Thank you for your answer. But I want to know the total force acting on a group of atoms. FCM command allows you to do that but I am not able to print it in a file.
LAMMPS does not have a built in function for that to my knowledge. Craig supplied a fix that, in combination with a group definition, will be able to give you the sum of forces of a group and output the resulting vector to a file. You could use his suggestion as a step-off to your own refinements. It would need to include something like inr group.cpp: Group::fcm().
For a group of atoms in a sliced layer i was using something like
fix 1 all ave/spatial 10 1000 x center 10 force.fx atom fx norm all
The file prints avg force/atom and no. of atoms in that layer
I am not sure whether it will help your case.
Narulkar, Rutuparna wrote: