Dear Lammps users,
I used compute group/group command to get the force on the group. The command is below.
compute friction_ion ion group/group water kspace yes
In theory, the force on group “ion” and group “water” should have opposite sign and same value. I wonder which force lammps calculates when using above command. The force on the group “ion” or the force on the group “water”?
The documentation does say that compute group/group computes “the force on the compute group atoms”. This is the group used to define the command – not “group2” which is given as one of the variables.
This information could certainly be given more clearly in the first paragraph. Having said that, it should be very fast to make a two-particle box (with create_box, create_atoms and so on) and confirm for yourself which force is reported.