I need to get an equilibrated graphene structure.
For this first I have created a 2D configuration of C atoms in the hexagonal lattice and tried to minimize the energy. I am using AIREBO potential.
Ideally I should get a perfect planar structure.
But when I use the initial coordinates such that the “z coordinate is constant”, I find that lammps is throwing an error message sayiing “Cannot use neighbor bins - box size << cutoff”.
If I perturb the z coordinate randomly by a small amount, the minimization proceeds. I want to know why it happens like that.
The second problem is, lammps doesn’t change the box boundaries while minimization. Hence I wanted to know the ideal lattice parameter by running the minimization for various lattice parameters and getting the energy. I find that when I increase the lattice parameter more than 10% of equilibrium value, the energy remains constant wrt lattice parameter though I am well with the cut-off. Also in the final configuration I see the atoms moving up and down in the z-direction which are not supposed to move.
Please help me in figuring our where I am doing a mistake.