For style lj unit, I don’t know if perm0 is vacuum permittivity. It is not stated in Lammps manumal. “perm0” is used in the following units:

�� charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2

�� dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2
�� electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon

In LJ units, I believe that param = 1.0.

The way LAMMPS does this is it has a parameter

qqr2e which it defines for each unit system,

that is used in equations like this:

eng = qqr2e * Qi * Qj / Rij;

to convert qq/r to energy units. For LJ, that

factor = 1.0.

In other words:

eng* = Qi* Qj* / r

where eng and Q are in reduced units.

Steve