Using addforce to create 1 bar pressure

LAMMPS LAMMPS General Discussion
1

Hello,

I am running a PIM-1 polymer simulation with water both side on it. There is also graphene wall both side on the water chamber to create a pressure of 1 bar on each wall so that the water can insert into the polymer. In a NVT simulation, to create the 1 bar pressure on the wall , I am using the addforce command( fix 1 wall1 addforce 0.0 0.0 3.5633e-5) and I have calculated the force for the wall in real unit 3.5633e-5 Kcal/mol-angstrom ( as I have 540 atoms in the each graphene wall and the wall area is 36.27 x 36.86 (angstrom)^2).

However, after running for almost 1 ns the water become utterly compressed and so the polymer as you can see from the picture. It is not supposed to be compressed in only 1 bar pressure. I am looking for suggestion if I am doing it correct for creating the pressure using the addforce command ?

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For your reference, I have equilibrated both the water and the polymer without adding any pressure & ensured that both of them have meet the correct density before applying the pressure. My lammps version is 21November 2023.

Regards,
Morshed Mahmud
Ph.D. Student
Rennes Institute of chemical science , France

2

If the system gets compressed more than expected and the resulting density is higher than expected, the logical explanation would be that your force is too large, probably by one or more orders of magnitude.

However, how to determine that force for your specific system is a question of general physics (and calculus) and not a LAMMPS problem.

3

That sounds ominous. When a system that has been equilibrated under 0 bars of pressure is suddenly subjected to 1 bar of pressure, it will be very unhappy, and unhappy systems (like unhappy people) often do strange and unhelpful things.

4

Sorry for the confusion. I meant without any external pressure to the wall. The system equlibrated in NVT ensemble and obviously not zero bar pressure was there.

However, I fix the problem using aveforce instead of using addforce. I interpreted the wrong meaning of the addforce command before.

Thank you everyone for their suggestions.