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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: Axel Kohlmeyer <akohlmey@...24...>
Date: Mon, 27 Nov 2017 06:58:53 -0500


On Mon, Nov 27, 2017 at 6:47 AM, Nader Ameli <nader.mechanic@...24...> wrote:

Dear Axel,


You are absolutely right. Once I ignored “fix setforce” command, the forces by variable fcm command appeared. I was using “fix setforce” command for making my tool as a rigid body. However, I found that “fix move” command could be used for moving my tool and keeping it rigid. It could be useful for the others that have such problem.

Could you possibly tell me what wrong is with this study? There are hundreds of articles that have used LAMMPS for modeling of machining

(e.g. http://lammps.sandia.gov/abstracts/cms11.html).

Please let me know if you see any faults in my script.


​i already told you what is the problem. ​specifically and in general. now you are adding lack of attention to the list.

axel.

 

 

Thanks

Nader


On Sun, Nov 26, 2017 at 6:49 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:


On Sun, Nov 26, 2017 at 9:56 AM, Nader Ameli <nader.mechanic@...24...> wrote:
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.

​ok. what you observe is the consequence of your input.
- your fcm(tool,*)​ variable functions *must* result in 0.0, since you are using fix setforce 0.0 0.0 0.0. if you want to get the force *before* setting them to zero, you have to query the setforce fix. details are in the documentation.
- there are still a lot of unknowns due to the incomplete input, but if you slam your workpiece with a constant velocity into your material, there *should* be large changes in the force. at typical simulation speeds, this is a very high speed impact and thus usually causing shockwaves.

overall, it looks like you are trying to perform and interpret atom scale simulations with a continuum mechanics understanding. that is bound to fail.

axel.

 

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@...92......> 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.

axel.​

 

 

Best Regards

Nader


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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.


------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
lammps-users mailing list
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https://lists.sourceforge.net/lists/listinfo/lammps-users




--
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.


------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
lammps-users mailing list
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https://lists.sourceforge.net/lists/listinfo/lammps-users




--
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.