[lammps-users] Using pair style hybrid ( lj/charmm/coul/charmm and another pair style with coul/long format )

Dear all,
I simulated a process, and the system has two different molecules.
Molecule A with 4 different atoms (atom 1\2\3\4); Molecule B with 1 atom (atom 5).
Atom 5 has a charge of “0”, Atom 1-4 have nonzero charges.
Three kinds of pair styles are used: lj/charmm/coul/charmm, lj/charmm/coul/charmm/implicit, and lj/charmm/coul/long
The lj/charmm/coul/charmm is used for the inside of Molecule A: 1-1, 1-2, 1-3, 1-4, 2-2, 2-3, 2-4, 3-3, 3-4, and 4-4.
The lj/charmm/coul/charmm/implicit is used for the contact between Molecule B and Molecule A: 5-1, 5-2, 5-3, and 5-4.
The lj/charmm/coul/long is used for the inside of Molecule B: 5-5.
I thought, due to the charge of Atom 5 (0), so whether lj/charmm/coul/charmm/implicit or lj/charmm/coul/long will only calculate the lj/charmm part. So these two formulas are the same. And also, Molecule A will not calculate KSPACE.
Then I used the command :
compute Apeatom A pe/atom kspace
compute Ape A reduce sum c_Apeatom
I found that c_Apeatom is not 0. I don’t know why Molecule A has a long-range Coulomb force. And them I change lj/charmm/coul/long to lj/cut/coul/long. Apeatom is still not 0.

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Your model is incorrect and your logic is flawed.

The kspace calculation will always apply to all atoms because it is a formulation where the interactions for atoms with all other atoms and their periodic replica to infinity is computed. if you don’t want coulomb interactions between atoms of type 5 you should also use lj/charmm/coul/charmm and not use kspace at all.
Your assumption of “molecule A will not compute kspace” is not correct.