External pressure over system

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Hi ; all of you
I have used the Lammps in the simulation.
System Contains:

  1. layer of graphene (500 atom of carbon) as piston (high pressure) and The area of graphene is 1290.5 A2
  2. 2598 molecule of water and 10 pair of NaCl in feed reserve
  3. graphene as membrane
  4. 780 molecule of water in permeate reserve
  5. layer of graphene (500 atom of carbon) as piston (Low pressure) and The area of graphene is 1290.5 A2

  • The first question
    I want to apply 1000 atmosphere of pressure to feed reservoir (and 1 atmosphere of pressure to permeate reservoir separately) and I want to calculate the pressure . Is it enough to consider number of carbon and area of carbon in each piston? Or I should consider the number of water molecules in each reservoir?
    I calculated for 1000 atmosphere (came below) but it did not apply even amount of 1 atmosphere in magnitude in simulation .and I think , I am wrong .
    ?kcal/mol.A=1000atm * 1/500 atom * 1290 A2 (area)* 6.022 e 23 atom /1mol * 1L/1 e27 A3 *0.101325 KJ/1 L.atm * 1Kcal/4.184 KJ = 0. 038 Kcal/mol.A

  • The second question
    I have calculated a pressure and run the simulation. It seems that the pressure is not constant during the simulation . pressure increase in feed reservoir, when number of water molecules decrease and pressure decrease in permeate reservoir ,when number of water molecules increase during simulation (Because of water passes through the pore). How can I apply constant pressure throughout the simulation?
    I think that the number of water molecules in calculation of pressure are important.is it Right?
    Best regards,

Ebrahim