Application of external electric field into Lammps reax simulation

Good day everyone,

I am working on a research project which involves using the reax/c simulation parameters in Lammps. For the convention system, I know an external electrical field can be implemented using ‘fix efield x y z’. I have done reading up about this and I understand that this does not technically create an electric field, but rather, introduces a force on each atom based on the atom’s charge. I would like to simulate the effect of an electrical field in reax, but when I use ‘fix efield’ the results are noticeably different than the same simulation in the conventional system. I have done a series of simulations with all the same parameters except the ‘x’ in the ‘fix efield’ but the results between these simulations are nearly identical.

I have done some reading on the lammps forums and I saw that it was mentioned that “if you want to model the reaction of a system modeled with reaxff under an external electrical field, you have to modify the reaxff code accordingly in a rather non-trivial manner”. Has anyone come across this issue and found a way to fix it? I would expect there to be a simpler way to incorporate an external electrical field with reax/c, similarly to the conventional system, but have yet to find a way to do it.

Any assistance or references would be very much appreciated.

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No there is no simpler way. You have to include the electric field into the charge equilibration. That looks at the polarization of the atoms due to the electrostatic potential caused by the environment. For fixed charges, that is not a factor so it is sufficient to only compute the force added to atoms, which is what fix efield does.

At that point the situation gets more complicated, because you cannot really have normal periodic boundary conditions in the direction of the electric field, because the potential would have to “jump back” when going from one side to the other side of the box, so similar like kspace style with slab correction, you would have to have a vacuum region where atoms cannot go to have that jump in the potential.

Thank you for taking the time to respond, I appreciate it. Any recommendations on how to go about adding the electric field into the charge equilibration? I’m pretty new to LAMMPS in general and I work well with examples. Would you be able to point me to any references I can read up on which explain this or any .in lammps files that use reax/c and an electric field? I have not been able to find many references regarding people being able to work around this. I could go into more details about the process we are using for our simulations if necessary.

No recommendations. You will have to study relevant literature on charge equilibration and Reaxff and then study the source code of the qeq fixes used. This is non-trivial.