# Applying pressure to a thick rigid wall

Hello all,

I have a question which seems to be trivial, but still I am not sure of it.

I want to apply pressure to a rigid wall with a thickness of a number of atomic layers, and the size of the simulation box’s side, i.e. (lx*ly). As I could understand, I first need to calculate the needed force, and then apply the force to that group. I am using LAMMPS 64-bit 20150616, “units metals”, and here is how I calculate the force and apply it:

variable P equal 1 #Let’s say I want to apply only 1 Pa
variable GP equal v_P/10^9 #Pa -> GPa (you may ask why, but let’s keep it as is!)
variable mF equal v_GP*(lx*ly)*0.01 #GPa * Angs^2 -> nN
variable F equal v_mF/1.602 #nN -> eV/Angs

Now the calculated force is applied to the target group (the rigid wall) via “fix aveforce”.

I don’t know if there is any miscalculation, but the results suggest that the system is facing a pressure of 1 MPa instead of 1 Pa (the desired value). Before making any conclusion out of my work, I need to be sure whether it’s correct or not, but I don’t know how to test it. So, do you have any suggestions? (Or do you see any mistakes which is hidden from me?)