
region simulation_box block 0 ${xmdim} -${ymdim} ${yplane4} -${zmdim} ${zplane3} units box
create_box 4 simulation_box

mass 1 12.0107
mass 2 28.085
mass 3 28.085
mass 4 28.085
region lower block 0 ${xmdim} -${ymdim} 0 -${zmdim} 0 units box
create_atoms  2 region lower
# -----------------Define potential function----
pair_style tersoff
pair_coeff  * * SiC.tersoff C Si Si Si
neigh_modify exclude type 1 1
# -----------------Define Settings -------
fix 1 tool setforce 0.0 0.0 0.0
displace_atoms tool move 0 ${initialdist} ${zmove} units box   #Displace tool to Adjust Depth of Cut
velocity tool set 0.0 0.0 0.0 sum yes units box
velocity mobile create 293 482748 temp new #Defining initial temperature of mobile atoms

fix 7 all nve
velocity tool set 0.0 $v 0.0 sum yes units box    #Starting of tool movement
variable forcex equal fcm(tool,x) #total force in x direction
variable forcey equal fcm(tool,y) #total force in y direction
variable forcez equal fcm(tool,z) #total force in z direction

# -----------------Machining Run------------
timestep 0.001
thermo_style custom step v_forcex v_forcey v_forcez
thermo $p
thermo_modify temp new
thermo_modify   lost warn

fix 12 mobile print $p "${forcex}" file TotalFeatures/forcex screen no
fix 13 mobile print $p "${forcey}" file TotalFeatures/forcey screen no
fix 14 mobile print $p "${forcez}" file TotalFeatures/forcez screen no

run  $m

On Sat, Nov 25, 2017 at 11:33 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:

On Sat, Nov 25, 2017 at 9:59 AM, Nader Ameli <nader.mechanic@...24...> wrote:

Dear LAMMPS users,

I am using “Tersoff” pair style for nanometric machining and want to calculate force between tool and workpiece groups. According LAMMPS manual, we could not use compute group/group. Hence, I used variable fcm command as bellow:


variable forcex equal fcm(tool,x)

variable forcey equal fcm(tool,y)

variable forcez equal fcm(tool,z)

fix        12        mobile print 50 "${forcex}" file TotalFeatures/forcex  screen no

fix        13        mobile print 50 "${forcey}" file TotalFeatures/forcey screen no

fix        14        mobile print 50 "${forcez}" file TotalFeatures/forcez screen no


In this regard, I have 2 questions:

1- The achieved forces from these commands are 0.0 during the process. Why is that? I made my tool rigid. Could it be the reason of this?

2- Logically, forces on tool should be equal with the forces on workpiece. Nevertheless, when I put workpiece group in my variable fcm command, the result has been changed and showed a very bad fluctuation:

variable forcex equal fcm(workpiece,x)


Could you please help me in this issue?

​there is not enough information provided to give any kind of meaningful help.




Best Regards


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