Hi all,
I am performing exploratory classical MD calculations for graphane in a rather unusual geometry. In this geometry, all H atoms are on the same side of the sheet, and the graphane sheet is planar. The C-C chemical bonds are about 20% longer than the equilibrium graphene bonds. This geometry has been predicted to be stable by DFT calculations (right now I don't have the reference at hand - I remember it is a PRB rapid comm - sorry for that).
Problem is, my total energy curves versus lattice parameter become linear when I make the lattice parameters smaller than the equilibrium lattice parameter for this configuration (which according to the current REAXFF parametrizations included with LAMMPS is, btw, smaller than that predicted by DFT). I am trying various REAXFF potential files present in LAMMPS, but none give me a description compatible with that from DFT.
Would anyone know of a REAXFF parametrization for highly stretched (that is, very far from equilibrium) C-C bonds? Or, maybe, some simple modification to the existent REAXFF potentials to get a description of these bonds - maybe some cutoff distance for C-C interaction that could be increased, for example?
Thanks in advance!
Hi Sriram, thanks a lot for the suggestion. i will check this out!
Hi Marcos,
As you mentioned this is a very unusual geometry (sp3-like but planar carbonaceous framework with stretched C-C) , chances are that existing ReaxFF potentials may have problems describing it.
Although ReaxFF parameters are transferable, they have been parametrized for a particular set of geometries, forces, etc. So you may need to think about reparameterizing to incorporate this particular peculiar configuration into your reference set (if it is indeed a stable geometry at the DFT level).
Best wishes,
Fedor
Maybe ReaxFF parameters fitted for Ni-C-H system developed by Prof. Adri van Duin is better than that in LAMMPS. In my test on that set parameters, the energy curves versus C-C bond has three minimums corresponding to C-C single, double and triple bond respectively.
Hi Goumans and Sriram,
Unfortunately it seems that I might have to really try and parametrize a new reaxff potential for my weird geometry, which doesn't seem to be a simple task. Sriram's potential might not help me much, as it doesn't include the C-H interactions.
Zongyang, thanks for the suggestion. Would you have the reference for the potential you mentioned? Looks like it has not been included in the December 2014 release of LAMMPS.
Best regards,
Marcos
ReaxFF parameters I mentioned is in supporting information of this reference (J. Phys. Chem. C, 2010, 114 (11), pp 4939–4949). If it is not good enough, you can replace parameters of carbon with that in Sriram's potential. One tips: the format of reax.ffield is strict in LAMMPS due to being read by Fortran code.
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