You were right. I could use ‘"region’" command without “lattice” command after putting “units box” keyword in “region” command.
I have another problem. GaN crystal broke perfectly.
I devided the region of GaN substrate into three kinds of regions, such as surface, damping, and fixed region.
To make this substrate, I used GaN tersoff potential, “nve” ensemble and “fix temp/rescale” command in the damping region to apply temperature effect.
Moreover, I used “fix set force” command to fix the bottommost layers.
Is there any problem?