shear stresses are different from liquid and sliding slab

Hi all,

I’m calculating shear stress of a liquid film confined by two solid slabs using LAMMPS version 11Aug17. The slabs slide in opposite directions and system is basically in equilibrium. Ideally, the shear stresses from the liquid and the solid should be comparable. However, the former is always lower than the later. Here’s how I calculated each of them.
(1) shear stress (sigma_xz) in the liquid is a sum up of per/atom stresses, that is, (c_p/(lxlyh_fluid))unit2GPa = 0.059 GPa.
(2) stress in the solid is calculated as (-f_4[1]+f_3[1])/2/(lx
ly)*unit2GPa = 0.088 GPa.
I wonder if there is something wrong with my calculations. It would be helpful if someone can provide some suggestions. Following is the detailed code: