Deformation problem

LAMMPS LAMMPS Beginners
1

I’m trying to plot a Cu stress-strain graph with a script I saw on the internet that uses the following

variable upper_vel equal 0.01 #  
fix zeroing_force_on_lower lower setforce 0.0 0.0 0.0
fix zeroing_force_on_upper upper setforce 0.0 0.0 0.0 
velocity lower set 0.0 0.0 0.0 units box
velocity upper set 0.0 v_upper_vel 0.0 units box

The problem is that deformation in the y-axis occurs only every 5000 steps.

  Step           Ly           Volume        v_strain      v_ytensor         Temp          TotEng         Press
         0   72.269405      113283.91     -0.00062304519  49.140522      0.009062623   -35189.468     -1473.1663
      1000   72.269405      113283.91     -0.00062304519  3202.0166      0.0045077294  -35189.075     -3627.102     
      2000   72.269405      113283.91     -0.00062304519  6261.7685      0.00024858932 -35187.937     -5616.1535    
      3000   72.269405      113283.91     -0.00062304519  9062.1089      0.0051519743  -35186.102     -7372.4878    
      4000   72.269405      113283.91     -0.00062304519  12050.205      0.00025137671 -35183.556     -9286.4894    
      5000   73.267782      114555.76      0.013183006    14711.52       0.0078857771  -35180.284     -10871.358    
      6000   73.267782      114555.76      0.013183006    17713.529      0.0079513926  -35176.336     -12772.354    
      7000   73.267782      114555.76      0.013183006    20568.951      0.012033005   -35171.707     -14496.099

Why might this happen? I know that the fix deform command exists but with this method I see that the author obtained good results.

Cu_u3.eam (35.7 KB)
in_cu.lammps (2.2 KB)
log.lammps (7.7 KB)

2

Hello,
The commands you have copy-pasted are setting the velocities of a group of atoms, but the output you gave shows an increase of the box volume. Most likely you did not copy-past all the relevant part of the script, because the “velocity set” commands wont impact the box volume.
Simon
edit: I just saw that you are using boundary s s p, so please ignore my answer

3

It’s because the box size is only adjusted on neighbour rebuild and since your system doesn’t change much, it’s very rare.

One workaround is to calculate the ly yourself, using bound keyword for variable command.

2 Likes
4

Thank you ! I was able to solve it with your answer.