Computing pressure fora group of atoms

Hi all,

I want to compute pressure for a group of atoms i.e. a sub-volume inside

a volume. I use the following command. I have a continuum background so i am

having a bit of trouble interpreting some of the continuum concepts from the

atomistic point of view.

I have a pps boundary condition and the top layer is fixed.

I am interested in stress field around the object of interest, in this case its a dislocation.

First I define the region:

region temppress block INF INF INF INF -50 50 units box

And then the corresponding group:

group press_plane region temppress

Followed by the two computes

compute grpstress press_plane stress/atom NULL

compute p press_plane reduce sum c_grpstress[1] c_grpstress[2] c_grpstress[3] c_grpstress[4] c_grpstress[5] c_grpstress[6]

Would i be right to compute the total pressure in that region as

( c_grpstress[1]+ c_grpstress[2] + c_grpstress[3] )/3*volume of temppress?

Does the same apply to shear stresses? Can one define a shear stress at all

as in c_grpstress[4]/volume of temppress ?

Many thanks in advance,


What you are proposing is correct, in the sense that this is valid way of extracting information from LAMMPS. Whether it is correct in the sense of being physically valid is not a well-posed question, as it depends on the details of your calculation. There is a vast literature on the relationship between atomistic and continuum definitions of stress. If you are interested in atomistic calculations of the stress field around a dislocation, I suggest you read the literature on that subject. You can not expect the responders on this list to provide you with detailed guidance on such a specialized and complex subject.

Hi Aidan,

Thank you for your response.

My question was actually generic in the sense that if one is region in an arbitrary region of stress
in a region say without the singularity as well, then would it be right to just define a box and perform the computes
as i had done. I think you answered that and i highly appreciate it and I need to think of whether it is physically valid.

I just took an example of the dislocation because I am working on it at the moment.



The question you are asking does not have a simple yes or no answer. Many subquestions arise from this. For example, what volume of material should be selected as the region used to sample the stress. Thinking about it is good, but it is not a substitute for studying the prior literature in this area.