Dear Dr. Kohlmeyer,
Thank you for the reply.
Regarding your question about double counting I am still thinking about it. Let me share with you by some details on my simulations. I am going to simulate bilayer graphene formed on SiC substrate. In the documentation file i met a paragraph about hybrid/overlay using for carbon nanotube simulation on Si substrate.
please point me to the exact location of that text. i suspect that you are either misunderstanding what is written there, or there is a misleading formulation in the documentation.
In the document authors suggested to switch off C/C interactions in the tersoff potential. In my calculations i used pair_coeff atom types none command in order to switch off the C/C interactions in the tersoff potential (see the fragment of the lammps script below). Thus, C/C interactions are described only by AIREBO potential. However, what is happening at the interface between graphene and SiC is quite interesting question. I have checked my simulation for a small simulation box via fix NVE and it works well. It is known from experimental data that the buffer layer between a SiC and graphene layers is reconstructed and if i will get such reconstruction after the MD simulations i think it would be reasonable to use such hybrid/overlay style.
what you are neglecting is the fact, that both Tersoff and AIREBO are manybody potentials and thus they have not only pairwise additive components but also their potential also includes terms that depend triples or quadruples of atoms. So in your SiC you have to include not only Si-Si, Si-C, and C-C interactions, but there also are Si-Si-Si, Si-Si-C or Si-C-C and so on. By turning off C-C interactions you also remove Si-C-C and thus you are no longer using a correct model. By mixing AIREBO and Tersoff the way you do, you also are including C-C and C-C-C and C-C-C-C interaction inside the SiC to be modeled by AIREBO instead of Tersoff, which also is a very bad thing causing inconsistent behavior.
/// FORCE FIELD START ///
pair_style hybrid/overlay tersoff airebo 3.0
pair_coeff * * tersoff SiC.tersoff Si C
pair_coeff 2 2 none
pair_coeff * * airebo CH.airebo NULL C
/// FORCE FIELD END ///
to alleviate this, please consider the following “rules”. 1) you should be using pair_style hybrid not hybrid/overlay, 2) for hybrid to work properly with manybody potentials, each subset has to be wholly contained, that means all of the SiC has to be modeled by just one type of potential, same for the graphene, which means that you need to have at least three (3) atom types (one for Si in SiC, one for C in SiC, and one for C in graphene). 3) to model the interactions between the manybody subsystems you must use a pairwise additive potential, e.g. lj/cut or morse. for that you need to look up suitable parameters (i don’t have time for that, so i am just putting placeholders below). then your setup would look something like this:
pair_style hybrid tersoff airebo 3.0 lj/cut 10.0
pair_coeff * * tersoff SiC.tersoff Si C NULL
pair_coeff * * airebo CH.airebo NULL NULL C
pair_coeff 1 3 lj/cut <epsilon_SiC> <sigma_SiC>
pair_coeff 2 3 lj/cut <epsilon_CC> <sigma_CC>