strength depends on vacuum size during tensile test

Hello all,

I did tensile test on a metallic glass slab (size z>y>x) along z direction, with free surface on x and y direction, under NPT at 100 K. When the vacuum layer in x and y set as 20 angstroms, the peak stress along z was about 1.8 GPa, and when the vacuum layer set as 100 angstroms in x and 500 angstroms in y, the stress was down to about 0.3 GPa. During this two tests all other parameters were same except the initial velocities of atom different. I expected the strength along z should be the same. Any explanation? Thanks.

C Tang

My setting file reads like:

If by “stress” you mean a pressure tensor component,

then the formula for pressure divides by the system volume.

LAMMPS has no way of knowing that some or how much of your system

is vacuum. You can look at the compute stress/atom command

which intentionally leaves out the volume term. Basically

it leaves it up to you to define the appropriate volume.


Thanks Steve. I forgot that…
So after replacing the nominal volume (box volume) with the actual system volume, the stresses from the two tests are identical.