I am using a pair_style coul/cut for my simulation with a system containing multiple species with different charges on them. Using pair_write command, I write the potential from LAMMPS for 1-3, 2-3, 3-3 ionic interactions and plot them. The PE and force values written for all the pair interactions turn out to be exactly same despite the fact that the charges are different. The manual says that the Coulomb interaction energy is calculated using CQiQj/(epsilon*r) so I was expecting the pair potentials to be different. Is there something I am missing?
See documentation for pair_write:
If the pair potential is computed between charged atoms, the charges of the pair of interacting atoms can optionally be specified. If not specified, values of Qi = Qj = 1.0 are used.
It’s done this way because LAMMPS stores charges per atom*, not per type. Thus Coulombic interactions usually do not vary “per type”. (If I recall correctly you can set different cutoffs between different type pairs – but that does not seem practically useful.)
*indeed, many LAMMPS atom styles have no charge at all!