Discrepancy of Electrostatic energy forms between Lammps and the original Charmm FF

Dear LAMMPS developers,

I have a quick question about the Coulombic energy form used in Lammps: C® = Cq_iq_j/(epsilon*r), in pair_style lj/charmm/coul/charmm http://lammps.sandia.gov/doc/pair_charmm.html

But it doesn’t have any explanation about the coefficients “C” and “epsilon” in the doc page and how to set their values.

Also, in the original electrostatic term in CHARMM FF (see Equation 1. in http://pubs.acs.org/doi/full/10.1021/ct9000608), their Coulobic energy reads: q_i*q_j / r .

Why there is such a difference?

Many thanks,
Best wishes,
Lili

Dear LAMMPS developers,

I have a quick question about the Coulombic energy form used in Lammps:
C(r) = C*q_i*q_j/(epsilon*r), in pair_style lj/charmm/coul/charmm
LAMMPS Molecular Dynamics Simulator

But it doesn't have any explanation about the coefficients "C" and
"epsilon" in the doc page and how to set their values.

Also, in the original electrostatic term in CHARMM FF (see Equation 1. in
http://pubs.acs.org/doi/full/10.1021/ct9000608), their Coulobic energy
reads: q_i*q_j / r .

Why there is such a difference?

​there is no difference in practice.

the LAMMPS expression is more general and considers the case of unit
conversion and allowing for different relative dielectric permittivity.

axel.
​​