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Re: [lammps-users] 回复： The difference between compute pressure andcompute stress/atom

# Re: [lammps-users] 回复： The difference between compute pressure andcompute stress/atom

 From: Axel Kohlmeyer Date: Mon, 28 Aug 2017 07:31:51 -0400

On Mon, Aug 28, 2017 at 4:17 AM, Julien Guénolé wrote:
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.​

On 28/08/17 06:49, 709832702 wrote:

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!

------------------ 原始邮件 ------------------

On Sat, Aug 26, 2017 at 11:41 PM, 709832702 wrote:

Hi,

Recently, I adopted the compute pressure and compute stress/atom to calculate normal stress under uniaxial tensile as shown in the attached Figure, here the tensile direction is with the periodic boundary, and the other two direction are the free surface, I found that the stresses are not equal at the beginning, although they are in agreement at the end of progress, so what is the reason, they are all based on the virial theory. I also test under all three directions are the periodic boundaries, this time they are nearly coincident.

first rule about ?figures in science: *always* include units!
?
since you don't explain with sufficient detail what you are computing for the figure, there is little to comment. if you did it correctly, both graphs should be identical, since they should show the identical information. the fact, that they don't coincide hints at you don't computing one of them correctly.

axel.

```--
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