# Fix Aveforce calculation of the units to generate pressure on piston

Hi Lammps-Users,

I just need a quick check on my understanding of the fix aveforce function. In my simulation I’m using it to apply a force to a graphene sheet that is thereby applying a pressure to a reverse osmosis system. I want to apply a pressure of 15 atm over a graphene sheet that is ~30 x 30 Å and ~450 atoms.

When I’m calculating the force I need to apply over the area of the graphene sheet I simply converted the 15 atm pressure into the force applied over the area, but because aveforce is a force applied per atom would the area I’m applying the force be over the area of the entire graphene sheet (gaps between atoms included) or only over the cross sectional area of each atom?

My intuition tells me it would be the area of the entire graphene sheet despite aveforce being applied per atom but I’m not entirely sure how I should approach the application of the force based on my desired pressure and apparently after going through these relatively simple conversion calculations twice now, neither number I got ended up being correct based on the output data I’ve obtained being rather absurd.

For 15 atm of pressure I calculated a desired force of ~1.5*10^-11 N based on using the area of the graphene, but this would be significantly different if I used cross sectional areas of the carbon atoms instead. I noticed that for units real force is in units of Kcal/mol-Å. This seems to imply to me that I need to be doing this on the basis of the area of the carbon atoms not the whole sheet since I can convert the mol unit into a per atom unit.

Out of curiosity is there a suggested alternative way to do what I’m doing instead of fix aveforce?