[lammps-users] Determing the AIREBO forces only

Dear LAMMPS users

I am trying to determine the friction forces contributed by water molecules moving past a nano-tube. The nanotube is essentially at rest with no initial forces. When a water molecule collides with the walls of the nanotube, I intend to record this impact force. Since I wish to allow the nanotube walls to be able to flex, I want to sum up only the C-C AIREBO interaction forces (both the REBO and LJ C-C forces), since this should equate with the collision forces of the water molecules.

I understand that LAMMPS only allows the computation of the total force. By summing up the total forces on the nanotube, I would obtain 0 force. Can any LAMMPS user suggest where I can modify the code to extract the sum of the C-C AIREBO interaction forces? I have looked into the pair-AIREBO.cpp file but am not sure where exactly to extract the appropriate forces. I am using the AIREBO potential with LJ but without torsion.

Appreciate any suggestions.

Huck Beng, Chew

Why would the C-C force be the force on the nanotube due
to the water? If you sum the total force on all nanotube
atoms, wouldn't that be the force due to external atoms (the


I did something similar with Tersoff potential for C-C and a LJ for C-gas inside the nanotube (not with LAMMPS). It was not easy to flex the walls as it seemed the long nanotube was not stable, at least for the Tersoff potential.

See NanoLetters, 6 (2006) 1893.


Chew Huck Beng wrote: