# fix deposit and fix qeq/reax or qeq/comb

Hi, all.
I tried to study a deposition problem using command. The reax or comb potential is choosen in my simulation. I hope to observe the charge transformation between the deposition atoms and substrate atoms. However, before the depositon process, the charge distribution of the substrate is non-uniform because of the space asymmetry on the initial calculating step. So, it is difficult to indentify the reasons of charge variation when depositon begins. Maybe it is due to the charge transformation between deposition atoms and substrate atoms, maybe it is due to space asymmetry.
How to make the charge distribution of substrate become uniform on the initial step? Or alternatively, is there a way to exclude the effect of space asymmetry during the deposition process?

Best,
Justin

I don't know what "space asymmetry" means. If you are
depositing charged particles, then you have a basic issue
that your system is not charge neutral at every step as you
add atoms. So that can lead to big variations in charge equilibration.
If you are depositing neutral atoms, then I don't see that
the charge equil will have any strange difficulties.

Ray may wish to comment on doing this with the COMB
or ReaxFF potential.

Steve

Hi Justin,
If by "space asymmetry" you mean that your two vacuum surfaces are not identical, and you are worried that you will not be able to know where the change in atomic charges on your to-be-deposited surface came from - whether the asymmetric slab of substrate itself or depositions of molecules, then you can try first equilibrate your substrate with qeq but without fix deposit, let the system fully relax - both positions and charges, then, and only then, carry on with the fix deposit. I do agree it would be best to deposit neutral species. I am not quite sure if qeq/reax requires a charge neutral simulation box but qeq/comb certainly does. Also if you have a slab that has non-identical surfaces, then it should be difficult to make the charge distribution of substrate uniform. Since the charges will equilibrate themselves based on surroundings.
Best,
Ray

Hi, Ray,
Thanks for your reply. Yes, in my simulation, I deposit neutral species. As you said, I have equilibrated the substrate with qeq but without fix deposit, let the system fully relax before deposition. It can be seen from the charge distributions of the substrate that the charge fluctuations on the two surfaces are much larger than that in interior region of substrate. Attached figure shows this phenomenon. Due to this big fluctuations, it is diffificult to identify where the change of charge came from when deposition begins.