[lammps-users] problem in calculation of thermal conductivity

You have 160 layers in z. The fix will swap KE between atoms
in the 1st and 81st layer (as I recall, not looking at the code this AM).
This assumes periodic BC in z. If you have no atoms in either of
those layers, then no swaps will occur. We should probably allow
the user to choose which layers to do swaps from, which could
work better for non-periodic systems.