Positive Coulombic Interaction Energy


In a system under study, I have some Al ions, and I want to measure the interaction of ions with each other using compute group/group command.
I am getting negative col energy and positive value for pair interactions. I was wondering if negative value for coulombic interactions between positively charged ions is correct/possible? Do you have any idea about this? Thank you.

Input file:

boundary p p p
units real
atom_style full
bond_style harmonic
angle_style harmonic
dihedral_style opls
improper_style harmonic
pair_style soft 12
pair_style lj/cut/coul/long 10.0 10.0

read_data data.data

group Al type 3

set type 3 charge +3.000000

pair_modify mix geometric tail yes table 12

compute Int_Col_Al_Al Al group/group Al pair no kspace yes boundary yes
compute Int_Pair_Al_Al Al group/group Al pair yes kspace no boundary yes

fix Int_Al_Al all ave/time 5 50 500 c_Int_Col_Al_Al[0] c_Int_Pair_Al_Al[0] file tmp_Al_Al.Int

Why not paste the complete input script? There is no information on the kspace style nor what energy you are referring to. “ecoul” between same-signed ions is strictly positive, while “elong” can be anything. With the system being non-neutral, elong does not make sense either. Anyway, I would say this is not a good test.


Sorry! I missed some parts. Other settings are:

kspace_style ewald 1e-4
special_bonds lj/coul 0.0 0.0 0.5
neighbor 1 bin
neigh_modify delay 0 every 1 check yes page 1000000
comm_modify mode single cutoff 5 vel no

I’m referring to the energies that are measured by compute group/group.

Therefore, what can we do for non-neutral systems? Specifically, how can we measure the interaction energies between different groups in lammps?
Can we measure “evdwl + ecoul” only between certain groups as the interaction energy?
Thank you for your help.