[lammps-users] Stress Calculation and Box Size Update


I am applying a tensile strain in Z direction with shrink wrapped boundary conditions in Z direction (by pulling topmost layer with a constant velocity of 0.2 A/ps).

This is how stresses in Z direction is calculated in GPa.

Compute 4 all stress/atom

Compute 5 all reduce sum c_4[1] c_4[2] c_4[3] c_4[4] c_4[5] c_4[6]

Variable press1 equal c_5[3]/(vol*10^4)

My neighbor list information is:

neighbor 2.0 bin

neigh_modify delay 5

Since my box size (so volume) gets updated only after some time steps, will I get correct value of stresses (every time step) in Z direction.

How could I determine the interval between timesteps, after which my box size (volume) will be getting updated.

Thank you very much for your time.


The shrink-wrapped box size will be updated whenever neighbor
lists are re-computed. So you can only control that indirectly. If what
you are asking is how to know the volume, then you can always dump
snapshots whenever you like, and compute the bounding box on the
atoms yourself. Note that the "volume" of a system with a free
surface (which may be rough) is an ambiguous concept from the start.