# 回复： The difference between compute pressure andcompute stress/atom

Hi Axel,

Both of the two curves represent the normal stress along tensile direction. The black line is that I used the “compute pressure” command to calculate the normal stress(pzz), while the red line I used the command “compute stress/atom” and use the “compute reduce” to calculate the normal stress(sigmazz), I think they are should be the same, but the result does not. I use these two method in the other materials, they are the same. So which method is better to calculate the normal stress during the tensile test.

Best wishes!

Compute pressure use the box dimension to calculate the pressure.
Compute stress/atom does not calculate any pressure, but
pressure*volume. Because you don't mention the use of Voronoi volume, I
assume you use here the volume of the box to get the value in pressure
unit. It seems to me relatively ill-defined to use the box volume with
stress/atom without a box fully periodic...

Hi Axel,

Both of the two curves represent the normal stress along tensile direction.

​in what units?​

The black line is that I used the "compute pressure" command to calculate
the normal stress(pzz), while the red line I used the command "compute
stress/atom" and use the "compute reduce" to calculate the normal
stress(sigmazz), I think they are should be the same, but the result does
not. I use these two method in the other materials, they are the same. So
which method is better to calculate the normal stress during the tensile
test.

​impossible to say. both should give *identical* output, if computed
correctly. since you don't provide enough information to assess how you
computed them, there is no way to tell. they could be both wrong.

axel.​

Compute pressure use the box dimension to calculate the pressure.
Compute stress/atom does not calculate any pressure, but pressure*volume.
Because you don't mention the use of Voronoi volume, I assume you use here
the volume of the box to get the value in pressure unit. It seems to me
relatively ill-defined to use the box volume with stress/atom without a box
fully periodic...

​compute pressure does the same thing. ...and the sum of all voronoi
volumes is the box volume, so for as long as this is computing the sum over
all per atom stresses, this should be comparable. in fact, the compute
stress/atom docs give an example for how to compute the same result as what
compute pressure gives.

axel.​

Thanks for the precision Axel! I wasn’t indeed sure whether the the Voronoi cell would be limited by the simulation box or the paire_style cutoff, in the particular case when the paire_style cutoff extends “outside” a non-periodic boundary. Julien