External applied pressure vs. confined fluid pressure

Dear LAMMPS users,

By using fix aveforce (0.02 Kcal/mol/A) on a 900 A^2 piston composed of 441 atoms it will give a total force of 8.82 Kcal/mol/A upon that piston, and a corresponding pressure of 0.0098 Kcal/mol/A^3 (or ~672 atm). Right?

Now my question is: if I confine a fluid between two pistons and compress it as described above, i.e., by applying an external force upon one piston, the pressure in the fluid would not be the mentioned ~672 atm (at least, close of it)? I have tested this story against 825 SPC/E water molecules and got an average pressure exactly 10 times smaller (it seems I am missing just a factor of 10…). What am I missing here? My input and system data go in annex in case someone want to take a look.

All the best,


system.data (302 KB)

in.press (1.79 KB)