shear stress calculation

Dear Lammps users,

I’m shearing my model with displacing the upper block with fix move command, fixed the bottom layer and let middel mobile under nvt.

And I using two method to calculate the stress (press)


compute stress all stress/atom NULL

compute Supper upper reduce ave c_stress[5]

compute vor all voronoi/atom

compute Sv all reduce ave c_vor[1]

variable stress_atom equal c_Supper/c_Sv/10 #bars to Mpa


compute Fx upper reduce sum fx

variable Fshear equal (c_Fx/lx*ly)*160217.66208 #ev/A to MPa

The result of (I) is 10 times to (II)

Can anyone tell me where is wrong?

Best wishes,


For method (II), why would summing the Fx force
over all atoms give a pressure?