Why is pressure tensor similar for different regions at this structure?

Hi everybody,

Would you mind write any comments on why is pressure tensor get same results for different regions for this structure?
Pressure tensor should have different values since number of molecules are different at regions. However, I don’t exactly know why I get exactly the same results for different regions.


config.data (2.7 MB)
config2.in (2.7 KB)

You are not computing per region pressure, but in both cases the global pressure.

I used fix command and mention group-ID (mine) as follows:

fix P_tensor1 mine ave/time 1 10 30 v_pxx v_pyy v_pzz v_pxy v_pxz v_pyz file ptensor1.profile

So, mentioning a group-ID here for fix command involves the calculation for this group. doesn’t it?

Could you please write me any guidelines?

a) it would be irrelevant, since you are referring to the global thermodynamic properties. you cannot retroactively distribute global properties to sub-systems
b) the documentation for the fix explicitly says it ignores the group id.

computing per group/region pressure is one of the more frequently discussed topics. thus you should find plenty of information in the archives. it is tricky business since you are trying to treat atomic scale data like continuum data, which is not quite the same. specifically the partitioning of the volume is non-trivial and the volume is only well defined for the entire simulation cell with all periodic boundary conditions. LAMMPS computes pressure from virial stress in a stress*volume formulation. That is easy to do based on the force computation that is already been done, but has known limitations.