Sorry if you get this twice, I made a small error in describing the test
job -- it is r=1.0 angstrom to r=12.0 angstrom with a step size of 0.01
angstrom/step. Best Regards, John

Dear Lammps-users,
I think I found a bug in coul/wolf. It doesn't look like the energy goes
to zero for r > rcut.

Here is a test job which scans the coulombic energy for an arbitrary dimer
with +1.0 charge on each atom for r = 1.0 angstroms to r = 10.0 angstroms
in increments of 0.1 angstroms per step.

Thanks for pointing this out. After examining the potential and the
original paper, I don't think it is a bug that energy is not zero when
using the Wolf Summation method when rij > Rc. Rather it is the nature of
the summation method.

Wolf sum is a method based on Ewald sum with the following
modifications/approximations: spherical truncation with a finite cutoff,
damping via the complementary error function, charge neutralized within
the cutoff sphere, and dropping off the slow converging error function
term by adding and subtracting a self energy term. The total energy from
Wolf after all that becomes a sum of a shifted Ewald (charge-neutralized)
potential and a self term (Etot = Eew - Eself).

Ewald energy (Eew) does go to zero when rij > Rc, but Eself is a constant
that depends on damping, Rc and charge only. This constant does not
vanish when rij > Rc, and it is why you see different energies when rij >
Rc with different damping and Rc values. Two atoms in a vacuum is an
extreme case for Wolf summation method, and the PotE does not go to zero
if you use Ewald sum with commands "pair_style coul/long" with
"kspace_style ewald". I don't think you can compare Wolf/Ewald with a
pure cutoff method like coul/cut.

By the way, the damping (0.3 you used) is a factor determining how fast
the complementary error function (erfc) falls off from one to zero with
increasing rij. Erfc falls off slower with a smaller value of damping
(0.1-0.2), and the energy/forces converges better.

For more info, Sections III and V of "D. Wolf, J. Chem. Phys. 110 17 1999"
have detailed descriptions.