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

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

 From: Axel Kohlmeyer Date: Mon, 28 Aug 2017 07:28:02 -0400

On Mon, Aug 28, 2017 at 12:49 AM, 709832702 wrote:

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.​

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.

Best wishes!

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--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.

--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.