[lammps-users] minimum image criteria

Dear all lammps-users,

I am trying to do a simple computation in lammps but I am really confused.

I want to compute the interacting force between two atoms in a periodic box. My script reads the initial positions from a data file and print the net force on each atom in a dump file. But the result changes when I change the size of the box. This is strange because the distance between two atoms is always less than half size of the box.

Here are the data files that differ only in box bounds.

Yes, LAMMPS does the min img and periodic BC correctly.
What cutoff are you using with pair SW?