QEQ point charge model

Dear LAMMPS users,

I am trying to understand how charge equilibration methods works in LAMMPS, particularly qeq/point (the simplest one). I have gone through the code, I stuck at the position where H matix is computed. According to Rappe and Goddard paper, the coulomb potential (J_AB) between two charge centres have been calculated as 14.4/r (14.4 - constant is added for the unit conversion Angstrom into eV), where r is the distance between the centres A and B.

Inside fix_qeq_point.cpp file, I don’t see this conversion factor, instead I see 1/2r (inside compute_H). Could you please explain me about this 1/2r factor? Is this enough for the conversion into eV?

In my case, I consider two atoms (for e.g., LiCl) and try to equalize their electronegativities by changing their charge values (using excel), and compared with the charge values that obtained from simulations. I have got different charge values compared to simulation values when I use (14.4/r), however, when I use 1/2r, almost end up with the close electronegativity values.

Your reply will be helpful and clearly understand the charge model.

Thank in advance



fix qeq/variant does not have a hard-coded unit system so there is no need for the 14.4 conversion from eV to Kcal/mol. The unit used in qeq/variant will depend on your units command. 1/2r is just a result of taking the derivative of q_i*q_j/r with respect to q_j (q_i is in another matrix). 1/2 is there due to double counting.