As I understand right now, there is no option in LAMMPS to add constant external electric field energy to charge equlibration with QeQ variants. How difficult would be to implement it? Are there plans to implement something similar in QeQ schemes in the future?
The reason I am interested in this, is that I think it would, for example allow to simulate a parallel plate capacitor setup where one of the “plates” is represented by the electric field and the other one is our system under interest, for example a copper slab. This would allow to investigate surface polarization effects, like surface charge distribution in the case of non-planar geometries. Also would allow to calculate the electrostatic force on surface atoms, when used with fix efield(which of right now only adds forces as I understand)
Kristian Kuppart, University of Tartu
It is not too difficult to add the contribution from external electric field to QEq. The fix efield in LAMMPS adds a force that is based on f = qE, so you can derive the energy term for the external electric field. Then take the derivative of the energy expression with respect to charge, which is then the charge force from the external field, and add it into the QEq expression.
Thank you for your answer. As I understand it, the energy contribution in the case of a constant field would be really simple, V_field=qEr, so if we take the derivative we should simply add Er to the equation, where E is the electric field and r is the position vector of the atom. Is there anything wrong with my reasoning from the first glance? Also in my proposed test model, would it be a problem if the total system would not be charge-neutral? If not, would it be necessary to somehow set the total charge manually?