I am new to GULP. I want to simulate bilayer graphene to calculate the phonons and the eigenvectors generated during the phonon calculations. In LAMMPS, I can do it easily. I can define the intralayer and interlayer potential by pair style and pair coefficient. Here, I am using tersoff for intralayer and LJ for interlayer potential. I am having difficulty doing this in GULP.

If anyone has experience in doing this, kindly let me know how to define different potentials for different interaction types. It would be a great help to me.

In GULP you can add together as many different potentials as you like so the only issue is how to handle the distinction between layers. There are 2 easy ways you can do this in GULP:

If you define the system in terms of “molecules” then each layer can be a different molecule and so the LJ potential then just has to be specified to be “intermolecular”
e.g.
lenn 12 6 inter
C C 1000.0 10.0 0.0 12.0
Defining molecules can be done by adding the keywords “mole”, “molq” or “molmec”. Because you have a periodic 2D sheet then you can use either “molq” or “molmec” (see help for reasons why)

If your system just has 2 layers (you can do this in 2D boundary conditions) then label the atoms in one layer as type 1 and the other as type 2 (i.e. call them C1 & C2). Then the input for the LJ part would just be:
e.g.
lenn 12 6
C1 C2 1000.0 10.0 0.0 12.0
and it would act only between the C1 atoms and the C2 atoms.

Hope that helps.
Julian

PS There are the Brenner (REBO), ReaxFF and pGFNFF (version 6.1) models which automatically work for graphene, though at present at gamma point only.