How to output the van der waals energy in graphene with AIREBO potential

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1

Ming,

I assume you do not have reactions between your graphene sheets? If that is the case why not run a simulation with a hybrid potential with REBO and LJ. Use REBO for each sheet, and LJ for between the sheets. Take the sigma and epsilon parameters for CC from Stuart’s 2000 AIREBO paper.

something like:

pair_style hybrid lj/cut 2.5 rebo rebo
pair_coeff 1 2 lj/cut 0.00284 3.4 2.5
pair_coeff * * rebo 1 CH.airebo C NULL

pair_coeff * * rebo 2 CH.airebo NULL C

where atom type 1 is for C in the first sheet and type 2 is for C in the second sheet. I imagine you could then get the energy from each pair_style separately this way.

2

Thanks, Dave! It works well, although I had to increase the lj/cut from 2.5 to 3.5 to make it work.

Thanks,
Ray

3

Dear Ray and Dave,

Thanks for your comments

1: Ray is right. The LJ parameters are totally different from AIREBO published values. Thus, I change the epsilon value from 2.84 meV (AIREBO published value) to 2.39 meV in AIREBO. And in the reference paper, the potentials are Berner+LJ, comparing with REBO+LJ in AIREBO, I believe they should give the same results when the LJ parameters are the same. Right?
2: the first and second methods are expected to give the same value. I obtained the E1, E2, E3 (energy for different layers) by running 1 step instead of 500 million step as shown in my script. Sorry for the misleading script. Therefore, there will be no interlayer sliding, interlayer separation. But they give the different values even so. Ray, you can check the energy difference quickly. Pls give some comments.
3: thanks Dave for the hybrid potential suggestion. I will check this method to get the LJ energy. My final aim is to model the self-folding of graphene. For the single folded graphene, we can define the interaction between two unfolded regions. But for multi-layer folded graphene, the definition of group, the setting of atom type and the specifying the interaction between different atoms are really mess. Any suggestion?

Best wishes,

Ming

4

Replies in-line, thanks.

Ray

Dear Ray and Dave,

Thanks for your comments

1: Ray is right. The LJ parameters are totally different from AIREBO published values. Thus, I change the epsilon value from 2.84 meV (AIREBO published value) to 2.39 meV in AIREBO. And in the reference paper, the potentials are Berner+LJ, comparing with REBO+LJ in AIREBO, I believe they should give the same results when the LJ parameters are the same. Right?

If I remember correctly, the paper used AFEM method, not Brener potential. Even if the Brener potential is used, they do nut guarantee the same results. See below.

2: the first and second methods are expected to give the same value. I obtained the E1, E2, E3 (energy for different layers) by running 1 step instead of 500 million step as shown in my script. Sorry for the misleading script. Therefore, there will be no interlayer sliding, interlayer separation. But they give the different values even so. Ray, you can check the energy difference quickly. Pls give some comments.

If inter-layer LJ is the only LJ contribution, then all methods should give the same result. However, you can not rule out the possibility that you have intra-layer LJ contributions, which will result in different energies.

3: thanks Dave for the hybrid potential suggestion. I will check this method to get the LJ energy. My final aim is to model the self-folding of graphene. For the single folded graphene, we can define the interaction between two unfolded regions. But for multi-layer folded graphene, the definition of group, the setting of atom type and the specifying the interaction between different atoms are really mess. Any suggestion?

Dave’s method may be the only one to obtain LJ energies from each inter-layer interaction on-the-fly. It is really nice, and you will get what you ask for if you put up with a little more complicated input script.

5

Thanks for your comments.

Best

Ming