I did tensile test on a metallic glass slab (size z>y>x) along z direction, with free surface on x and y direction, under NPT at 100 K. When the vacuum layer in x and y set as 20 angstroms, the peak stress along z was about 1.8 GPa, and when the vacuum layer set as 100 angstroms in x and 500 angstroms in y, the stress was down to about 0.3 GPa. During this two tests all other parameters were same except the initial velocities of atom different. I expected the strength along z should be the same. Any explanation? Thanks.
My setting file reads like: