how to set the Bjerrum length in lammps?

I don't know. Try emailing Mark Stevens (after reading
his paper, which might well explain it). msteve at


Please check this as well:

If you understand the meaning of the Bjerrum length, then you must be
aware that for a given temperature, it is totally equivalent to define
the dielectric constant of the solvent, as they are inversely
proportional. It is just a matter of computing by yourself the
dielectric constant you wish to impose to your implicit solvent in
order to get the required Bjerrum length.