[lammps-users] Stress/Strain

Dear LAMMPS users,

Am trying to calculate the Yong’s modulus of a thin sheet.My sytsem is equilibrated and uniaxial loading (Y direction) is introduced via "fix deform " command.
As a out put “per atom stress tensor” is received.
Some literature used LAMMPS to calculate Yong’s modulus explain each “atom stress” and “time” have to be averaged to gain the Cauchy stress.

Am confused with this (how to do this) and can anybody explain how can I get the stress strain diagram with these information.Are there any good paper (materials) which describe these details?

I really appreciate your help.

Thanks a lot.


You can time average the stress components via the fix ave/time command.
Then you can plot stress vs strain from data output to a file. LAMMPS
doesn't produce plots if that's what you're asking.


Hi Steve and all,
Thanks a lot Steve.
I have few more doubts.
If I use stress/atom command Lammps will provide the virial stress tensor and I think unit will be bar*volume.
Since it is a per atom quantity I used compute reduce command with sum to convert it into global value and then used fix ave/time command. Here am only interst the stress in " Y" direction.

  1. Is this a correct approach?
    02.Do I need to convert stress tensor value when I draw strain -strain diagram?
  2. Regardless of “Nrepeat” am getting a same output value for Y stress. This means do I need to do any normalization ?
    I really appreciate your assistance…

Thanks a lot.

The units of the per-atom stress and global pressure are
detailed on the doc pages for the 2 computes and on
the units doc page. If you sum the per-atom stress
with compute reduce, you simply get the global pressure
(minus a scaling factor - see the doc page).