pair-write the lj/cut/coul/long potential

Dear LAMMPS Users,

I'm trying to create a tabulated potential for K-Cl interaction, to make sure my potential is reasonable, I first created a tabulated potential that includes lj interaction and coulombic interaction and compared with that of LAMMPS pair style: lj/cut/coul/long. The lj parameters I used are: epsilon=2.5 eV, sigma=3.145 A and the charges for the two particles are +1e and -1e respectively. However, I found the potential I created is different from that of lj/cut/coul/long outputed by pair_write command. I have attached the plot where the plot of the function is also provided. As you can find, the table I generated has an energy minimum value way lower than that from pair style lj/cut/coul/long.

My further analysis seems to suggest that the difference comes from the coul/long interaction described by lammps, its amplitude is always smaller than the simple coulombic function E=-14.41/r. Can anyone please advise what causes this difference?

Thanks very much!

Chun Tang

lj-cut-coul-long.png

From the pair lj/cut/coul/long doc page:

Styles lj/cut/coul/long and lj/cut/coul/msm compute the same Coulombic interactions as style lj/cut/coul/cut except that an additional damping factor is applied to the Coulombic term so it can be used in conjunction with the kspace_style command and its ewald or pppm option. The Coulombic cutoff specified for this style means that pairwise interactions within this distance are computed directly; interactions outside that distance are computed in reciprocal space.

In other words the “long” potentials multiply the 1/r term by a damping factor
that goes to 0 at the cutoff. Any reference on PPPM or PME will explain this,
it’s an erfc() function.

Steve

Dear Steve,

Just want to confirm: So does this mean that the output potential provided by pair_write command does not include the long range interaction dealed by pppm summation?

Thanks,
Chun

Dear Steve,

Just want to confirm: So does this mean that the output potential provided
by pair_write command does not include the long range interaction dealed by
pppm summation?

it *cannot* because that interaction is not a pairwise interaction.

axel.