thermal conductivity calculation problem

Please keep the list in the loop.

yes, static means stable, not flat. If one side is cold,
the other hot, the temperature profile can't be flat.

The heat flux tallied by fix thermal/cond is just a cummulative
number. You divide by elapsed time and that is the flux rate.
It's just 2 numbers. You don't need LAMMPS to do it for you.