I m not sure what it calculates, but the total energy per atom should be around -7eV/atom. This quantity should be larger than -7eV/atom, if it is calculated for T>0.

I do not understand the question. Kcal per mole is simply a unit for energy. Just divide the number by Avogadro's number, so you get the energy for exactly 1000 atom system in Kcal unit. But what is the difference in interpretation of that number anyway?

And with LAMMPS, you can choose the unit you want by using the command "units". So, if you don't like kcal/atom, use another one.  By the way, all results should be equivalent (with different precision, of course).

Thank you guys specially you Abolfazl.

Maybe this helps:

69.7 pN is about 1 kcal/(mol-Angstrom)