Two questions about the COMB potential

Hi Jun,

The setup looks good, but I would keep qeq/comb to apply to all. First
reason is that your g_heat and g_fix are all Cu atoms, which take very few
iterations to equilibrate charges, second reason is that unless your
g_surface is very thick your g_heat still has the chance to oxidation.

Just to clarify, Cu oxidation results in the paper was performed with
in-house code using the COMB2011 variation. While in LAMMPS, we have only
the COMB2010 variation. You can still perform Cu oxidation with LAMMPS
COMB, but the results will not be the same as in the paper.


Hi, Ray,
I have tried to simulate the oxidation of Cu surface using 4 groups of atoms as I descried. The commands are as follows:

fix 1 all nve
fix 2 all qeq/comb 1 0.001

fix 3 g_heat temp/rescale 10 300 300 0.01 1.0
fix 4 g_fix setforce 0.0 0.0 0.0

However, when the simulation proceeds, the charges of some atoms become infinity, so the iteration will be endless. Is it related to the following two questions?
(1) In my simulation, the initial positions of oxygen atoms are 30A away from the Cu surface. Could I use “fix qeq/comb” command to these oxygen atoms at the initial time since they are so far away from Cu surface? I doubt that the “fix qeq” command could only be used when the oxygen atoms are close to Cu surface, is it right?
(2) In my simulation, the calculating time step is set as 0.0001ps. Is it acceptable?


Hi, Ray. In my simulation, I use “fix deposit” command to deposit the oxygen atoms one by one on the Cu surface. Therefore, the number of simulation atoms continuously increases due to deposition. As the simulation proceeds, the charge equilibrium is hard to be accessible. I have a question to consult you: is the “fix deposit” command compatible with “fix qeq/comb” command?

Thanks again for your so many explanations. You are so kind.