How can I achieve my idea with Reaxff forcefiled in lammps

Dear lammps users

I want to use lammps to do my simulation following this idea.

First, I construct a cluster which contains lots of stacked molecules, one type of molecules are placed at the outer shell of the cluster, while another type of molecules are made up of the core of the cluster (Just as the attached pictures describes). And then, I will use oxygen molecules to oxidize this cluster with the Reaxff forcefiled, meanwhile I hope to keep the core-shell structure unchanged because I want analyze the oxidize process from the shell to the core. The cluster contains the stack molecules which hold together by Van der Waals’ force, and they will fall apart during the oxidization processes at high temperature without doing something to the cluster, thus I cannot achieve what I want.

My question is how can I achieve my idea with Reaxff forcefiled in lammps?


Fan Li


What is the physics reason for why the molecules should

stay together during/after oxidation. If that effect is

not part of the force field or some externally applied

force, then the MD simulation will not do it either.


Hi Steve

The cluster represents C black which is intact and contains stacked molecules at high temperature, and this is also proved by experiments. But, which kind of force keeps them together is unclear.

You mentioned that externally applied force could keep them together. How can I apply such force in Lammps, meanwhile it is better that this kind of force has no effect on the oxidation mechanism which I want to analyze.


Fan Li

If particles in the cluster have a LJ interaction

with each other with a deep(er) well (set by epsilon)

they will effectively stick together. If/how that

will alter the oxidation via ReaxFF, I don’t know.



How thick is your shell? Oxidation and diffusion are slow processes and are often beyond the MD timescale. ReaxFF is also an expensive and slow potential. These two factors combined, you might want to reconsider your approach. Also, during the oxidation of the core shell structure, roles played by the center carbon black does not seem to be important. You need to discuss with your colleagues and advisor on the overall approach of your simulation.


Hi Ray

The thickness of the shell is about 0.8~1nm which is as same as the core.

My original intention is to compare the oxidation mechanism between two different models with the same number of atoms (2400 atoms). The first one is the core-shelled C black, and the second one is only made up of molecules 2 (see attachment “structure”). Just as you said the center carbon black is not important, so the two models do have to be totally oxidized, and I think the 0.8nm of the two structures being oxidized is enough to compare the results.

I will increase the concentration of the O2 molecules and the reacting temperature to increase the reaction rate (I will use 3000 K for the oxidation process), and those two methods are often used in Reaxff simulation to accelerate the simulation.

But, I have encountered a problem. Just as Steve said the stacked molecules in the cluster have LJ potential and the molecules can be hold together by LJ potential at 1000 K, however it falls apart at higher reacting temperature (3000 K) which makes me cannot achieve my goal, meanwhile falling apart at 3000 K also does not conform to the experimental observation.

My question is how I can solve my problem in lammps (for example, applying externall force and using hybrid forcefield ), meanwhile it is better to have no/limited effect on the oxidation mechanism.

I have talked to my advisor, but his advice is meaningless. So, it is extremely grateful if you can comment on my idea and tell me how to achieve it in lammps from your experience.


Fan Li


What happens if you describe the entire structure with only ReaxFF and not hybrid it with LJ?