Dear LAMMPS users,

I seem to be getting unreasonable results when using the pair style lj/long/coul/long with the 31Mar2017 version of lammps (and older versions).

In particular I was simulating the wetting behavior of water droplets on rigid substrates. When I use a lj/cut potential for the substrate-water interaction I see that there is a strong dependence of the wetting-transition temperature, which converges with Rcut. However when I compute the transition temperature with lj/long/coul/long the result doesn’t agree with the largest hard cut-off simulation but rather with the simulation that has the same hard cut-off Rcut as the real space cut-off Rreal in the lj/long one. It seems as if the long-range part of the forces is missing (although I notice a computational slow-down when using lj/long).

To confirm this I looked at a simpler system: a single atom above a slab of rigid atoms, interacting with lj/cut or lj/long/coul/long respectively. I computed binding curves of this atom for different choices of Rcut/Rreal. In the picture (link: ) you can see that the curves for lj/cut and lj/long/coul/long are identical (slightly shifted in x-direction because otherwise they exactly overlap) if Rcut=Rreal which again suggests that the long-range part is missing. Am I right in the assumption that the binding-curve for lj/long/coul/long should in principle be independent of the real space cut-off used?

Here is the part of my input concerning the interactions:

#Eps=0.68, Sigma=2.5027, nCut varies

pair_style lj/long/coul/long long off (v_nCut*v_Sigma) pair_coeff 1 2 {Eps} {Sigma} # C-mW pair_coeff 2 2 0.0 {Sigma} # C-C

pair_coeff 1 1 0.0 ${Sigma} # mW-mW

kspace_style pppm/disp 1.0e-4

kspace_modify force/disp/real 0.0001

kspace_modify force/disp/kspace 0.002

kspace_modify mix/disp none

neigh_modify delay 0 every 1 check yes

which results in the following output about PPPMdisp:

PPPMDisp initialization …

Using 2 structure factors

Dispersion G vector (1/distance) = 1.41955e-15

Dispersion grid = 2 2 2

Dispersion stencil order = 5

Dispersion estimated absolute RMS force accuracy = 0.000120517

Dispersion estimated absolute real space RMS force accuracy = 0.000120517

Dispersion estimated absolute kspace RMS force accuracy = 0

Dispersion estimated relative force accuracy = 3.62934e-07

using double precision FFTs

3d grid and FFT values/proc dispersion = 216 2

I read the corresponding parts of the manual and also tried to choose the kspace settings by hand, but to no avail, i.e. there seems to be no long-range part no matter what I do. Is there something wrong with the way I am using these commands or could there be a bug?

Best wishes,

Martin