Dear LAMMPS users,
I found that long-range Coulomb potential (lj/cut/coul/long) with kspace_style ewald
generates forces between atoms, even if the atoms do not have interactions.
Is this an intended behavior of LAMMPS?
The tested code is very simple with only two atoms, as follows,
atom_style charge
atom_modify map array sort 0 0
units metal
boundary p p p
read_data lmp_manual_in_mm1_2.data
pair_style hybrid/overlay lj/cut 12.0 lj/cut/coul/long 12.0 12.0
pair_coeff * * lj/cut 0 0
pair_coeff 2 3 lj/cut/coul/long 0 0
pair_coeff 1 3 lj/cut/coul/long 0 0
kspace_style ewald 1.0e-4
mass 1 22.989769274086011563440479221754
mass 2 35.449999990880691314032446825877
mass 3 12.010999996910237896940998325590
set atom 1 charge 0.8
set atom 2 charge -0.8
set atom 3 charge 0
thermo_style custom step etotal temp lx vol press pxx pyy pxy
thermo 1
dump 4a all custom 1 dump.ewald_mm1 id type q x y z fx fy fz
#fix f1 all nve
run 0
data file is shown below. (The name is “lmp_manual_in_mm1_2.data”)
2 atoms
3 atom types
0.0 20 xlo xhi
0.0 20 ylo yhi
0.0 20 zlo zhi
Atoms
1 1 0 10 10 10
2 2 0 10 10 14
The dump file of the result, dump.ewald_mm1 shows
ITEM: TIMESTEP
0
ITEM: NUMBER OF ATOMS
2
ITEM: BOX BOUNDS pp pp pp
0.0000000000000000e+00 2.0000000000000000e+01
0.0000000000000000e+00 2.0000000000000000e+01
0.0000000000000000e+00 2.0000000000000000e+01
ITEM: ATOMS id type q x y z fx fy fz
1 1 0.8 10 10 10 1.89175e-18 1.57951e-18 0.0795596
2 2 -0.8 10 10 14 -1.89297e-18 -1.8637e-18 -0.0795596
This gives some forces in z direction that are not negligible.
If I calculate with “lj/cut/coul/cut”, short-range interaction without Ewald sum, the forces are just zero.
Would you please kindly tell me if there is a reason why force appears even if there is no atoms that have non-zero interaction in the system?
I checked this behavior in lammps-stable_29Sep2021 and 2022-09-18 version of windows executable (LAMMPS-64bit-latest.exe).
I would be happy to hear your opinion.
Best regards,
K. HISAMA