pressure elements profiles

[jude vishnu]

Hello all,

Has anyone calculated local pressure tensor along a direction of the simulation box?

I have a system of polymers with fene and WCA interactions..

My plan is to use neighbor list   of freud to calculate forces on particles from their trajectory.

For this I find the neighbors with 1 sigma distances and calculate forces and the distances and distance vectors.

Now for each particle I find the force on each of them due to neighbors (using neighbor list).

Bond list from gsd file will be used similary for FENE calculations.

I then find virial on each particle separately.

Now I bin the virial of each particles by using position of each particles along a direction. I can then sum over the virial of each particle in a bin to get total pressure tensor of that bin.

The problem now is that the result I get from this does not match with on the fly calculation in hoomd 2.9.6. I am attaching my code here. I think I am doing something wrong in force calculation and may be I should used different kind of neighbor list. Any input would be appreciated. 

[gyorgy...@gmail.com]

Hello,

This is not really an answer to your question just a remark. 

I think that you cannot compute a local pressure tensor from the atomic virials like that. Sure, you can sum up and average them, which will tell you something about some local stresses, but I would not call it really stress (or pressure). The virial expression gives you a correct pressure tensor only when computed in the whole box. If you localize the pressure by summing up the virials in a given volume then the pressure tensor you get from this calculation will be wrong. The problem stems from localizing a quantity (pressure) which is inherently non-local.

To compute local pressure tensors one can use Irving-Kirkwood type expressions or the Method of Planes - or surely others that I am not aware of. 

This is my understanding of the problem, but I am happy to change my opinion if you have a different view,

Best regards,

Gyorgy

[jude vishnu]

Dear Gyorgy,

Can you please refer me to any papers which uses Irving-Kirkwood type expressions or the Method of Planes to calculate local pressure in simulations if you know of any?

Regards,

Jude

[gyorgy...@gmail.com]

Hi Jude,

Off the top of my head I can recall these papers:

Method of planes: https://journals.aps.org/pre/abstract/10.1103/PhysRevE.52.1627

Irving-Kirkwood method: https://doi.org/10.1063/1.1747782

This is perhaps a good summary of the problem: https://pubs.acs.org/doi/abs/10.1021/acs.jctc.6b00576

Check these papers, maybe you find more interesting ones through references in the above articles.

Hope this helps

Best regards,

Gyorgy